Your gardening answers with Paul Rogers.
COLUMN: Your gardening answers
`When temperatures exceed 90 degrees, should I leave my plants alone?"
If by "leaving the plants alone" you mean not spraying, fertilizing, pinching or otherwise stressing the plants, by all means leave them alone! Plants that are subjected to temperatures above about 86 degrees (the actual temperature varies among different varieties) are under temperature stress and may be damaged by chemical treatments, whether it be fertilizer or pesticide.
Does that mean that we should not be misting or watering the plants? Moisture evaporating from leaf surfaces cools the plant. Watering allows the plant to continue to function. By all means, the more expensive and more meaningful the plant is to you, the more necessary it is that you prevent the plant from excessive or prolonged wilting.
Disregard the old idea that water on plant foliage in the sun causes burn spots on plant foliage, as it does not. Otherwise we would observe brown spots on plants after a shower followed immediately by bright sunshine. There are plants (such as those with hairy leaves) that will leaf-spot from cold water droplets at any time.
Any chemicals, even usually benign fertilizers, are made hyperactive by high temperatures and can burn plants. In addition, water is often used as the "carrier" when applying liquid fertilizers and pesticides, and the water evaporates too fast, thus leaving concentrated amounts on plant foliage.
It's also wise for gardeners to limit their range of horticultural activities when temperatures are high. Keep yourself and your plants well hydrated with clear, clean, unadorned water.