University of Illinois Extension

 


Sharon Yiesla
Unit Educator, Horticulture
Lake Unit

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Did the Cold Hurt My Plants?

The recent bout of sub-zero temperatures, coupled with no snow cover, has raised the question “Did the cold temperature hurt my plants?” For the most part, the answer will be no. Our winter-hardy plants are in the middle of their dormancy and will not be affected by the sub-zero temperatures.

At this point, the only damage we might see would be frost cracking on trees with thin bark (crabapples, young maples, fruit trees) and possibly some drying out of the evergreens. Let’s look at both of these situations and the possible consequences.

Frost cracks are vertical cracks that occur in the bark of trees that have thin bark. This damage happens in winter on cold, clear, sunny days. The sun heats the bark and the wood directly under the bark, causing them to expand slightly. When the sun sets or the sky clouds over, the temperature of the tree drops quickly. The bark cools more quickly than the wood. Therefore, the wood contracts more slowly than the bark and the bark rips open in a long crack.

These cracks usually become apparent in early spring, when homeowners start spending more time in the yard. Frost cracks may or may not need any intervention. If the crack is a clean one with neat edges and no loose bark, the tree may be able to heal the wound with no intervention from the homeowner. If the edges of the crack are ragged and torn, or there is loose bark, it is best to try to ‘repair’ the wound.

Before attempting to repair a frost crack, it is important to understand a few things about how trees “heal.” First, trees do not usually close a wound. Instead, the tree tries to seal the edges of the wound by forming a callus layer. Secondly, bark that is loose, cannot be re-attached. It will not grow back on to the wood, so there is no reason to try to tie or strap the bark back on.

If a frost crack needs repair, start by cutting away any loose bark. Use a sharp knife to turn ragged edges into smooth ones that the tree can heal with callus. Do not cut deeply into the tree’s wood. You need only cut a clean edge into the bark. Frost cracks usually only affect the bark and seldom damage the wood under the bark. As you cut clean, sharp edges, shape the wound into a vertically elongated ellipse. A vertical shape allows water to drain out of the bottom of the wound instead of settling into it.

There is no need to apply any type of wound paint. Research shows that this has no benefit. If the tree is healthy, the wound’s edges will begin to callus during the first growing season after that crack appears. Over the years, the callus layer will grow and after many years, the wound may close entirely.

Many trees sustain a frost crack and live for many years. Frost cracks may re-open in winter and close again in summer. While frost cracks are seldom fatal, they can serve as an entry way for other problems such as insects and diseases. Frost cracks should be inspected regularly for any changes.

Desiccation, or drying, of evergreens can occur when soil moisture is low, temperatures are cold and winds are blowing. Since evergreens keep their needles or leaves in winter, they can lose water all winter.

To keep this problem at a minimum, several steps should be taken throughout the year. Evergreens should be watered regularly (about one inch per week) during the growing season, whenever rainfall is inadequate. This includes late autumn and early winter. As long as the ground is not frozen and can accept water, the evergreens should be watered. This will help them maintain good vigor throughout the winter season.

There is relatively little that can be done in winter. For smaller evergreens, a burlap barrier can be erected to minimize the effect of cold winds. This may not be particularly attractive, but the benefits may outweigh the poor appearance. The burlap barrier may also be useful if the evergreens are situated in an area where they receive salt spray from local roads. The burlap can help keep the salt off of the needles which will help reduce damage.

Gardeners also need to be diligent in spring as the soil thaws. Since spring is usually cool, it is often thought of as being moist. This is not always the case. As winter turns to spring, keep an eye on the weather. If spring rains do not come or are inadequate, begin to water evergreens again. They will most likely need the water, after this long, dry winter.

To achieve healthy, vigorous evergreens, give good care all year round. Do not let maintenance become a quick reaction to a one-time adverse condition.

 

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