Seedlings

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!)

What are seedlings?

As a seed germinates, an embryonic root (radicle) goes down, an embryonic stem (hypocotyl) goes up, and seed leaves (cotyledons) begin to emerge. The portion of development that happens underground (or in the dark) is called skotomorphogenesis or etiolation. When light is added to the equation, development switches to photomorphogenesis. Photomorphogenesis implies that photosynthesis is occurring, first in the seed leaves and then in mature leaves. Seed leaves rarely look like mature leaves, being more rounded, but the number of seed leaves can tell you if a plant is a monocotyledon (single blade-shaped seed leaf), or a dicotyledon or a eudicot (two seed leaves). If a seedling does not receive adequate light, the hypocotyl will grow too long to support the young plant and it will fall over or become too “leggy” to be healthy.

Monocot (left) and dicot/eudicot (right) by Peter Halasz CC BY-SA 2.5

Factors of seedling growth

Seedlings are initially dependent on their stored food reserves. Once photosynthesis begins, things really get growing. Temperature, light, and moisture are the major factors in seedling development, but wind and other forms of physical contact (thigmomorphogenesis) are critical to developing a strong plant. Most seedlings need 14 hours of daylight to grow. Compacted soil can make it difficult for tender new roots to anchor themselves in the soil or to find enough nutrients. One way you can give your seedlings a boost is to feed them with fish emulsion. You can also apply mulch, which will shade the soil, add nutrients, and reduce competition from weeds. (Unfortunately, it may also attract and protect sowbugs and earwigs). Remember that any major, sudden changes can be devastating to a seedling. Erosion, in particular, can expose delicate seedlings to more stresses than they can handle.

Hardening Off

Seedlings that are transplanted too early in their development into an unforgiving environment will often exhibit wilting. Underdeveloped root systems are not able to absorb water and nutrients as readily as more mature plant structures.

Young plants are more likely to thrive if they are allowed to become acclimated to new conditions gradually. This is called hardening off. To harden off seedlings started indoors or recently purchased plants, take them outside for a few hours each day. Ideally, they will be placed in a quarantine area that allows you to see if they are carrying any pests or diseases before they are introduced to your other plants.

Initially, seedlings should be protected from wind, with filtered or morning sunlight, gradually increasing the amount of time and sunlight by an hour or two each day until they are outside all day. If temperatures allow, plants can then be left outside overnight.

Acorn seedling (Amphis) CC BY-SA 3.0

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.