Extension Educator, Horticulture
I'm not sure how I survived winter before I discovered long johns, flannel sheets and Polartech®. Martha may knit plant cozies but most plants don't have it as easy as we do.
Tremendous variability exists within and between plant species in their tolerance to cold. However we can do a lot in plant selection, site selection and plant maintenance.
A few basic tips to get plants through the winter:
Evergreens in particular should enter winter well watered. Adding a three-inch layer of mulch can conserve soil moisture. All species of trees and shrubs entering winter without adequate soil and tissue moisture or with low food reserves become more susceptible to low temperature injury. In addition, problems such as improper pruning or fertilizing, or defoliation during the growing season (insects, disease) may increase susceptibility to low temperature injury.
Sometimes it isn't really the cold temperatures that cause problems, but the rapid fluctuations between warmth and cold and how dormant the plants are when the fluctuations occur. Also some plants hate our winter winds.
Evergreens and broadleaf evergreens such as holly, boxwood or rhododendrons are especially prone to winter burn from winter winds. Evergreens continue to lose water through their leaves even in cold weather. Winter winds can cause desiccation injury to plants, causing above ground plant parts to dry out because water cannot be replaced from frozen soil. Windy conditions, sun and warm days can speed the process.
Symptoms of excessive water loss may be browning of the leaf margins or rhododendrons may show leaf rolling. Stem death may occur but may go unnoticed until spring. On needle evergreens, desiccation injury shows up as yellowing or browning of needles during winter or early spring (some varieties naturally do this but quickly recover in spring).
Proper site selection of broadleaf evergreens is particularly important. Pick a site where they are protected from north and west winter winds and late afternoon sun such as the east side of a building.
There are ways to reduce desiccation injury of evergreens. Plants existing in exposed areas or along a south or west foundation may be loosely wrapped in late fall with burlap or canvas materials to slow desiccation. Pine boughs stuck in the ground around the plant will also help to slow winter winds. Commercially available anti-desiccant products such as Wilt Pruf® can be sprayed on evergreens and broadleaved evergreens to reduce the amount of water lost through leaves.
Successful boxwood plantings follow all the tips I just mentioned. Boxwoods make beautiful small hedges with their slow growing dense foliage. Their green leaves in winter provide a wonderful eye vacation from our usual winter grays and browns.
Boxwood need well drained soil. Since they are surface rooters they appreciate mulching with wood mulch or leaf mold to keep the root zone cool and moist in summer and protect from drying in winter. They need protection from wind and late afternoon sun.
There are many cultivars of boxwood. Look for ones that are cold hardy and will maintain good green color in winter. A couple that have performed well are 'Winter Gem' and 'Wintergreen.' 'Chicagoland Green' also known as 'Glencoe' reportedly maintains better green coloration under harsh conditions of winter than the others mentioned.
Be sure to mail in your Master Gardener application today.