Beware of Frost

Cold temperatures cause many different plant problems. Some are unavoidable but others can be greatly reduced if the proper measures are taken. Fall frosts stop the growth of annual plants including tomatoes, squash, and all other warm-season vegetables. Fall frost is also hazardous to some trees and shrubs. Frost injures the woody growth of plants that grow to late in the season. Well-acclimated plants stop growing in mid-summer and harden off before the cold arrives but grapes, tea roses, nectarines, and other tender plants may keep growing. Much of the damage that we attribute to "winter kill" is actually due to improper hardening off of the plants in the fall. This is particularly true of young, succulent growth. Ultimate low temperatures also cause severe damage to trees and shrubs. Plants are rated according to their cold hardiness and prolonged cold may destroy the plants. Plants that normally grow in warmer areas are particularly prone to damage. Plants growing in pots are also more likely to be damaged than plants growing in the soil. Soil is warmer so the roots are able to survive better if they are planted in the ground. In addition to keeping plants healthy it is important to minimize frost heaving. The alternate freezing and thawing of soil has tremendous impacts upon the soil structure and aggregation. There are some beneficial aspects of freezing and thawing in clay soils but rapid freezing will tear the roots from the plant and damage it severely. Water actually expands about 10% in volume when it freezes. This freezing and thawing will cause the soil to heave or lift up which may force shallow rooted plants out of the ground or tear the roots from more deeply rooted ones. This type of freezing will damage perennials, roses, newly planted lawns and winter annuals and biennials. The best prevention for this kind of damage is a good snow cover. Snow is an excellent insulator and also prevents drying. Since we have no control over the snowfall we substitute mulches. Anything that serves as a good insulator including leaves, straw, or wood shavings will work providing they do not pack down. Styrofoam containers that cover the entire plant are also available. As mentioned previously, the function of mulches is to keep the plant and soil cold not warm. Soil that remains frozen will not damage the plants as the freeze-thaw cycle does. Organic matter also helps hold the soil together further lessening the damage. Larry A. Sagers Regional Horticulturist Utah State University Thanksgiving Point Office