The problem of winter injury on plants and avoiding it can be a tricky one. After the recent snowstorms, I’ve seen many trees and shrubs with broken branches and downed limbs, the result of heavy snowpack and high winds. Besides going out there and physically knocking the snow off the sighing branches (not quite so easy for a full-grown tree), there’s not much that can be done to prevent this type of winter injury. Remedying it would include restorative pruning to eliminate broken branch ends and making a clean, smooth surface over which the tree’s callus tissue can grow.
Photo: Limbs may break under heavy snowpack, like this Pitch Pine; Riverhead
But two other types of plant injury may not be as obviously attributable to old man winter - freeze injury and winter desiccation. Many of the scorched-brown Cryptomeria, Leyland Cypress, and other NON-green evergreens that I saw last year, succumbed precisely to this. Get a head start this season by understanding the problem of winter injury and knowing what you can do about it!
For many plants, it’s not the actual number on the thermometer that causes the problem, but the time of year that the thermometer is down. The critical times of the year for cold or freezing temperatures are in the fall before plants have hardened-up, in the spring after plants have lost their hardiness, and whenever cold temperatures follow a particularly warm spell during the dormant season. The time of year cold weather hits is important because of the plant hardening or cold acclimation process. When plants acclimate every year to the cold weather, water within their cells is pushed out to the small spaces between the cells. If the water stays inside the cells and freezes, the cells burst and are destroyed. (Remember from your high school chemistry class that water expands when it freezes!) So if freezing weather comes abruptly, the plant doesn’t have time to get the water inside its cells out and freeze injury can occur.
Another type of winter injury is winter desiccation. Plants, especially evergreens planted late in the fall, can suffer from drought even during the winter time! Normally there is lots of moisture in the soil during the winter because of lots of precipitation and no plant transpiration. But a relatively warm winter day with bright sunshine and high winds can increase the temperature of the leaf surface of evergreens enough to cause transpiration and water loss. If the soil is still frozen or even at a cold temperature that restricts water movement into the roots, the plant can suffer from winter drought-stress or winter desiccation.
Photo: Winter desiccation caused this Cryptomeria to brown and die; Riverhead
You can tackle winter desiccation by:
1. Irrigating during a relatively warm winter day, especially when there has been no precipitation for a while and it will get cold again the next day.
2. Applying a 2-3 inch layer of mulch to insulate the soil and keep it moist.
3. Reducing wind by constructing temporary wind barriers.
4. Situating the plants in partial or full shade (as long as they don’t require full sun) so that their needles do not heat up as much in bright sun on a warm winter day.
5. Especially for broadleaf evergreens, plant earlier in the season, so that they can develop a more extensive root system and be less sensitive to drought before winter comes.
You can also try applying an antitranspirant at the beginning of the winter and following up with repeat applications. There really aren't enough studies on antitranspirants and cold hardiness to form general recommendations. In other words, they MAY help, especially when combined with other strategies.
Unlike winter desiccation, freeze injury is much more difficult to prevent. There are some studies that suggest that maintaining soil fertility (and thereby plant nutrition) will improve winter hardiness. This does NOT mean to fertilize all your evergreens with a heavy application of nitrogen fertilizer in the fall – too much nitrogen too late in the season can spur tender growth which is especially susceptible to winter injury. Instead, make sure that your soil fertility is maintained throughout the year and your soil pH is optimal for the types of plants you are growing. Calcium especially, may be an important nutrient for proper cold acclimation. Calcium can become unavailable to the plant in highly acidic soil.
So let’s hope that we get no freezing temperatures and crazy snowstorms again in late winter/early spring, and that our plants are off to a good start for 2011!
