Some call this time of year the “deep freeze,” some “hard-faced moon,” and in antiquity, the “king is dead,” referring to vegetation as king, to be followed in spring quickening by “long live the king.”
Is anything really going on with our landscapes during this cold, short-sunlight time?
Under our feet, the soil food web is active — bacteria, arthropods, protozoa and fungi — breaking down organic matter, changing some matter into forms that provide energy for other members of the food web, producing compounds that help bind soil into aggregates, converting forms of nitrogen that may later feed plant roots and perhaps even conquering potential plant disease.
Folks used to think these microscopic critters were inactive in winter, but a recent study by scientists at the Swedish University of Agricultural Sciences have shown they are almost as active in winter as in summer, and have potential for major impact on degradation of soil carbon. That would assist those folks in reducing the carbon affecting climate change on earth. Another storage strategy to reduce carbon’s effect is no-till gardening and farming. No-till farming has been practiced by some farmers, but that method will likely become more widespread in the near future.
What’s happening with trees and shrubs? Cold weather causes sap in some woody plants to drop to the root zone, where it lies waiting for warmer days, rising and falling, while other woody plants wait for a spring rush of sap from root zone to branch and twig tips to help swell buds of flowers and leaves. Those buds were formed on most trees late last summer (midsummer or earlier on many fruit trees), but now they lie quiescent, waiting for their variety’s necessary cold hours to pass and warmer weather to grow plump and begin to break into flower or leaf.
Cold weather draws moisture from cells, ensuring that freezing water crystals won’t destroy the cells. This desiccation occurs even on leafless trees in winter. Mature leafless apple trees can lose about 300 grams of water per day in winter. Broadleaf, needled and scale-leaf evergreens continue to transpire moisture too throughout winter, the loss more pronounced on warm days than cold. Those of us responsible for landscapes hope sufficient rains and/or melting snows supply sufficient moisture to trees and shrubs to make up for that parched result. Keeping those trees alive over winter is one reason we advocate giving trees a long drink of water in autumn. Bright sun and drying winds speed up moisture loss.
Tree roots are very difficult to examine in the best of times, but researchers have found that as long as soil temperatures are above 20 degrees F, roots can function and grow even though the air temperatures are frigid. They may repair damage, too, but mainly they are quiescent, resting and ready to support above-ground growth and development. It takes prolonged freezing temperatures to drop temperatures of soil. Snow blankets help insulate soil against the cold that could damage roots. Mulch, shredded leaves and dropped needles also help keep roots sufficiently warm to prevent dieback in our area since snows usually don’t persist for long.
During this cold time, some earthworms and nightcrawlers dig sufficiently deep into the soil to be safe from freezing temperatures. “Red wriggler” worms used for worm compost can’t or won’t do that so they die, but they leave cocoons of eggs that will survive and hatch in warmer weather.
Some beneficial and destructive insects winter in crevices in tree bark, in garden debris, under decks, in nested pots, etc. sometimes at surprising distances from the garden. When the daytime temperatures permit, spray dormant oil on trees and shrubs to smother overwintering insects and their eggs. Be sure to read the product label before applying any spray, and don’t spray dormant oil on blue-needled trees or shrubs, for the “blue” color is due to a waxy coat that will be dissolved by oil. The tree or shrub will eventually recover its blue tinge.
I have not used a dormant spray on all trees and shrubs in past years because I have a good supply of beneficial insects that live in my yard (lady beetles now are clustered together in the foothills keeping one another warm). I will, however, have my garden helper spray pear trees when the temperature permits because my pear trees have been invaded by pear leaf blister mites that live on those trees’ buds through winter.
Send garden questions to firstname.lastname@example.org or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.