Frost Damaged Plants May Need Pruning, but Wait until Spring

Recent freezing temperatures may have injured some citrus trees and other frost-sensitive plants. But since the full extent of injury won't be known for several months, UC Cooperative Extension (UCCE) horticulture advisor Ed Perry suggests gardeners wait until spring before pruning and removing damaged trees and plants.

The frost injury to plants depends upon a number of factors, including species, age, health, soil moisture and location. Frost injures plants by causing ice crystals to form in plant cells, making water unavailable to plant tissues and disrupting the movement of fluids. Frost-damaged leaves appear water-soaked, wither, and turn dark brown or black. Unprotected, sensitive young trees may be killed, but frost rarely kills mature trees in California.

"While you may be tempted to prune out damaged branches right away, it's best to wait until spring when new growth will show you the extent of the injury," said Perry. "Always allow plenty of time for new growth to take place, so that the damage is clearly defined. Earlier pruning often results in leaving some limbs which might continue to die back, and the removal of limbs which might recover."  

If a one- or two-year-old grafted fruit tree has been killed almost to the ground, it's important to determine whether the dead wood extends below the bud union, something best done in April or later. "If only the roots survive and grow back, you may be left with an unproductive rootstock instead of the true-to-type fruit tree you originally planted. In this case, the tree should be replaced," Perry said. "If there is enough live wood above the bud union to grow a strong shoot, a new trunk will develop."

For herbaceous and shrubby plants, patience is also the best course. While you may be tempted to remove the damaged leaves, they provide some protection from future frosts. Wait til the danger of severe frost in your area has passed and new spring growth has begun before deciding what to prune or remove..  

Courtesy of Jeannette Warnert, University of California Cooperative Extension

With the very cold weather that we've had lately, I'm sure you've been busy protecting your favorite plants.  But what about your pesticides?  Even the organic and low-toxicity pesticides favored by Master Gardeners are subject to deterioration by freezing temperatures. Freezing of liquid pesticides can result in separation of the active ingredient from the solvents or emulsifiers, or inactivation of emulsifiers, which may lead to crystallization or coagulation of the pesticide.

Be sure to read the Storage and Disposal instructions on your pesticide label. Many dormant oils and fungicides such as Liqui-Cop say “KEEP FROM FREEZING”. Pesticides which contain BT to kill caterpillars should be stored between 20-90°F.

The freezing point of many pesticides is lower than 32°F due to the hydrocarbon solvents or inert ingredients. However, it’s a good idea to assume that they shouldn’t be subjected to freezing temperatures for a substantial period of time. Pesticides that cannot be frozen must be placed in a heated or adequately insulated area to avoid sub-zero temperatures.

If your pesticides have frozen, techniques for thawing and redissolving are important, since a pesticide, once frozen, can plug spray equipment, result in poor product performance and/or damage crops if the proper thawing and mixing procedures are not followed.

Before attempting to thaw a frozen pesticide, the container should be checked to make sure that it is not ruptured or cracked from the expansion of the frozen liquid. If sound, the container should be brought to room temperature (placed in a heated room or south side of a sunny building away from children, livestock and pets) for the thawing process, which may take several days. Once the liquid has thawed, the container can be rolled, shaken or otherwise agitated to get the contents into a uniform suspension. The container should also be inverted several times to ensure the product is completely dissolved. Pesticide manufacturers caution that if a pesticide cannot be totally redissolved (crystals are still present), the pesticide should not be used.

Adapted from “Cold Weather Storage and Handling of Liquid Pesticides” by Greg Johnson, Pesticide Education Specialist, and Robert Hendrickson, Program Assistant with Montana State University Extension Service (and they should know about cold!)

Retail nursery and garden centers are among the top sources of pest management information for home gardeners. To help provide customers with the latest pest information from the University of California, the UC Statewide IPM Program created the Retail Nursery and Garden Center IPM News, a three times yearly e-newsletter for retail nursery and garden center employees, managers, owners, and affiliates. 

This is also a great resource for Master Gardeners.  The latest issues have information on a downy mildew affecting impatiens, 2 new stink bugs, deer resistant plants (fact or fiction?), brown widow spiders in Southern California and much more.

If you'd like to sign up to receive an electronic copy, check out their website.

http://www.ipm.ucdavis.edu/RETAIL/retail-newsletter.html

Welcome to our new information-sharing blog for North State Master Gardeners and gardening enthusiasts.  We plan to share "how to" information, notices of upcoming events, photos, recipes, answers to frequent garden questions and more.