2005 Year in Review

Cold Temperatures Take Their 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.

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

Surprisingly, temperatures right at freezing (32 degrees Fahrenheit) seldom cause as much damage as those that dip below 28 degrees. 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 plant 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 sufficiently warm enough yet 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’s still possible to have a bumper crop.

On the negative side, though, 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. Layering sheets or light blankets over a strawberry bed, or lightly covering the flowers with loose straw, may provide some protection at night. Avoid using plastic because it conducts cold and provides little or no insulation.

Fruit trees and strawberries aren’t the only plants to be affected, adds Robson. On trees and shrubs, the green chlorophyll pigment hasn’t been produced; as a result, these plants may tend to turn red or yellow. This can result in reduced growth and more stress on the plant if seed production is heavy. A classic example is the maple trees producing their winged seeds now.

Red-leafed plants have a deeper color, which might be appealing in the landscape. The same holds for yellow plants, though warmer temperatures will soften the color. Homeowners who buy colored foliage woody plants may notice by summer that their prized specimens have more green in them.

While the redder barberries and maples and more yellow colors on the spirea and honey locusts might be considered a positive attribute, the plants do have the potential of suffering sunburn, especially on the new tissue and during bright spring days. Chlorophyll acts somewhat as a sunscreen for plants, protecting the leaves during the spring and summer.

Cold temperatures can affect emerging leaves and those developing deep into the shoot tips. 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. The effect will be 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.

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. In most cases, though, the damaged leaves will remain on the tree.

“Small plants can be covered with several sheets or light blankets during the cold evenings,” says Robson. “Just make sure you remove the coverings during the day.”

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 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 with a milk jug or clay flowerpot for the evening. 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 leaves 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 instead of arrow-straight. The spears, though, are perfectly safe to eat.

“The cold spell may have killed some insects and weeds such as crabgrass but not enough to rejoice over,” says Robson. “Under the right conditions, insects can quickly build back up, and weed seeds will continue to germinate.”

Robson concludes that annual vegetables or flowers 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.

by  Editor, theCity1.com
May 6, 2005