No Pesticides in Hot Weather

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Most gardeners clearly understand that applying pesticides on a rainy or windy day is not going to be very effective, but heat also be a factor to consider. On those really hot days, some pesticides, and that includes organic ones, can damage the plant at least as much as the pest. 

Always Read the Label

That you ought to read a pesticide label before applying it should be obvious, but gardeners who regularly use the same pesticide often figure they know it well and skip this step. When the weather is hot, though, get out your reading glasses (or a magnifying glass: sometimes that small print is really small!) and give the label a thorough read over. If the product should not be applied in hot weather, it will say so!

You’ll find that even “safe” insecticides, like insecticidal soap, neem and horticultural oil, can kill or damage plants when applied in hot weather. Most emulsifiable pesticides warn not to apply them at temperatures above 90˚F (32˚C). 

Dusts and powders are less likely to cause damage to plants at high temperatures than oils and soaps, but still, read the label!

Damage is most likely on young growth and flowers, less so on mature leaves and stems. and may include spotting, dead zones, stunted growth, deformed fruit or even the death of seedlings.

Of course, insects and diseases are not going to wait until temperatures drop. Fortunately, you can apply many of these temperature sensitive products early in the morning or in the evening, when temperatures are likely cooler. Or maybe wait a few days if the weather is expected to cool down

Vapor Drift

Damage to the plant being treated is only one factor to consider. Many pesticide products are volatile and will be carried much further in hot weather then at normal temperatures, thus reaching non-target plants, beneficial insects or even fish in a nearby pond. Often the cut-off temperature for these products is 85˚F (30˚C). Usually, too, the label of such products will warn not to apply them at all during hot weather, even during cooler hours, as they can vaporize hours after application.

Herbicides Too

The information above applies to not only insecticides, but also herbicides. 

Herbicide damage to tree leaves. Photo:

One man recently wrote that the lower leaves of his trees were twisted and contorted with brown margins and was convinced some disease or insect was involved, but I thought it odd that only the lower leaves were damaged. I asked if he had sprayed a lawn herbicide during a recent heat wave. He said no, then recalled serving lemonade to a sweaty lawn care company employee obviously suffering on a very hot day. It turned out he had sprayed a herbicide on the grass below and it had drifted up to the foliage of the trees. 

These companies are not supposed to apply such products during hot or windy weather, but … they do. If they show up at your place during a heat wave, send them packing!

Home Remedies

There are no labels you can check for home remedies and indeed, many of them are fairly innocuous, but even so, they could still cause damage. I’d suggest not applying any home remedy at temperatures about 85˚F (30˚C), just to be certain.

Heat and pesticides: they just don’t mix!

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

2 comments on “No Pesticides in Hot Weather

  1. Some of your Cs should be Fs: 90˚C (32˚C) –>90˚F (32˚C)

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