It’s all about ethylene

The lowly ethylene molecule is responsible for many changes within a plant. Ethylene is used by plants in respiration and to inhibit the movement of auxins, a plant growth hormone. By manipulating auxin levels, plants use ethylene to stimulate seed germination and adventitious root growth. It is also used to bend the plant toward the sun (epinasty). This ripening, or aging, hormone also triggers abscission (leaf and ripe fruit drop), chlorophyll destruction, flower drop, and all the signs of senescence (deterioration related to aging).

The physics of ripening

Fruit is food for seeds and seed spreaders. When a fruit falls to the ground, it provides easy access to important nutrients to the seed or seeds contained in that fruit. It also attracts animals that may help the plant disperse its seeds over a wider range. There’s no sense in attracting fruit eaters if the seed isn’t ready. Just as the seed(s) approach maturity, a series of chemical changes take place within a fruit. That’s when ripening kicks in. We all know that, as a fruit ripens, it becomes sweeter and softer. Generally, fruit becomes less green and more colorful as it ripens, too. This is because enzymes are breaking down the chlorophyll. [One study found that birds of different continents prefer different colors of ripe fruit.] This sweetening and softening is the result of certain enzymes breaking starch down into various sugars, such as fructose, glucose, and sucrose. [Any time you see a word ending in -ose, it’s a sugar.]

Some fruits stop ripening as soon as they leave the parent plant, while others continue ripening. Those that continue ripening are called climacteric. Common climacteric fruits include bananas, apples, mangoes, melons, and apricots. These fruits are frequently stored before appearing on grocery store shelves. Non-climacteric fruits, such as grapes, citrus, and strawberries must be harvested when ripe.

Stratification is a process that fools seeds into thinking they have experienced winter, spring, or both, to help them break dormancy.

Traditionally, stratification referred to the practice of layering seeds with a moist growing medium, such as vermiculite, peat, perlite, sawdust, composted bark, or potting soil. As seeds germinated, they would be removed to a more permanent growing space. There are three types of stratification: warm, cool, and variable.

Learn from your plants
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Many plants have evolved to use cold temperatures as a period of rest and warmth as a trigger to gear up for germination in spring. We still don’t completely understand the magic that happens within a seed. [Part of me hopes we never do!] Simple starches, sugars, and genetic information are somehow transformed into a living, breathing, growing organism. It’s really amazing when you think about it. We can look to those natural processes to get more out of our gardens. There’s no sense bucking millions of years of evolution. Even better, we can put all that evolution to work for us. Keep in mind, however, that some seeds need the extra time for the embryo to fully develop. Pushing them to do too much too soon only weakens them.

Benefits of stratification

​By artificially stratifying seeds, you can decide when they will germinate. This can help you control when a crop might reach harvestable size. It can also help avoid predictable pest or disease infestations. It can also be sued to extend your growing seasons. Since seed dormancy can be quite variable, with some seeds taking over a year or two to germinate, stratification can be used to provide a more favorable environment for new seedlings.

Preparing seeds for stratification

Start with firm, certified disease-free seeds. These seeds will need a little moisture for their period of hibernation, but too much moisture sets the stage for decay. Seeds destined to be stratified need to be soaked for 24 hours. Then shake off excess water and place them in a plastic container with three or four times the seed volume of some type of growing medium. Some people use folded paper towels, which you can certainly use. Professional growers prefer sphagnum moss or vermiculite. Whichever you prefer, add water to the plastic bag and allow the medium some time to absorb all that moisture. Vermiculite absorbs water quickly, while moss may take 8 to 10 hours. Next, squeeze the bag to get rid of most of that water, the same way you would squeeze out a sponge. Gently shake the seed-medium mixture, to distribute the seeds and to incorporate air, before placing the bag in the appropriate temperature-controlled environment.

Be sure to write the seed name and the date stratification was begun on the bag. The next day, pour or squeeze out any excess water. Then, let nature take its course. Periodically check your seeds for signs of rot, desiccation, or germination. Rot should be completely wiped off and the seed allowed to dry out before continuing stratification. Otherwise, the seed can be planted after all signs of decay have been removed. Dried out seeds need more water and closer monitoring. Germinating seeds need, you guessed it - planting!

Cold stratification

Most perennial woody shrubs and trees require cold stratification to germinate. Lettuce, milkweed, delphinium, and violets can also benefit from this process. Cold stratification mimics the conditions of a cold (31 to 41°F), wet winter and is generally used on seeds that naturally ripen in late fall or early winter. Your refrigerator provides the perfect place to cold stratify crops intended for spring planting. You can leave moistened seeds in the refrigerator for 1 to 4 months to get the desired effect. Just make sure they are not sitting in water, or they will rot. Also, do not plant cold stratified seeds in autumn. The double whammy of cold will be more than most seeds can handle.

Warm stratification

Warm stratification is used on seeds from trees and shrubs that naturally ripen in early fall. This is the majority of plants grown in gardens and landscapes. Warm stratification mimics the conditions of spring with warmth (68-86°F) and moisture. The moisture softens the seed hull, entering the seed and providing the water needed by the embryo to complete its development. Moistened seeds can be placed in a plastic container on top of the refrigerator for 60 days, or until 20% germination is seen. When warm stratifying seeds, you will need to keep a look out for mold, which will need to be wiped off regularly. Some growers apply fungicides to seeds being stratified. I do not.

Variable stratification
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In some cases, seeds need a combination of warm and cold stratification to stimulate germination. Starting with warm stratification, moistened seeds are given 60 days to soften the seed hull before being placed in a cold environment.

To determine whether or not stratification is needed by your seeds, find out how it grows in nature.

Discarded limbs, scattered petals, seeds and fruits thrown to the ground - abscission!

Abscission is the intentional shedding of body parts. Some lizards do it with their tails, we do it with our teeth, and mushrooms do it to their spores. All plants use abscission as a normal part of their lifecycle, but for different reasons and in different ways.

Anatomy of abscission

The area where abscission occurs is called the separation zone, or the abscission zone. When abscission occurs, enzymes are released that breakdown the structural cellulose and the adhesive pectin within cells in the abscission zone. The timing and methods of abscission depend on the reasons behind the action.

Forms of abscission

There are four basic forms of abscission:

• Conservation - damaged leaves or twigs are dropped to reduce water loss in drought, or in response to shade tree decline
• Defense - plants may drop heavily infested leaves or blossoms to rid itself of some pests
• Seasonal - deciduous trees shed their leaves each fall to avoid snow load in winter; fruit drop occurs to focus resources on fruit yet to come; dandelions and other flowers and fruits shed seeds to continue the species
• Senescence - natural aging

Artificial scarification

Commercial agriculture relies heavily on crops that grow and reach maturity at a consistent rate. It’s the only way to take advantage of the economies of scale provided by heavy farm equipment and large acreages. Scarification can help farmers get crops to grow uniformly by triggering seeds to germinate at the same time with these artificial forms of scarification:

• Chemical scarification - household bleach, vinegar, sulfuric acid, and potassium nitrate have been used to chemically carve access into a seed’s innards
• Mechanical scarification - the outer seed coat is cut or nicked with a file or knife, rubbed with sandpaper, given a gentle whack with a hammer, or placed in a container a shaken within an inch of its life [Research has shown that this ‘percussive scarification’ results in 90% germination, as opposed to 50% germination from chemical scarification.]
• Thermal scarification - boiling water, smoke, and fire can be used to compromise the integrity of a determined seed

Before you plant a seed, learn as much as you can about its natural life cycle. This information can help you decide if scarification is a good idea.

Just be careful not to harm the seed within (or your fingers)!

Our lovely fall colors are caused by a deciduous tree’s inability to maintain chlorophyll levels within its leaves. Chlorophyll, being green and abundant most of the time, masks the other colors that are alway present within a leaf. Shorter days and cooler temperatures trigger the tree to form a layer of cork at the base of each leaf, blocking the flow of water and nutrients and interfering with the leaf’s ability to produce chlorophyll. Eventually, the other colors can shine through. The veins of a leaf are the last part to turn color because it is the last place nutrients were available.

The Calvin cycle describes what happens to light energy after it has been absorbed into a leaf.

Put on your steampunk magnifying glasses because, today, we are going deep into the molecular level of leaves to learn how they make energy from sunlight.

Also known as the Calvin-Benson cycle, Melvin Calvin won a Nobel Prize in Chemistry for figuring this out. That was in 1961. Until Melvin’s research was complete, everyone thought that photosynthesis consisted of chlorophyll interacting directly with carbon dioxide to create edible organics (sugars) for plants. Instead, his research taught us that light energy causes chlorophyll to trigger plants to produce those organic compounds for themselves!

So, how does photosynthesis work?

Photosynthesis

Photosynthesis occurs in two steps. The first step is the light dependent reaction. This is when the sunlight is absorbed and transformed. To do this, electrons are torn from water molecules, creating oxygen as a waste product. When this happens, hydrogen (H) is released and used to create two compounds: nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP). The second stage of photosynthesis is the light independent reaction, or the Calvin cycle. This is when the ATP is converted into glucose.

Calvin cycle

​Each stage of the Calvin cycle has its own enzyme. Enzymes are chemical catalysts that trigger change. After light energy has entered a plant through the stoma and been converted into ATP and NADPH, the light-independent (or ‘dark’) aspect of photosynthesis can begin. There are four stages to the Calvin cycle:

1. Carbon fixation - a five-carbon enzyme molecule (RuBisCO*) snags a molecule of carbon dioxide (CO2) and turns into a six-carbon molecule
2. Reduction - The six-carbon molecule is split in half by an enzyme, using energy from ATP and NADPH molecules; one half becomes a sugar phosphate molecule, which is later combined to make starch and sucrose; the other half moves on to the next step
3. Regeneration - Again using energy from ATP and NADPH, the three-carbon molecule becomes a five-carbon molecule and the process starts over

​For your chemistry buffs, here's the equation and a graphic:

In the illustration above, atoms are represented as black (carbon), white (hydrogen) red (oxygen) and pink (phosphorus).
​
For the rest of us, try this story for a more memorable form:

​Rubio marries Connie. Connie has twins. One twin becomes a cook. The other works the family business and marries a girl just like mom. And they have twins. And so it goes….

​Factors that interrupt the Calvin cycle

Any interruption in photosynthesis leads to chlorosis, or yellowing. Chlorosis can be the result of insufficient light (epinasty), disease, a lack of mycorrhizae, or sunburn. Other factors that interrupt the Calvin cycle include:

• Excessive heat
• Insufficient magnesium uptake - it may be present in your soil, but other factors can limit uptake
• Overspray of herbicides and other chemicals
• Soil pH - too much acidity

If you see chlorosis, it means your plant is starving. By learning about the Calvin cycle, you may be better equipped to figure out what is wrong with your plants. Get growing!


* RuBisCO - ribulose bisphosphate carboxylase/oxygenase; believed to be the most abundant enzyme on Earth (Wikipedia)

Okay, so what is evaporation?

Technically, evaporation is the process of turning a liquid into a vapor without forming bubbles. As soon as bubbles happen, we call it boiling. There shouldn’t be any boiling in the garden, but  you can bet there’s a lot of evaporation going on! But how does the liquid actually turn into a gas? It ends up, at the molecular level, as something is heated, all those molecules move around more. Then they start banging into each other. Eventually, they start to fall apart, transforming into a gas. That’s evaporation.

Plants and evaporation

If you’ve been reading The Daily Garden for a while, you may recall a post called evapotranspiration. If not, give it a read. It’s really short. Okay, so plants absorb water through their roots and move it around in the xylem, and then lose water through tiny holes on the underside of leaves, called stoma. This process is called transpiration. You may be surprised to learn that plants only hang on to 2 or 3% of the absorbed water. The rest is lost to evaporation through transpiration and guttation. [Think of guttation as sweating.]

Evaporation and irrigation

The bulk of water loss through evaporation happens to the soil around your plants, leaving them high and dry. Farmers use complex equations involving rates of evapotranspiration to decide how much water to give their crops. We don’t have that much technology in our gardens or landscapes, but we can use what we know about evaporation to reduce water loss and water waste. Here’s how:

• Use drip irrigation and soaker hoses
• Water at night or in the early morning
• Avoid watering during the heat of the day
• Water deeply and infrequently, to encourage deeper root systems

Just because a plant wilts during the hottest part of the day does not mean you need to run outside and turn on the hose. Wait to see how the plant is able to recover in the evening. If it stays wilted, then water it. If it perks back up, then it has not lost too much water through evaporation.

Etiolation describes how plants become long and white due to growing in too little light.

This condition is called blossom drop. Blossom drop can be caused by several factors, most of which are perfectly normal. Others, not so much. Generally speaking, plants kick unfertilized flowers to the proverbial curb. Here are some species-specific causes of blossom drop. 

Citrus June drop

Most citrus trees produce far more flowers than they could bring to maturity. When the tree decides it has enough fertilized flowers, usually around June, they discard excess blossoms, which is perfectly normal.

Cucurbit blossom drop

The first flowers on your melons, winter or summer squash, and cucumber are generally male. These male flowers drop naturally after a brief appearance. If female blossoms start falling off, it is usually because of thrips damage, poor soil fertility, environmental factors, or inadequate pollination. Adding insectary plants, such as yarrow and bee balm, to your landscape will attract more bees and other pollinators. You can also allow onions, carrots, and fennel to go to seed. These plants will all provide pollen and nectar to beneficial insects that should increase pollination rates. If that doesn’t work, you can always try hand-pollinating.

Bean blossom drop

Temperatures over 90°F will cause bean flowers to abort. Blossoms drop also occurs with insufficient irrigation and poor air quality due to smog or fires. If you know your summer temperatures will exceed that threshold, try planting beans earlier or later in the growing season.

Tomato and pepper blossom drop

Tomatoes and peppers often drop their blossoms when environmental conditions are unfavorable. These conditions might mean any of the following:

• Excessive nitrogen
• Fusarium wilt
• Insufficient irrigation
• Hot, dry winds
• Overcrowding
• Potassium, phosphorus, or nitrogen deficiencies
• Tarnished plant bug feeding
• Temperature extremes
• Verticillium wilt

How to reduce blossom drop

Use these handy tips to reduce blossom drop in your garden:

• Irrigate regularly
• Maintain good soil fertility
• Position plants far enough apart that pollinators can find flowers
• Protect plants from strong, hot winds

The good news about blossom drop

When environmental conditions cause blossom drop, most plants will produce a second batch of blossoms.

Epinasty refers to how leaves and stems turn downward when their tops grow faster than their bottoms.

While many plants move to follow the sun's path each day (phototropism), sometimes plant movements are more random. These are called nastic movements. Epinasty is a nastic movement.

Temperature impacts every stage of plant growth and fruit production.

We all know that most plant life processes stop when it gets really cold outside, but did you know that the same thing happens when it is hot?

Temperature sweet spots

Each plant species has a minimum, maximum, and optimal temperature range for its different life processes: germination, seedling growth, vegetative growth, and reproductive growth. Some plants, such as peas, lettuce, broccoli, and spinach, prefer cooler weather (60 °F), while squash, melons, peppers, tomatoes and corn prefer hot weather (80 to 90 °F). The normal  (phenological) responses to temperature are what make our harvest possible. And sometimes we get 90 °F weather in April.

Plant responses to heat and cold

Just as the symptoms of not enough water look a lot like the symptoms of too much water, plants respond to both temperature extremes in very similar ways. (Their options are limited.) At first, they close the tiny pores, called stoma, on the underside of leaves to retain moisture. If this isn’t sufficient protection, they wilt. Then leaves begin to dry and burn at the tips and edges, and tender new growth can die. Prolonged exposure can lead to twig and branch dieback, or complete loss of the plant.

What happens when it gets too hot?

Excessive heat (above 90°F) can lead to water stress, sunburn damage, and blossom drop. It can also slow photosynthesis, reduce pollen production and release, and interfere with pollination. Pollen loses its viability at temperatures over 95°F and seedlings die at 125°F. While  most areas never see temperatures that high, sunlight reflected off of hot pavement can easily scorch nearby plants. In crops such as corn, a sudden heat wave can reduce your harvest by 80 to 90%! Studies have shown that high temperatures can reduce the amount of time a plant takes to produce fruit, lowering both size and quality. This is because high temperatures stop plants from producing the proteins responsible for the ripening process.

Plants also tend to stay smaller. You cannot control the weather, but there are things you can do before, during, and after planting to help your garden and landscape plants survive temperature extremes.

When water isn’t enough

Deep roots and good health go a long way toward helping a plant protect itself. Shallow root systems dry out quickly and are more sensitive to heat and cold. Watering plants deeply and allowing the soil to dry out between waterings encourages roots to go deeper. And sometimes water isn’t enough. A thick layer of aged compost or mulch placed around trees, shrubs, and other plants can stabilize soil temperature and provide a slow-release of nutrients. Just be sure the mulch does not touch the trunk.

​Avoid using fertilizer when extreme temperatures are expected, and be sure to leave as much leaf cover as possible. Excessive pruning can make plants vulnerable to heat and cold. Artificial protection from the elements can take the form of shade structures, row covers, pergolas, umbrellas, and shade cloth. Blocking even a little sunlight on a hot day can help your plants get through the worst of it without sunburn damage or water-stress.

An ounce of prevention

You can eliminate much of the work related to temperature extremes by selecting plants best suited to your microclimate (native plants are an easy choice), and place them in locations suitable to their species. And be sure to install plants at a time of year when they will have enough time to develop a strong root system. Finally, have your soil tested by a reputable lab, to find out exactly what you are working with, and continually strive to improve soil structure.

Healthy soil makes healthy plants. Healthy plants can tolerate heat and cold better than plants marginalized by poor soil, insufficient irrigation, or improper care.

Why has someone wrapped the stems of our bananas? It’s all about the gas!

Put ethylene gas to work for you

​Placing a piece of fruit in a paper bag allows you to take advantage of ethylene gas. The paper holds the ethylene gas closer to the fruit, speeding the ripening process. Plastic bags do not work, as they trap moisture that can lead to rotting. To slow the ripening of neighboring fruits, many sellers place waxed cloth or plastic over the stem end of bunches of bananas.

Other sources of ethylene
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Ethylene gas is not just from plants. It is manufactured for agribusiness. It is also the byproduct of your car’s engine, natural gas leaks, welding, and some manufacturing processes. The discovery of ethylene gas occurred over 100 years ago, when someone noticed that trees growing near gas street lamps kept dropping their leaves faster than other trees. 

Maybe that’s why my store-bought strawberries go from nearly perfect to inedible overnight.

Respiration probably isn’t what you think it is, especially when it comes to plants.

Most of us think of ‘respiration’ as the breathing in and out that we do to oxygenate our blood. But that’s only part of the story. Respiration refers to any process within a living thing that uses a gas exchange to generate or release energy.

Plant respiration

Plants respire through root hairs, outer stem cells, and tiny holes on the underside of leaves called stomata. The stoma can be opened for respiration or closed to conserve water. [In extreme heat, plant respiration can be reduced by as much as 50% due to stomata closure.] The oxygen that enters through the stoma then moves to individual plant cells, where it is used in several different cellular functions. These chemical reactions produce carbon dioxide, which is released by diffusion. Iron and potassium are important components of this process.

Respiration as energy production

We all know that plants absorb water through the roots and create sugars in the leaves using photosynthesis. Those sugars travel through the phloem to the mitochondria of most plant cells, where they are oxidized, or broken down, by oxygen molecules. The oxygen breaks the sugar molecules into carbon dioxide, water, and storable energy, called ATP (adenosine triphosphate).

Respiration and photosynthesis

Everything switches while photosynthesis is taking place. When a plant is actively producing energy from light, carbon dioxide is inhaled and converted into sugar, and oxygen is exhaled. This is called the Krebs Cycle. This means that your plants are providing you with oxygen during the day, and taking it back at night. Thus, the Eternal Balance of oxygen and carbon dioxide in nature is maintained.

You may be surprised to learn, as I was, that respiration without oxygen (anaerobic) is called fermentation. When plants try to respire without oxygen, they make alcohol!​​

Everything dies. The programmed death of cells within a plant is called apoptosis.

Sooner or later, the end comes for all living things. In the world of plants, the timing of death is used as a major classifying factor. That’s why we say a plant is perennial, annual, or biennial. Within the world of plants, there are three types of death:

• Apoptosis - planned release of certain proteins that cause the uninterruptible death of a plant
• Autophagy - ‘self-eating’; the absorption of a plant’s own tissues leading to death
• Necrosis - uncontrolled method of death, such as being stepped on, or hit by a car

Annual, biennial, and perennial

Annual plants complete their entire life cycle in one growing season. That may sound like a raw deal, but the male luna moth only gets three days and no mouth. (See, perspective is everything.) Within an annual plant’s DNA is a series of instructions that drive the plant to produce seeds for the next generation before the seasons change for the worse. Biennials get a two year cycle to accomplish the same ends.

Perennial plants are, well, perennial. They keep on going. Some perennials last 10 to 20 years, or upwards of 50, in the case of most fruit and nut trees. Some perennials, however, have seen the coming of electricity, Christ, and even the Bronze Age, and the invention of writing! If you count clones as the same plant, there is one Tasmanian shrub, King’s lomatia (Lomatia tasmanica), that has been around for somewhere between 43,000 and 135,000 years! It boggles the brain. Even these ancients will eventually succumb to senescence. Senescence refers to the stage in a plant’s life when its metabolism slows prior to death.

The process of apoptosis

The programmed cell death of a plant begins with instructions from the mitochondria, activating certain proteins, called caspases. These proteins trigger other proteins that lead to cell shrinkage, bulging (called ‘blebbing’ - how cool is that?), and fragmentation of the nucleus and DNA. The individual cells technically commit suicide in response to predetermined growth and survival factors.

The end is nigh!

So, what does all this have to do with your garden? First, understanding that some plants are preprogrammed for a short life, you can select the plants that best suit your purposes. Do you want trouble-free perennials, such as rhubarb and asparagus, that will come back, year after year, or do you prefer the more tender annuals of cucumber, peppers, and corn? Taking into account a plant’s lifespan can help you to design your garden and landscape more effectively.

And, hey, with words like blebbing and senescence, your friends are sure to be impressed!

Grapes first grow to full size and then veraison begins. This is when acidity drops and sugar levels rise. This is also when grapes start to change color. Scientists do not know what triggers veraison, but now you know a garden word that most other people do not!

Photoperiodism describes the way living things detect and react to the changing lengths of nights and days.

Just as many of us feel more energized in summer and more likely to hibernate in winter, other living things are hardwired to behave certain ways in different seasons. In the world of plants, this ensures the proper timing of flowering, fruit set, building food stores, and preparing for dormancy, making it more likely that a plant will be able to pass on its genetic information.

Light/night detection

Many angiosperms (flowering plants) contain photoreceptor proteins that can detect and signal information about the length of each consecutive night. This is done by two different proteins: one absorbs red light and the other absorbs far-red light. These proteins then trigger seed germination, internode elongation (stem length), and flowering. While we call certain plants long-day or short-day varieties, we should technically be calling them short-night and long-night plants, respectively.

Growth responses

Plant responses to hours of darkness are categorized as either obligate (mandatory) or facultative (likely to occur). In most cases, a plant with an obligate photoperiodic response is triggered to flower at a particular length of darkness, while a facultative plant may be more likely to flower within a range of nighttime hours. Temperature, plant age, environmental conditions, and plant health also play roles in the way plants use this accumulated information. In some cases, plants can be obligate at one temperature and facultative at a different temperature. To further complicate things, some plants need long nights followed by short nights to start flowering!

Long-day (short night) plants

Long-day plants are triggered to flower as nights get shorter. This usually means they tend to bloom in late spring and early summer. If a long-day plant is exposed to more than 12 hours of darkness each day, it will not bloom. Lettuce, peas, barley, and wheat are long-day facultative plants, while oats, henbane and carnations are long-day obligates. Spinach (13 hours), potatoes, California poppies, dill (11 hours), asters, and coneflowers are all long-day plants.

Short-day (long night) plants

Short-day plants require long periods of continuous darkness to develop flowers. If these plants are installed near a motion-detector light that goes on and off during the night, they are less likely to flower. Some short-day plants include sorghum, onion, cotton, soybeans, poinsettia (10 hours), chrysanthemum (15 hours), and marijuana.

Day-neutral plants

The flowering of day-neutral plants is not affected by light. Many of these plants evolved close to the equator, where day length changes very little. Tomatoes, corn, cucumbers, roses, and other day-neutral plants respond instead, to changing temperatures, other environmental conditions, or as a function of plant development.

Artificial light and darkness

Commercial nurseries use photoperiodism to ensure that there are plenty of flowers for specific holidays by artificially creating longer and short nights. To lengthen daylight hours, grow lights can be added outside of daylight time, much the way hens are exposed to artificial lighting to keep them laying eggs during winter. To simulate longer nights, plants can be covered, to block sunlight.

In your own garden, you can use photoperiodism to place short-day plants where they will not be exposed to nighttime lights. You can also cover plants that tend to bolt in summer, or that need longer nights to start flowering. 

If you want to read more about the science behind photoperiodism, check out Photoperiodism in Plants (Thomas & Vince-Prue, 1997). 

An interesting note on photoperiodism: when leaves of one plant have been exposed to the correct amount of light, and are then removed from the parent plant and grafted onto a plant that has not received enough light, flowering is triggered in the new plant. This is because the leaves release plant hormones that initiate flowering after the correct amount of darkness has occurred.

Seedlings are the young plants that emerge each spring as warmth and moisture help them to convert stored food (endosperm) into new growth. Learning more about seedlings can help you get the most from your garden. (You’ll get an awesome vocabulary while you’re at it!)

Hardened off plants can then be installed in the landscape or garden, with a significantly higher chance of success. Be sure to use the information on seed packets and plant labels to determine the proper way to plant and manage your seedlings.

​Thinning seedlings

Too many plants in one place means none of them get enough of what they need. Overcrowding and undesirables used to be eliminated by simply yanking them from the earth. We now know that this isn't in anyone’s best interest. First, it damages neighboring young root systems. Also, it removes millions of beneficial soil microorganisms that help plants find the food they need. Instead of pulling, it is more productive to snip unwanted plants off at soil level. The roots will gradually die (unless it is a particularly tenacious weed), giving the microorganisms the time they need to migrate elsewhere. Weed seedlings can be eliminated with a nice thick (4” or more) layer of mulch.

Seedling pests & diseases

Seedlings often need protection from birds, squirrels, slugs and snails. This can sometimes be easier if you are using raised beds or containers, but the addition of trellising, row covers, and protective wire or netting in any garden location can help keep keep some of these pests away from seedlings. Since seedlings are so tender, they are favorite foods of many garden pests:

• Cutworms emerge from the soil at night and sever seedling stems; they can often be found near missing seedlings in the top 1-2” of soil
• Earwigs also come out at night and may chew leaves, stems, or take the entire plant
• Slugs and snails chew irregular holes in leaves or remove them completely
• Flea beetles are small, shiny jumping bugs that chew dozens of holes in leaves or removes them completely
• Root maggots leave tunnels in older plants and can destroy seedlings; look for brown pupal cases
• Sowbugs, or woodlice, will eat tender seedlings
• Springtails normally feed on decaying organic material, but they have also been known to feed on the roots and leaves of young seedlings; the damage looks like tiny, irregular holes on the leaves and little pitted areas on roots and leaves

Nematodes and boring insects can also damage or destroy seedlings. Fungal diseases, such as Fusarium wilt, Verticillium wilt, damping-off disease, and stem rot, as well as bacterial diseases, can block the developing vascular system of young seedlings, causing them to wilt and die.

Most plants produce an abundance of seeds for good reason: growing up is hard to do. Many seedlings never make it. They are eaten, desiccated, drowned, stepped on, chewed up, or try growing in a location that doesn’t work for them. As a gardener, you can improve the odds of survival for your seedlings by providing an environment rich in nutrients, moisture, sunlight, and protection.

Unlike our glandular bodies, each cell within a plant is able to produce its own hormones. These hormones dictate flower, leaf, and stem formation, the timing of leaf drop, seed growth, flowering, flower gender, and, the really Bog One, fruit development and ripening. These chemicals also tell which tissues to grow down (roots) and which tissues to grow up (stems), and even how long a plant will live.

​Appropriately enough, the word hormone comes from a Greek word that means to set in motion. Phytohormones, more commonly known as plant growth regulators, or PGRs, trigger certain cells to respond only at certain stages of the cell’s life. New, undifferentiated meristem tissue produces a lot of phytohormones. After the cells have used all that they need, any remaining phytohormones are moved to other locations within the plant where they are needed, or they can be inactivated and stored for later use. In some cases, phytohormones are cannibalized for their various parts, or chemically destroyed within the plant.

What do plant hormones have to do with gardening?

For one thing, it means that every time you prune out a branch or stem, you are altering the hormonal activity within your plant. The auxins that prohibit bud development further down a stem are mostly found at a growing tip. Remove that tip and hormone levels change, allowing more buds (read fruit) to develop further down the stem.

Secondly, upright, vertical stems tend to produce leaves over buds, because they contain a phytohormone called auxin. Bending a vertical stem into a horizontal position suppresses auxin development, allowing several flower-bearing buds to develop that otherwise would have remained dormant. Bottom line: more fruit.

Also, some bacteria create the plant hormones auxin and cytokinin, which can result in tumor-like crown galls.

Classes of phytohormones

There five basic classes of PGRs. Please don’t let the chemistry scare you off. After reading the descriptions, you will find a handy poem at the bottom to help you remember:

1. Abscisic acid (ABA) - growth inhibitor; closes the stoma of water-stressed plants; prevents seeds from germinating in unripe fruit, or when weather is too cold; slows growth in mature areas of the plant to promote new growth elsewhere; used by nurseries to keep plants dormant during shipping
2. Auxins (IAA; indole-3-acetic acid) - travel downward from shoot tips to stimulate root formation; encourages plants to grow toward light sources (phototropism); inhibit bud formation lower on stems; toxic to plants in large doses, especially to dicots; synthetic versions are used as herbicides, such as Agent Orange, and as root stimulants on cuttings
3. Cytokinins - move up the plant to promote promote axillary (side or lateral) bud development, cell division and cell formation; delay aging; affect internodal length and leaf development; counteract auxins (when a plant is injured, higher auxin levels increase root development/repair, while higher cytokinin levels increase stem development and repair)
4. Ethylene - a gas that is produced when plants are growing rapidly, especially in the dark (underground); at first, more ethylene gas is produced than can escape the plant, causing  shoot development; as levels decrease, leaf development occurs; this gas helps new shoots push their way through the soil; stimulates fruit ripening and rotting (which is why one ripe banana makes all the unripe ones ripen faster!) and leaf drop; some producers spray unripe fruit with ethylene to cause the appearance of ripening (but the flavor aspect never really catches up)
5. Gibberellins - mobilize nutrients within seeds to stimulate germination; cause stems and leaves to grow long; initiates bolting; stimulates pollen tube growth; counteracts abscisic acid

​See if the poem below can help you sort all this information out:

 
Germinate with gibberellins, then ethylenes break ground
Auxins push them higher, abscisic acid slows things down
Cytokinins make them longer, keep them young and stronger 
Always working in the garden, in flowers, shrubs and trees
I wish there was a cytokinin made especially for me!

Okay, so it doesn’t exactly roll off the tongue. How about trying your hand at creating an easy pneumonic and share it with your friends and fellow gardeners!

Dew normally accumulates on plant surfaces when there is enough moisture in the air and temperature differences that create different rates of evaporation and condensation. Guttation is closer to sweating.

Under normal conditions, plants close the tiny holes, or stoma, found on the underside of leaves. These stomata are used in respiration. When there is too much water in the soil, water pressure builds up in the roots. This forces xylem sap out of tiny leaf edge structures called hydathodes. When this xylem sap dries on a leaf, it often leaves a white crust. This crust is mostly sugars and potassium.

Research has shown that corn seeds treated with neonicotinoids create guttation drops that contain active ingredients with insecticidal properties. These chemicals are posing a serious threat to native bee populations. As bees drink these sugary droplets, they are poisoned and die within minutes.

Self-pollination occurs when the flower of one plant can pollinate a flower on the same plant.

Nearly every piece of fruit we eat (and many fruits that we call vegetables) would not exist without pollination. When creating a foodscape, space constraints can be a deciding factor in plant selection. While plants that cross-pollinate, those that use flowers from two different plants of the same species, often produce bigger and better yields, botanists have nurtured many self-pollinating varieties that can be used on balconies, windowsill gardens, and small yards.

How does pollination work?

Pollination refers to the transfer of pollen (sperm cells) from the anther to the stigma of a flower (angiosperm) or the ovule (gymnosperm), and the resulting fertilization of an embryo. Plants that use cross-pollination expend a lot of energy attracting pollinators. They use bright, colorful flowers, sugary nectar, and sweet aromas to attract the beneficial insects and bats that carry genetic information from plant to plant. Plants that rely on self-pollination tend to have smaller flowers because most of the pollen transfer occurs by falling onto the stigma as the flowers close. Wind, bees, moths and butterflies, birds, bats, and even rain may also play a part, but infrequently. If self-pollination occurs within the same flower, it is called autogamy. When it occurs between different flowers on the same plant it is called geitonogamy.

Pros and cons of self-pollination

The most obvious advantage of planting self-pollinating varieties is that you will only need one of them. It also means you can get a harvestable crop without the help of bees or other pollinators, which might be in short supply in your area. The disadvantage of self-pollination is that it limits genetic diversity and may reduce overall plant vigor. Just as inbreeding in mammals increases the chance for disease and deformity, plants can respond the same way.

Edibles that self-pollinate

Most sour cherry, peach, nectarine, citrus, and pear trees are self-pollinating. If you shop around, you can find self-pollinating plum, almond, and avocado trees, as well as some grape varieties. Many vegetables self-pollinate, including peppers, peas, beans. tomatoes, and eggplant. Fruit cocktail trees are one way to reduce space requirements while still taking advantage of self-pollination. These trees have several varieties grafted onto the same root stock, so they are technically self-pollinating.

Why is this important?

Understanding which plants self-pollinate can help you select the best plants for your garden. Fruit and nuts trees, in particular, are an investment of time and money. If you only have room for one, you have to make sure that it can self-pollinate if you are going to get a harvestable crop.

Be sure to read plant labels and get your plants from reputable suppliers. It can take a year or two to learn that a tree advertised as self-pollinating was a marketing scam and then it is pretty damn hard to dig it out and start over.

Genetic survival strategies

Plants are all about reproduction, and they have different strategies to ensure the survival of their genetic information. While perennial plants invest energy into maintaining individual plants over time (and some can live for thousands of years), biennials use the first year to collect resources and the second year to produce seeds. Annuals go from seeds to seedlings to mature plant to seed production in a single growing season, and then they die. This means that they do not have the luxury of missing a chance at being pollinated. Brighter, more fragrant flowers increase those chances. Other annuals, especially those found in the desert, spend most of their life as a seed. These plants, ones that go from seed to seed in only a few weeks, are called therophytes. One big advantage to being an annual is this lifecycle interrupts many pests and diseases that might otherwise wipe out a species.

Annuals as food

Many of our food crops are annuals that must be replanted each year: corn, peas, beans, melons, squash, and most cereal grains are annuals. Some biennials are grown as annuals, for convenience sake, or because they cannot tolerate locale microclimates. These plants include celery, parsley, and carrots. Other common edibles, such as tomatoes, sweet peppers, and sweet potatoes, are actually perennial plants, but most gardeners treat them like annuals. Under the proper conditions, these plants can continue producing food year round for several years.

Seeds from annuals

As your annual plants end their growing season, you can prepare the the next year by collecting seeds. Choose seeds from the strongest, healthiest, most flavorful produce, and dry them out of direct sunlight. Some seeds may need chilling hours. Other seeds may need a short trip to the freezer (such as beans) to kill off any internal pests.

Budbreak occurs when new buds begin to open.

These buds may open to become leaves, flowers, or twigs, but timing is everything when it comes to budbreak. Open too soon and tender shoots freeze. Open too late there isn’t enough time for flowers, fruit, or other new growth to mature. Very often, when to fertilize or treat for pests and diseases is determined by budbreak.

What causes budbreak?

Budbreak (or bud break) occurs in response to a combination of factors. Lengthening days and warmer temperatures are the two main stimulants, but plant variety, genetics, age, and health are big factors, too. As soil temperatures rise, osmosis causes water to be drawn upward into the plant through the xylem. This water is carrying minerals, sugars, organic acids, and hormones from the roots. Those hormones stimulate the buds to ‘break’ open.

Fertilizing and budbreak

Many fertilizer instructions will say “apply fertilizer two weeks after budbreak”. This is because plants are using a lot of energy and nutrients in creating all this new plant tissue. Roots store the initial nutrients used, but young leaves may not be able to produce enough energy. Fertilizing in the early part of the process provides the nutrients needed for the best health and maximum production.

Pruning and budbreak

Most fruit and nut tree pruning is done while trees are dormant (apricot and cherry the only exceptions in San Jose, California, due to Eutypa Dieback). Pruning before budbreak makes a lot of sense. Bottom line: there’s no sense allowing plants to put effort into twigs and buds that are going to be pruned out anyway.

Budbreak and disease analysis

Erratic or reduced budbreak can indicate a number of different problems. Monitoring plants in mid- and late-winter and early spring can help you determine problem areas, such as:

• A period of early warm weather
• Cutworms, mealybugs, ants, mites, or thrips feeding on new buds and shoots
• Excessive cold
• Insufficient chilling hours
• Nutrient deficiencies
• Overcropping (growing so intensively that soils are depleted)
• Root damage caused by nematodes
• Severe water stress from the previous growing season
• Too much nitrogen
• Too much water the previous season

Budbreak as a production indicator

When and how budbreak occurs can give you a good idea of what’s ahead. Early budbreak normally indicates earlier flowering and fruiting. Reduced bud break means less fruit thinning later in the season. Uneven budbreak may cause the harvest to be spread out over a longer period.

How to promote healthy budbreak

Healthy plants are generally able to recover from temperature, water, pest, and disease stresses well enough to generate a good crop of buds each spring. Maintaining that good health requires proper pruning, irrigation, feeding, and regularly monitoring for pests and diseases.

Commercially, substances are applied to stimulate simultaneous budbreak, for easier management. They also use fans and overhead sprinklers to protect against frost damage. This isn't realistic for the home orchardist. If your buds break too soon, as my almond tree did this year (in late January!), there isn't much you can do, except enjoy the shade this summer and hope for a better crop next year.​

Chilling hours are the accumulated periods of cold temperatures that allow many fruit and nut trees and shrubs to provide us with their bounty each year.

We are all familiar with the buds and leaves of spring, the prolific growth of summer, and the harvest of autumn, but fruit and nut trees and shrubs (and strawberries!) are working through winter, too. Colder winter temperatures are part of a deciduous tree’s natural lifecycle. In preparation to survive potentially freezing temperatures, many fruiting trees and shrubs produce a hormone that initiates a state of dormancy.

What are chill hours?

Just as many seeds require vernalization (a period of cold temperatures) to germinate, many fruit trees have the same need for hours between 32°F and 45°F each winter, called its chilling requirement. In this temperature range, the growth-inhibiting hormone responsible for dormancy begins to break down, allowing trees and shrubs to produce buds that ultimately become the leaves and flowers of spring. Fruiting trees and shrubs that do not receive enough chilling hours in a year will generate fruit and leaves erratically. Any fruit produced will be lower in both quantity and quality. Also, insufficient chilling hours can extend bloom time, making delicate buds and flowers vulnerable to diseases like fireblight and brown rot.

How are chilling hours calculated?

There are different models used to calculate chilling hours, but they all take the same basic information into account:

• Chilling does not occur below 32°F.
• The ideal range for chilling occurs between 32°F and 45°F.
• Temperatures above 60°F can reverse the number of accumulated chilling hours.
• Fall and winter chilling hours are more effective than spring hours.

The Utah model provides chilling hours, while the Dynamic model provides chilling portions. Whichever model you use will give you a better idea of the best varieties for your microclimate.

How many chill hours do my trees need?

Different species need varying amounts of accumulated chill hours. Within each species, each variety has its own needs, as well. For example, northern varieties of blueberries have chilling requirements of 800 to 1,000 chilling hours, while southern varieties may only need 150 to 200 chilling hours.

Your local chilling hour station

Universities work with the USDA to provide valuable information to farmers and orchardists. You can access this information to learn the cumulative chilling hours in your area. The accuracy of this information will depend on where you live and how far you are from the nearest recording station. Click on the county and town closest to you for historical averages or current season figures for hours below 45°F and those between 32°F and 45°F.

Almonds are a staple crop in California. Part of the reason is most almond varieties only need 200 to 300 chill hours. Northern California receives an average of 800 to 1,500 chill hours, while southern California only gets 100 to 400 chilling hours.

Ultimately, your fruit and nut crop will depend on pollination rates, weather, plant age, soil nutrition and structure, irrigation, and chilling hours. You can get the most out of your fruit and nut trees and shrubs by selecting varieties best suited to your microclimate and the average number of chilling hours each year.

We’ve all heard some seeds or plants described as heirlooms and others hybrids, but what do those terms really mean?

Both hybrids and heirlooms come about through naturally occurring cross-pollination, as opposed to genetically modified organisms (GMOs), which are created in a lab using altered DNA strands.

Pre-industrial agriculture

Before agriculture became an industry, people grow a wider variety of plants for food. That biodiversity helped offset inclement weather, diseases and pests, and other threats to a failed crop and the resulting starvation. Corporate agriculture, on the other hand, feeds countless millions by generating a smaller variety of uniform plants that consistently grow at specific rates, that can be sprayed with a variety of pesticides, herbicides, and fungicides, ship well, and store well. As many of you know, taste and texture often suffer s a result. ​

Pros and cons of heirlooms

Heirloom seeds are those that have been handed down, person to person, in a specific geographical region, for a very long time. Also, heirlooms are open-pollinated, which means pollination occurs naturally, by wind, birds, animals, and insects, and not by human efforts. Heirloom varieties are at least 50 years old (some say 100 years), and many of them have been grown consistently, in the same locale, since before WWII. These plants have evolved to take advantage of local microclimates and beneficial insects.

​Heirloom seeds are hand selected by gardeners from the very best plants each growing season.  Many heirloom plants do not have the uniformity or long term storage capabilities of hybrids, but growers (myself included) claim that the flavor is significantly better. Heirloom crops have more variety in size and shape than hybrids, but they always grow true to their parent plants. Heirlooms are more genetically diverse, making them more durable as a species, and less susceptible to local pests and diseases. Heirloom offspring are fertile and can reproduce.

Pros and cons of hybrids

Hybrid plants are intentionally created by cross-pollinating different varieties of a species. The intention of hybridization is to take advantage of the best characteristics of each parent plant, creating what is known as hybrid vigor (heterosis). This vigor only lasts for one generation. Hybrid seeds do not grow true to their parents and they lack vigor and genetic diversity. This lack of diversity is what caused the Irish Potato Famine of the 1840s. If all the plants are identical, they are equally susceptible to pests and diseases. A single threat can be devastating.

Creating a hybrid that grows “true” to the desired characteristics takes years of diligent effort. Plants are often pollinated by hand or grown in greenhouses or pollination bags that block contamination from outside pollen to ensure that pollination only occurs between the desired plants. The majority of the fruits and vegetables you see in grocery stores are hybrids. Harvests are very consistent in size and shape. Hybridization is done for many specific characteristics:

• Improved disease resistance
• Easier growing
• Early maturity
• Better flavor
• Bigger size
• Longer storage capabilities

Unfortunately, we often sacrifice aroma, flavor, and texture in exchange for longer storage.
​
When shopping for plants and seeds, one way to know if it is a hybrid is to look at the Latin name. If you see the letter “x” between words in the name, it is a hybrid. For example:

Raspberry (Rubus idaeus) crossed with blackberry (Rubus ursinus)
  creates Loganberry (Rubus x loganobaccus)

*Check labels for the letters V, F, N, T or A. These symbols indicate a resistance to verticillium wilt, fusarium wilt, nematodes, tobacco mosaic virus or alternaria stem canker, respectively.

Understanding the difference between heirlooms and hybrids can help you make the right choice if you want to collect viable seeds from your harvest for next year’s planting.