Horticulture News: Recognizing damage caused by cold snaps

By Adam Leonberger, Published:

I’ve received quite a few calls the past few weeks about cold damaged plants.

Hopefully you were able to cover or mulch your susceptible outdoor plants, or move container plants to protect them from the cold weather we received.

Depending upon the plant species, damage could have ranged from the loss of a few blossoms in a low-lying field; to barely visible leaf burn on early spring vegetables and flowers; to the death of above and below ground plant tissues. 

Depending on the crop and location, some aesthetic and economic loss from cold injury can occur every year. Losses can result directly from damaged or killed plants, and indirectly from reduced quality or delayed maturation.

Low temperatures can affect plants in several ways.

First, temperatures near the minimum for plant growth will reduce the plant’s rate of metabolism and growth. If the temperature, and therefore the metabolism, remains low for an extended period, plant quality will suffer, and death may occur.

Another type of injury occurs if the temperature falls below freezing.  Plant tissues that freeze generally appear dark green and water soaked at first, later becoming blackened and necrotic. Also, freezing temperatures in spring after the buds have begun to swell will often result in abnormally twisted and curled new sets of leaves.

Some vegetables, such as the cole crops (cabbage, broccoli and cauliflower) and onion sets, respond to cold weather by producing a seed stalk. This process, called “bolting,” occurs when young plants are exposed to low temperatures for several days.

Quality impacted

This greatly reduces the quality of the affected crop. Bolted plants should be discarded; cutting the flower stalks off will not prevent the deterioration in flavor and quality. 

The cold temperatures may affect the flower buds of many fruit and flowering trees. The further developed the buds are, the more susceptible they are. During a freeze, the most advanced buds may be injured, while the less developed ones may survive. If injured sufficiently during the pre-bloom or bloom stages, the buds will dry up and eventually drop. 

Evergreens may exhibit the most noticeable cold damage. Branch and leaf death often appears to be random. In the case of conifers, needles may turn brown and fall off green stems. As for spring frost-injured shoot tips, they will die back and, provided the tree or shrub has adequate energy reserves, new growth will emerge from latent buds on the twigs. Broad-leaved evergreens, such as hollies, often have marginal leaf burn.

The signs of cold damage can be confusing, since some damage may not be evident until months later. Cut a few buds open from several places on the tree and examine them. If the center of the bud is darkened or black, the cold has killed the bud.

Check several trees in the area to obtain a representative sample. Scrape affected branches with a pocketknife or fingernail. If this reveals brown tissues and not green under the bark, then you have experienced severe damage.

What now?

Be patient. Some plants may appear dead, but they are not. Injury to foliage and tender shoots should be visible within a few days, but it may be several months before damage to larger limbs can be determined.

After about a month or so, you can prune out any dead twigs or branches. On woody plants, leaf buds, which are more cold hardy, will often grow and fill in areas where dieback has occurred. If the buds have not survived, prune dead branches back to living (green) tissue.

Do NOT (repeat not) fertilize trees and shrubs. This results in shoot growth. The plant needs to begin growing slowly and to replace damaged tissues before it begins putting out growth. The plant will not be able to support the water needs for itself during the hot, dry weeks of July and August if there is an abundance of fresh, new growth. Late fall fertilization is best for most woody plants

For more information on this and other horticultural issues, contact the Franklin County Extension Office, 502-695-9035, or Adam.Leonberger@uky.edu.

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