Gardeners should use caution when applying herbicides

The weather this spring has been unusually variable cool, blustery, rainy days followed by warm sunny days and cool evenings. We can anticipate a really hot spell or two soon, with temperatures near 100 degrees. The unpredictable weather is problematic if we need to apply herbicides to kill off the explosions of weeds that have sprung up this year or when we treat for pest insects in our gardens.

A few days ago, a neighbor told me of an argument she’d had with her gardener who wanted to spray the weeds in her lawn with a glyphosate herbicide on a breezy spring day when dark rain clouds were looming in the sky above. The gardener assured her it wasn’t going to rain soon, but it did, and she wondered what the effects would be of spraying an herbicide just before a rain shower.

Although the label on this herbicide states that it dries completely within 30 minutes and that there’s no pesticide run off after that time, spraying any chemical during windy or wet conditions is not recommended. Spraying herbicides during wet, cool weather also dilutes the chemicals’ effectiveness; herbicides are most effective at full strength during warmer weather when the treated plants are growing rapidly.

During windy conditions, herbicides can drift onto nearby plants and also vaporize into small particles that can rise up into tree leaf canopies. Plant damage from herbicides can occur at distances much further away from the targeted plants (or weeds in this case) than many people realize.

Run off from pesticides and herbicides has been found in drinking water supplies throughout the Central Valley. Home gardeners must use common sense and caution to avoid tainting our water. Do not spray chemicals when rain is forecast; avoid spraying hard surfaces like cement driveways and stone patios when treating for weeds-try to spray the weed only; do not rinse out pesticide or herbicide containers into gutters or onto hard surfaces; apply chemicals when the wind has died down-in the early morning hours or early evening; hand pull weeds or use a wiggle hoe to cut them back when they are small, before seed heads form.

Applying pesticides and herbicides during hot weather also entails risks. Read labels carefully; manufacturers’ will advise if their product should not be sprayed when temperatures are above 90 to 95 degrees. Horticultural oils that are sprayed in April and May to smother the immature “crawler” stage of hard scale insects on citrus should not be applied when temperatures are high; the hot oils can burn leaf tissue. Neem oil and insecticidal soaps when sprayed during hot weather to control soft-bodied pest insects such as whiteflies and aphids can also burn leaves and flowers. High temperatures can cause the chemicals to vaporize as well, reducing their effectiveness.

Wait until cooler weather returns to spray pesticides, even the least toxic products. Try non-toxic methods while you’re waiting-use a blast of water from the hose to knock down aphids, buy a bucket of ladybugs or green lacewings to prey on your soft-bodied pests, use sticky traps or vacuum up whiteflies, and protect your plants from stress by keeping them well-watered and lightly fertilized.

This story was originally published April 13, 2017 at 4:30 AM with the headline "Gardeners should use caution when applying herbicides."