In winter, trees, shrubs and perennial plants go dormant. They stop growing and in some cases die back to ground level.
The single signal the leads plants to begin the process of going dormant is day length. When daylight hours shorten in fall, growth regulator hormones begin to build up in plants, slowing their growth. It happens consistently every year, regardless of temperature, rainfall or soil fertility because change in day length is constant from year to year. Once the dormancy process starts, cool temperatures, water stress and low fertility may speed the process. Warm temperatures, too much water and excessive fertilizer too late in the season won’t stop the dormancy process, but it can slow it down so much that the plant is caught off-guard by an early hard freeze that it can’t survive, or that parts of it can’t survive. This is why it is important to stop fertilizing plants and to cut back on watering, except for evergreen plants, by mid- August.
There are two stages of dormancy. The first is called endo-dormancy. Endo is a Greek word meaning “inside.” Thus, something inside the plant, those hormones, is causing dormancy. While the plant is endo-dormant, it begins counting chilling units, which are hours of time spent above freezing. Temperatures from 40°-50° are most effective. Each plant needs a certain number of chilling units before it can move on to the next stage of dormancy and wake up in spring. Chilling units needed range from 300 to 1500 or more, depending on the plant. Plants native to Wisconsin require an average of 1,000 chilling units.
Once the necessary number of chilling units is reached, plants enter a phase called eco-dormancy. If conditions are right, the plant could actually break dormancy and begin growing, but now something in the environment, usually freezing temperatures, are preventing it from growing. This is the most dangerous part of winter for plants. An early warm-up could prompt them to begin to grow, leave the dormant state, and lose their ability to adjust to cold temperatures. Then when cold weather resumes they are killed. This is also the part of winter, usually late February to late March, when you can prune branches of forsythia, crab apple, pear and other flowering fruit trees and bring them indoors to force them into early bloom.
Knowledge of the fact that daylight hours trigger dormancy has a practical application as you dream your way through seed and nursery catalogs this time of year. It’s important to order plants and seeds grown in a latitude similar to Wisconsin’s. Here’s why: say you buy a tree produced in North Carolina and say this tree begins the dormancy process when daylight shortens to twelve hours. That happens much earlier in the south than it does here, so the tree will continue to grow late into the fall, failing to start the dormancy process in time for our cold winter weather. Without time to move into dormancy before winter, the tree will die.
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