Early Fall Cold Snap Can Lead to Damage on Landscape Plants

Early Fall Cold Snap Can Lead to Damage on Landscape Plants

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

In our area we typically have relatively long periods of mild fall before we finally get a hard freeze.  In most years, the leaves senesce and normally fall off the tree as leaves do in autumn.  However, occasionally an early cold snap will take us and our trees and shrubs by surprise.  One very obvious sign of the sudden end to the growing season is all the leaves left hanging on the trees.  Just like us, they weren’t ready for the severe cold.  They didn’t have time to form the “abscission layer” that develops at the base of the leaf stem and causes the leaf to fall. The leaves stay attached to the trees.  This is a common trait of trees such as oaks, but unusual for many other trees and shrubs.  Some of these leaves will hang on for most of the winter, but others will fall every time we have a strong windy day. All will drop off by spring when the new leaves start to develop.

How Plants Get Ready for Cold Winter Temperatures

While these persistent leaves are not harmful to the plants, they are a sign that many trees and shrubs were not ready for record breaking severe cold temperatures.  Plants go through a physiological process to be able to avoid damage from cold temperatures.  This begins in late summer and early fall as the days grow shorter and the temperatures begin to decline.  Plant hormones formed in response to these changes trigger the cessation of plant growth, stimulate the formation of the abscission layer, and bring about physiological changes that allow the plant to withstand colder and colder temperatures.  In response to gradually cooling temperatures, the plant attains its winter hardiness through this physiological process called “hardening” or “acclimatization”.  By midwinter it has attained its ultimate hardiness, which is genetically determined.

The potential problem caused by our recent record-breaking cold spell is that many plants were not fully acclimated and hadn’t achieved their ultimate winter hardiness.  While they might be able to withstand 15 degrees, 10 degrees, or even 5 degrees in mid-winter, they may have sustained damage with the cold temperatures coming so early.  It won’t be until the next spring and summer that the amount of potential damage can be assessed.

Symptoms of Damage

For gardeners, the next spring and summer are the “moment of truth” for appraising the extent of possible damage.  The first clue that damage actually occurred will be the failure of a plant to leaf out and grow.  In some cases, leaves may form while flowers fail to develop because of injury.  There may also be plants that die abruptly after appearing unscathed with leaves and flowers growing normally.  On these plants, the buds were not injured but stem and branch tissues were severely damaged or killed.

When cold is sudden and sustained over a period of several days, roots of plants also may be damaged.  This usually takes longer, perhaps several months or more, to become evident on established plants.  Symptoms include outright plant death or gradual thinning and dieback of the crown.

Help for Injured Plants

Once spring arrives, there are some things gardeners can do to help an injured plants. First, be patient. Wait until late spring, after there has been sufficient time for the plants to fully leaf out, and then prune out dead wood.  Be sure to prune properly.  Don’t make stub cuts.  Prune back to a bud, stem or trunk with live green healthy wood.  Only remove dead and severely damaged wood.  Follow that with tender-loving care, proper watering, and mulching to retain soil moisture.  Injured plants are already stressed; so don’t add to their stress. 

Experts disagree about fertilizing winter-injured plants.  Some advise against it, others recommend it. One thing you don’t want to do is encourage excessive growth with heavy fertilization because injured roots and transport tissues may not be able to support this growth.  I recommend watering properly and fertilizing only lightly in the spring.

Minimizing Cold Temperature Damage

There’s not much we can do to protect our plants from record-breaking low temperatures in the fall or winter.  But we can minimize losses to our landscape and gardens by selecting plants hardy to our region.  Many landscape plants that we commonly use are “exotics” and are better adapted to climates where they’re considered natives.  Exotic plants respond differently to local climatic clues than do native plants.  Native plants have a greater chance of surviving winter cold temperatures.  However, gardeners like to test the limits and try growing many different plants... so even if we are planting “exotic” trees, shrubs, and perennials, we should make sure they’re hardy for our zone.

Several late-season gardening practices can help plants avoid winter injury, whether they’re natives or exotics.   It’s important not to promote late-season growth. Avoid fertilizing trees, shrubs, and perennials late in the growing season.  Don’t prune plants late in the growing season.  Wait for trees and shrubs to become fully dormant.  Late winter is the best time to prune.  Don’t drought stress your plants, but do cut back on the watering as the weather cools.  They certainly don’t need as much water in late summer and early fall as they did during 100-degree heat.  (It’s still important though not to drought stress your plants, especially needled and broad-leaved evergreens, as they go into winter.)