Dormancy is a plant’s response to adverse conditions, such as too much cold, too much heat, or not enough water. During dormancy, a plant’s growth, development, and reproduction are slowed or halted until conditions improve. Hibernation refers to a mature, viable seed that has not germinated because of unfavorable conditions.
Dormancy is a protective measure that increases a plant’s chances at long term survival. Imagine what would happen to your apricot crop if all the buds emerged in November. There wouldn’t be many honey bees or other pollinators around, and the fruit, if it could form at all, would probably end up frozen mush on the tree, even here in the Bay Area.
Dormancy is classified as either predictive (endo-dormancy) or consequential (ecto-dormancy). Consequential dormancy is what happens in response to external conditions, while predictive dormancy occurs because of internal processes. The perfect example of predictive dormancy is the way deciduous trees and vines pull nutrients from their leaves and let them fall. This is caused by chemicals that inhibit growth and help the plant prepare for winter temperatures. These chemical changes are usually triggered by shortening hours of light and lower temperatures, and sometimes by rainfall levels. The same is true for seeds. Plants will remain in predictive dormancy until they have accumulated enough chill hours. After that point, they are in consequential dormancy, also known as delayed-dormancy, until spring comes.
Winter dormancy and chill hours
Many fruit and nut trees (and wheat!) require a certain amount of winter dormancy to produce a good crop the following summer. Somehow, somewhere within these trees is an amazing mechanism that tracks the number of hours of dormancy that are spent above freezing temperatures. These are called chill hours. Depending on the species and variety, the chilling requirement can be anywhere between 500 and 1500, or even more. This is why it is so important to select plants that are suited to your microclimate. You may do everything else right, irrigation, fertilizing, weeding, and you still won’t get a crop if your plant needs more chill hours than it can get in your garden. Also, all temperatures are not the same when it comes to fruit trees and chill hours. The most effective chills are between 40oF and 50oF. Temperatures below or above that range are less effective, and temperatures above 60oF can interfere with accumulated chilling hours.
Dormancy and cold hardiness
While plants are dormant, they are able to acclimate to colder temperatures. This is called cold hardiness. Once a tree or shrub has entered dormancy, it can withstand subfreezing weather and still put out buds in spring. Berries, such as raspberries and blackberries, also need a cold period of rest to perform better in the next season.
Tree care during dormancy
Winter provides the perfect time for pruning and many pest management activities. In the Bay Area, trees are usually fully dormant in December and January. Delayed-dormancy runs from February, when buds start swelling, through the point where buds reach the stage where their tips are green. Use periods of dormancy for these garden tasks:
*WARNING: DO NOT APPLY OILS 30 DAYS BEFORE OR AFTER SULFUR APPLICATIONS
Fertilizers and dormancy
In most cases, fall and winter are not the time for fertilizers. Nitrogen, in particular, can stimulate new bud and leaf growth, despite colder temperatures. These tender new tissues are doomed from the start and can create points of entry for many pests and disease.
Frost damage: prevention and response
As frost damages the aboveground portion of many plants, our first instinct is to remove the ugly, damaged leaves and stems. While that may make things look nicer, it is actually harmful to many plants. The root system may still be able to absorb many of the nutrients stored in those frozen leaves, and the damaged plant tissue provides a blanket of sorts, to protect the roots against even colder temperatures. If you really can’t stand the look of frost bitten plants, you can prevent the damage with protective fabric, or cover dormant plants with straw.
For the most part, periods of dormancy are normal, natural, and good advice for the rest of us.
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