The last few years we haven’t been seeing the possible extremes of living in our hardiness zone of 6b in Westminster. Our winter lows have been more like the average range given for zone 8. Yet, here is in January is one of those weather anomalies called a “polar vortex”–essentially dense, cold air that has detached itself from the North Pole and escaped from the binding power of the jet stream that has slipped down into the United States bringing extremes of frigid weather and winds.
This is a good time to review some information about cold damage to plants. Thank you Ag Extension writers everywhere!
Woody trees and shrubs and many herbaceous perennials, once hardened off and dormant, can tolerate very low temperatures. Buds are often protected by bud scales which protect them from water loss and damage. Gradual exposure to low temperatures affects plant metabolism and promotes resistance to freezing. However, if warm temperatures return for a few days in mid to late winter, a plant’s hardiness can be adversely affected, especially if severe temperatures follow on the heels of the warm up.
Frost cracking and sunscald can occur on thin barked trees like red maples or peach trees. The bark and underlying tissues on the south/southwest side can be heated up on cold, bright days. When the affected area returns to ambient air temperature at sunset or from cloud cover,the bark may split longitudinally (frost cracks) or be otherwise damaged. One damaged, the split may recur seasonally and be an entry point for pests and disease organisms.Sunscalds may lead to tree cankers. One measure that may be taken to lessen the problem is to use white latex paint on the bark of young trees in late fall to early winter, to reflect the sunlight and keep bark temperature from wide fluctuations. Seasonal tree wraps may also be used during the winter and then removed for the growing season.
The stem tissues of plants are actually more hardy to cold than are the roots. This is especially true for containerized plants let to the mercy of elements. The airier the soil, the deeper cold can penetrate, so in this case clay soil is less damaging. Moist soil is better than dry soil to avoid cold injury to roots. One type of root injury is frost heaving. Repeated bouts of freezing and thawing end up working less established plants right out of their planting sites, exposing the roots and killing the plant. This can be a problem in clay soil. (I had a number of fall planted quart sized shrubs affected by frost heaves last year.) This can be controlled by proper site selection of soil that drains properly and by application of mulch to insulate the soil and control temperature fluctuations.
Dessication injuries can be a cold weather issue with broad-leaved evergreens like rhododendrons on windy days when the soil is frozen.Too much moisture is lost through the leaves/needles and browning occurs with possible death of buds and leaves. Properly watering before the onset of freezing weather will help as will using windbreaks or screens as protective measures. There are anti-dessicant products that can be applied as well.
Otherwise, plant for your zone, using hardy plants keyed for those conditions. If you are pushing the zone boundaries, make sure to plant in a more protected area.
