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!)