Thursday, January 20, 2011

Plant Survival in Winter by Elaine Lockey

Winter is a true test for the hardiness of the plants in your garden. If you’ve nurtured your plants during the other seasons, chances are your plants will provide you with another glorious year of, well, doing what plants do best! An ideal season for plants would be a cool fall slowly growing colder with a constant freezing cold winter and gradual warming into spring. Since this is not reality of life in the Rockies, plants must adapt to rapid and giant temperature swings, drought and sometimes even too much snow.

How do your plants make it through such cold times?  Snow is an excellent insulator for plants against cold air temperatures.  It also helps prevent soil heaving which is caused by the alternate freezing and thawing out of the wet soil. Plants that have heaved up out of the soil are exposing their roots to drying and extreme freezing which can kill them. Using mulch will also perform a similar function to snow as insulator and reducing the temperature extremes of the soil.  It also helps hold moisture in the soil which is critical to the survival of the plant’s root system, aka, the plant. A simple mulch that you might still have on hand is the evergreen boughs off your old Christmas tree.  Cut those and lay them around plants for a pretty and easy mulch.

Low winter temperatures can affect plants that are not hardy for our area.  The US is divided into Plant Hardiness Zones.  The USDA updates this map to help gardeners and growers determine what kinds of plants can generally be grown where. Using plants that are labeled for your appropriate zone will help reduce the loss of non hardy plants.  It’s really hard when you’re looking at a garden catalog from a warmer climate to take this into account but doing so will lessen your heartache.  As you can see from the Colorado zone map, the higher the zone number the warmer the temperature that the plant thrives in.  If you do fall in love with a plant in zone 11 you can try it as a houseplant as that’s where many houseplants come from!

Even hardy plants can occasionally be taken by surprise by sudden drops in temperature or extreme cold snaps.  Keeping plants healthy going into winter will help them survive some of the shock. Avoid heavy nitrogen applications in the fall to allow plants to harden-off properly and additionally reduce fall watering for the same reason.

Plants in containers are especially vulnerable to temperature swings.  Make sure that they stay watered during the winter and mulch pots or keep them on the north side of your house to keep temperatures more constant. Winter watering of all of your landscape plants is essential, one to two times per month.  See the recent blog by Carol King on this topic:

If you do get plant die-back be sure to prune out those damaged areas in the spring to avoid plant disease.  Plants that have died over the winter should be removed. Always a good excuse to go shopping!

Winter burning of broadleaved evergreens, such as on Pyracantha and Oregon Grape Holly and needled plants such as spruces and pines, is also called dessication.  This drying out of the needles and leaves is caused by the plant losing moisture faster than it is getting it from the soil.  Windy areas can contribute to this and also sunny spots. Proper winter watering can help reduce this issue.  Sensitive plants can be protected by burlap or other materials or planted in areas with protection from drying winter winds or too much winter sun.

How much moisture do we get from snow?  The common myth is 1” of water per every 10” of snow. As skiers know, snow can be highly variable – fluffy and dry to heavy and wet.
The National Snow and Ice Data Center answers this:
“The water content of snow is more variable than most people realize. While many snows that fall at temperatures close to 0 degrees Celsius (32 degrees Fahrenheit) and snows accompanied by strong winds do contain approximately one inch of water per ten inches of snowfall, the ratio is not generally accurate. Ten inches of fresh snow can contain as little as 0.10 inches of water and as much as 4 inches of water, depending on crystal structure, wind speed, temperature, and other factors. The majority of U.S. snows fall with a water-to-snow ratio of between 0.04 and 0.10.”

So you can’t judge a snowfall by its appearance.  I think that’s how the saying goes?
And what makes that that fluffy snow that Colorado is known for? “Studies in the Rocky Mountains have shown that the fluffiest, lowest density, or water-to-snow ratio (0.01 - 0.05) snows typically fall with light winds and temperatures near -9 degrees Celsius (15 degrees Fahrenheit). At colder temperatures, the crystal structure and size change. At very cold temperatures (near and below -18 degrees Celsius or 0 degrees Fahrenheit) crystals tend to be smaller so that they pack more closely together as they accumulate, producing snow that may have a density of 0.10 or more.”  (

Heavy snow can cause breakage of tree limbs and distortion of plants.  You might have even seen entire trees toppled over after a heavy snowstorm.  Proper pruning of trees and shrubs will help reduce most major breakage. One of the keys is to prune out branches that have very narrow angles to the main stem or trees with co-dominant stems. The wider the angle of the branch to the stem, generally the better that it can withstand heavy snow or ice loads.  If you do get broken branches from snow, prune those out to reduce further damage to the tree. Those of you with rodent problems may be displeased to hear that another negative to a heavy snow can be actually be an increase in problems with voles and rabbits by providing better cover for them and safety from predators.

Winter is tough on plants and animals but with some good general care of your plants you can help see them happily through winter and into spring.