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John Fulton
Former County Extension Director



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In The Backyard

Horticulture columns and tips done on a timely basis

Frost and Cold Damage - from David Robson and Martha Smith

Posted by John Fulton -

Cold Temperatures Take a Toll on Plants

Recent cold temperatures have affected many yard and garden plants and may continue to do so for the next month, says David Robson, University of Illinois Extension horticulture educator. For most of Illinois, the average date of the last spring frost is between April 15 and April 25. Yet here we are with still a chance of damaging cold temperatures the last week of April. This is why planting dates between May 1 and May 15 are given. Now, plants that are already in the ground will have to withstand Mother Nature's chill.

Flowering plants generally are the hardest hit, with fruit trees suffering most as the temperatures keep dropping.

On perennials and woody landscape plants, cold temperatures can affect their emerging leaves, adds Martha Smith, U of I Extension horticulture educator. Cold injury shows up first as blackened edges around some leaves. Other leaves may have a twisted or distorted appearance, often confused with weedkiller injury. Cold injury can be seen throughout the entire plant from top to bottom and side to side, as opposed to the more localized chemical injury.

Some leaves may look shredded or in tatters. This can be blamed not just on the cold temperatures, but also on blowing soil particles that destroy the tender emerging tissue.

Robson and Smith give this overview of cold injury and what you can do to protect your plants.

Fruit Plants

Temperatures right at freezing (32 degrees Fahrenheit) seldom cause as much damage as those that dip below 28 degrees, especially on fruit plants and flowers. Cells freeze at lower temperatures, rupturing the cell walls and causing the plant to collapse. The centers of flowers turn dark brown to black. Once that happens, the flower won't be capable of producing fruit.

Flowers on the outside of the plant and those closest to the ground freeze first. Soil temperatures are not yet sufficiently warm to provide any insulation.

Flower buds are protected more than fully opened flowers and can tolerate colder temperatures. However, once the flower is showing color, cold damage is likely to take place.

The good news is that most fruit trees tend to produce an extraordinary amount of flowers. If half to three-quarters of the flowers are lost due to a cold snap, it is still possible to have a crop.

On the negative side, pollinating insect activity from bees and wasps is reduced at colder temperatures. Bees are seldom active at temperatures below 55 degrees. Flowers may be open, but unless the bees pollinate them, no fruit will develop.

Strawberries in bloom can suffer more than fruit trees. Check the flowers—if the center is dark, the fruit has been hit with a cold spell.

So how can you protect fruit plants from cold night temperatures?

On a strawberry bed, you can lightly cover the flowers with loose straw, or you can cover the planting with bed sheets or light blankets. Carefully lay the cloth material on plants to prevent damaging the tender shoots.

Avoid using plastic because it conducts cold directly to the plant tissue that it touches and provides little or no insulation. If you must use plastic, think along the lines of a greenhouse or a tent. Support the plastic sheeting with braces to keep it off the plants.

Small plants and fruit trees can be covered with several sheets or light blankets during the cold evenings to trap heat from the ground. Just make sure you remove the coverings during the day. If you are able to cover your fruit tree, a 100-watt incandescent light bulb placed in the tree, under the cover, will add a small amount of heat.

Sprinkler irrigation can also be considered. Water freezes at 32 degrees by releasing heat, and water melts at 32 degrees by absorbing heat. At this temperature, both actions are occurring and maintain a 32-degree temperature. To be effective, sprinkling irrigation must be constant throughout the cold period until air temperature rises above 32 degrees. This method does have some risk for fruit trees since ice can build up if temperatures dip below 28 degrees; the added weight may cause branch breakage. Sprinkler irrigation is more practical for strawberries.

Encouraging air circulation prevents the cold from settling. For some areas, you could put a fan outside to have a breeze blow across the plants. Make sure the fan is supported so it doesn't topple over.

Trees and Shrubs

Most of our ornamental plants haven't fully leafed out, or they are just starting to leaf out. Many plants initially produce flowers; cold temperatures can kill the flowers that plants initially produce, resulting in fewer seeds. This seed loss won't cause any noticeable injury to the plant.

Ornamental plants in bloom may turn black and abort the flowers, causing flowers to fall off the plant. However, most shrubs and trees should continue to leaf out.

New growth may be damaged as cells expand and break. The results are brown or black-colored leaves that will drop by early summer.

Most woody plants have secondary and tertiary buds if something happens to the initial growth. If the primary buds are damaged by cold, these buds will take over, and the plant will continue to leaf out.

Nothing can be done to protect mature trees and shrubs. Most of these plants will continue to produce new leaves as temperatures climb; these new leaves will appear typical. Damaged leaves may turn yellow and fall. But in most cases, the damaged leaves will remain on the tree.

Keep in mind that heat rises, while cold settles on the ground. The canopy of trees may be several degrees warmer than the ground, giving the plants a little extra warmth.

Vegetables

Some vegetables including broccoli, cabbage, lettuce, peas and spinach aren't bothered by the cold unless temperatures drop into the low 20s for an extended time. Others plants including tomatoes, peppers, asparagus and rhubarb are more susceptible to the cold.

Vegetable plants may take on a light yellow or purple color because the cold temperatures affect nitrogen and phosphorus uptake. However, as temperatures rise, the typical green color should develop.

Gardeners can cover transplants for the night with a milk jug, a clay flower pot or any type of bucket weighted down with a brick or large stone. Incorporating manures into the vegetable garden before planting helps generate some heat. And, mulching with black plastic can also provide some insulation for the plants.

Rhubarb that is wilted and limp from the cold should not be eaten. Remove those stalks and wait for new growth. If leaves and stems are stiff and erect, the stalks are safe to eat.

Asparagus shoots will become distorted and twisted. But, the spears are safe to eat.

Vegetables that appear sickly from cold injury may never fully recover even if they don't die. It might be better to sacrifice those plants to the compost pile and buy new, healthy ones once the air and soil temperatures start to rise.

Flowers and Ornamentals

Bulbs generally are the only flowers blooming right now, and they can tolerate some freezing temperatures. But, the shoots of other spring flowers can be susceptible to cold injury. The vast array of perennials such as hostas and peonies can be protected with bed sheets or light blankets. Again, remember to remove the covering during the day. Plants in containers can be moved inside a garage or shed for the evening. If you don't have space inside, you can move the containers closer to the house where the ambient heat may keep them a couple degrees warmer than the open.

Robson and Smith say that the damage from cold injury depends on how cold it gets, the length of the cold spell, and the type of plants involved. Homeowners who have some prized plants may want to take precautions. Other may decide to take a wait-and-see attitude and watch for damage in the next week or so.


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