Dormancy and Winter Injury

By Jan Cashman • Posted on October, 28th 2009

by Jan Cashman

We tell kindergarten students when they tour our nursery that plants “go to sleep” in the winter.  This is a simple explanation that small children can understand, but what does dormancy really mean?   What happens to plants in the fall?  Dormancy is a state of “rest” plants enter to survive the freezing  temperatures of winter.   Some plants will also go dormant in periods of drought.  For instance, a Kentucky bluegrass lawn will turn brown, but survive in a dormant state, if you don’t water it; cactus in the dessert can survive months of drought by going dormant.

Deciduous Trees and Shrubs

The brilliantly colored fall leaves of deciduous trees and shrubs are the first signs of dormancy.   During the spring and summer, green chlorophyll in the leaves of plants absorbs energy from sunlight that is used to transform carbon dioxide and water to carbohydrates for the life of the plant.  In the fall, triggered by shorter days and lower temperatures, chlorophyll in the leaves starts to break down, the green color of the leaves disappears, and yellow and orange pigments (carotenes and xanthophyll), also present in the leaves, become visible.  Quaking aspen and birch show only yellow pigments.  Other chemicals changes can occur to form anthocyanin pigments which produce the red and orange leaves of maples.

This fall, our drastic low temperatures in early October (Some places broke the record low by 13 degrees!) prevented the leaves of most trees and shrubs from turning these beautiful colors.  Instead, the leaves seemed to ‘freeze dry’ on the trees and turn brown.

When these extremely cold temperatures occurred, our trees and shrubs had not reached full dormancy yet.  There may be damage, but we won’t know for sure until next spring how much damage.  Flower buds, formed in the summer, are the least able to stand cold and, therefore, the first to be hurt.  The low temperature a plant’s flower buds can tolerate before they are damaged varies with each plant.   For instance, a lilac’s flower buds could withstand lower temperatures than, say, a forsythia’s; a hardy Norland apple tree’s flower buds could withstand lower temperatures than a peach tree’s.  Leaf buds, also formed in the summer, can be damaged by cold, especially if the cold occurs before the bud develops a protective hardened scale around it.  Severe cold can even ‘freeze back’ the tree’s branches.

In the fall, when the plant reaches its full dormancy or rest, it will not grow, even if the weather turns warm, until it completes its required time of dormancy,  different with different plants.  When this time of dormancy is completed and the weather warms up, the plant can begin growth.  Winter injury of trees most often occurs when the dormant period has been met, we get a warm spell, and then it turns cold again.   Warm Chinook winds, common here, can be hard on our trees.

We can never totally protect our trees from weather changes in the winter that might damage them.  But, wrapping their trunks up to the bottom branch in the fall with a light-colored tree wrap will reflect the sun and keep it from warming the bark too much on a sunny day.   When the bark is warmed to above freezing during the day, followed by colder nighttime temperatures, the cells can burst and cause injury.

Although they slow or stop growing in the winter, roots are the most susceptible plant part to cold damage.  Conveniently, the ground acts as an insulator for roots.  Snow cover adds more insulation to the plants’ roots.  The roots of plants in a pot (above ground) are easily hurt by cold temperatures.

Evergreens

Even though evergreens are always ‘green’, they do lose some needles every year; needles stay on evergreens for 3 to 4 years before the needles closest to the trunk drop, called ‘fall needle cast’.  Evergreens sustain winter injury in a different way than deciduous plants because the needles on the tree continue to transpire.  Dry winds and cold ‘desiccate’ or dry out needles in the winter.  Because the ground is frozen, moisture cannot be replaced up from the roots as it would be in the summer.   Also, freezing and thawing of the needles can burst needles’ cells, turning them brown.    To prevent this ‘burning’ in evergreens, plant tender varieties such as dwarf Alberta spruce, arborvitae, and yews in shady, sheltered spots.   Or construct a shade out of burlap between posts to protect the south and west sides of the trees in the winter.  Anti-desiccant sprays such a Wilt Pruf put a protective barrier on needles to keep them from drying out.  Apply them once in late fall and again in February.

All landscape plants, trees, shrubs, evergreens, and perennials, benefit from a deep watering in the fall once they have gone dormant, around November 1, so the roots freeze in moist soil.  Dormancy is nature’s way of protecting your trees and shrubs from our harsh winters.   You can aid this process by protecting and watering your plants this fall.