Gardening Columns

This hosta was flattened by recent subfreezing temperatures.
The foliage won't recover, but hopefully the plant will
have enough energy to produce new leaves.

Baby, It's Cold Outside

First published on garden.org on April 12, 2007, by Suzanne DeJohn

Last week's cold snap was a doozy. Record low temperatures throughout the Southeast and in parts of the Midwest devastated the plants of farmers and gardeners alike, freezing tender new growth and killing fruit buds. Although official damage estimates haven't been released, some experts are predicting the worst. For example, it's feared that South Carolina, the nation's second-largest peach-producing state, may have lost its entire peach crop. Last year, South Carolina produced 100 million pounds of peaches; the economic effects of this freeze will be felt far and wide.

The Extent of the Damage
The freeze was particularly damaging because it came on the heels of several weeks of sunny, mild weather that had encouraged plants to grow and bloom ahead of their usual schedule. Even hardy plants can be harmed by below-freezing temperatures if they've broken dormancy and started to grow. Freeze damage occurs when water in plant tissues freezes and the resulting ice crystals rupture cells. The foliage on frozen plants often looks watery and wilted once it has thawed. Fortunately, most plants will recover. Damaged foliage will wither and plants should begin to grow new leaves. Your perennials may look dead but they'll probably bounce back. The same goes for most trees and shrubs.

Flowers and fruits are another story. Many fruit trees set their flower buds in the previous year, so trees whose buds were killed by the freeze won't produce more flowers this year. And no flowers means no fruit. Peaches, apples, pears, and cherries were all affected. Strawberries were also hit hard. With the unseasonably warm weather in March, many plants had started flowering and even producing fruit, all of which was killed. The strawberry plants will produce more flowers but these secondary flowers will yield fewer, smaller fruit.

What To Do?
Unfortunately, there's nothing you can do to help plants recover from the freeze. Be patient and give plants time to develop new shoots before assuming they're dead. Trees may look ragged until they drop damaged leaves and begin to resprout. Hardy perennials and annuals will probably recover but tender plants, such as tomatoes and petunias, probably didn't survive and will need to be replaced.

You may find trees with cracks in their bark. When trees break dormancy, water begins moving upwards; if the freezing temperatures last long enough to penetrate the wood, this water will freeze and cause cracks in trunks and branches. Trees with severe cracks may not survive.

The normal methods gardeners use to protect plants from late frosts probably wouldn't have been enough. Row covers, for example, may protect plants down to temperatures in the high 20s, but once temperatures drop into the teens and low 20s they just won't trap enough heat to keep plants from freezing. A thick layer of dried leaves or straw or other fluffy mulch topped by that row cover might have done the trick. Many of us, afflicted with spring fever, begin tidying our gardens in early spring by raking out leaves and cutting back dead foliage on perennials. This year, procrastinators may win out if the garden "debris" left on the beds -- nature's mulch -- offered enough protection to save new shoots.

You may have heard of fruit growers spraying water on their crops when a late freeze threatens. The technique requires precision in the timing and amount of water applied, and probably isn't an option for home gardeners. The protection isn't provided by the ice that collects on the buds, but rather by the continuous spraying and freezing. When water freezes it gives off a small amount of heat. As more water is applied, the ice layer gets thicker, but the temperature of the plant tissue under the bud remains at or just below the freezing point -- warm enough to prevent damage. As long as the rate of water being applied and the rate of freezing are in balance, the tissues won't freeze. As soon as the addition of water ceases, the temperature of the tissue will begin to drop until it reaches air temperature. Of course, an extended spraying session will result in a thick buildup of ice that can break branches and otherwise damage plants.

Despite our best efforts, occasionally Mother Nature seems to want to let us know she's still in charge.