My arborist is a great advocate of fall watering. We had several years of drought recently, and in the late fall he would always advise customers to soak the roots of their trees and shrubs before winter set in. In the spring, he would warn of potential damage because we had inadequate snow cover over the winter, unless we turned on the hose.
Things have changed a bit. 2008 was the wettest year on record in our area, and both this winter and the last we have had plenty of snow cover, enough to satisfy even my arborist.
So what is so wonderful about snow cover? Why do I rejoice when I see my shrubs buried under the white stuff?
In the fall, in temperate zones, most herbaceous perennials and most trees go into dormancy. This is a period of minimal cellular activity, a strategy employed by plants to survive times when conditions are harsh, such as droughts and freezing winters when sunlight is minimal. In many plants, the aboveground parts die back to the ground; in deciduous shrubs and trees, the leaves die and fall. However, the plant is still alive and still active at a reduced level. In the case of evergreens, some cellular activity in the leaves continues; while they do not grow during dormancy, they still engage in photosynthesis and respiration. As transpiration takes place and the tree loses water through its leaves, it is vulnerable. If it can not take up water to replace what it loses, it may suffer desiccation and winterkill.
Below the ground, in particular, a dormant plant is still active. Throughout the fall, as it enters dormancy, the plant has been storing nutrients in its roots or other structures. Roots can continue to grow during dormancy, and they continue to function as long as the ground surrounding them is not frozen, taking up water as long as it is available. But roots are vulnerable to damage from frozen ground, and particularly to cycles of freezing and thawing, which can result in frost heave that may break and tear them.
Snow is a form of water. As long as the roots of a tree or shrub are still active, they need water. Evergreens especially can suffer from a dry winter with little snow. Both sun and blizzard winds increase the transpiration rate of evergreen leaves during the winter, and snow can help the trees replace this moisture. Compared to rain, snow ususally melts and soaks into the ground slowly, so that less moisture is lost to runoff. When my arborist urges his customers to water-in their trees before winter, he is attempting to forestall the harm that a dry, snowless winter can do to them, as much as any other drought.
Besides moisture, snow provides trees and plants with a layer of protective insulation. Depending on its texture, a given volume of snow can be as much as 99% air. Like a down quilt, the snow with its trapped air serves as an insulating blanket to cover the ground and keep it from freezing. In addition, when the moisture from the snow soaks into the ground, the moist soil holds heat more efficiently than dry soil and retards frost penetration, thus protecting the vulnerable roots. In the case of evergreens, in particular, a good snow cover is the most valuable winter protection the trees can have.
But what if it doesn't snow? What if the white stuff never falls? In that case, it will be up to you to make sure your trees have the two critical elements of moisture and insulation. For moisture, there is always my arborist's advice, to water the trees in late fall, after they have securely entered dormancy. Watering too early may stimulate late growth, which won't have a chance to harden off before winter. Also, the nutrients necessary for producing late growth will be lost to the plant instead of being stored for the next year. When watering trees, don't put the hose right next to the trunk. The feeder roots, that take in the water, are usually located at the tree's drip line. Water deep and long.
For insulation, mulch makes the best alternative to a good snow cover. As with watering, don't pile your mulch up against the trunk of the tree, where it will only attract such destructive rodents as mice and voles; spread it over the root zone, if possible out to the drip line, where the insulation will do the most good. In the case of a dry winter, the mulch will also help hold the water in the soil. And in case the snow does fall, the mulch will help hold it, while the snow will only add to the insulating value of the mulch, like a double layer of warm puffy quilts over your your plants while they sleep until springtime awakes them.
(Editor's Note: This article was originally published on February 1, 2009. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)