How to tell when squash, tomatoes, other veggie plants have heat stress

Leimone Waite
Master gardeners

Q: My squash plants and other vegetables have suddenly developed brown edges around some of the larger leaves. Is this a disease? Is there anything I can do to prevent this from spreading to the whole plant?

A: I am assuming this development was recent and was caused by heat stress, as we had our first heat wave a couple weeks ago. Plants can experience heat stress when temperatures are above 90 degrees for a prolonged period, or if temperatures are above 104 degrees even for a short time. The heat stress is compounded when it’s windy and the soil starts to dry out.

Plants that have large leaves, like squash, will dry up the outer margin of the leaf as a survival mechanism when they become water or heat stressed. This allows the plant to preserve a smaller leaf surface that can still photosynthesize and produce food for the plant but reduces the amount of water that the leaf will need. Keeping plants well-watered should keep this from happening again.

To minimize heat stress in plants during hot summer months it is best to water deeply in the morning. However, wilted plants should be watered as soon as possible to minimize damage caused by lack of water. Mulch around plants to help conserve water in the soil.

Providing shade can also help to minimize heat stress and keep vegetables producing even when temperatures are hot. Shade plants with a shade cloth or other shade type structure. If providing shade with something that completely blocks the sun, such as a patio umbrella, it is best to block the afternoon sun only.

Providing a wind break can also help to reduce heat stress.

Here are some other signs of heat stress in plants.


This is when the leaves of the plant droop due to a lack of water pressure in the plant. It’s common for herbaceous (non-woody) plants that are suffering from heat stress to wilt during the heat of the day and then recover once the sun sets.

In the heat of the day, many plants may wilt or droop but by evening or the next morning they are once again perky. This gardener recycled an old tire for a garden border. the can help keep burrowing pests at bay.

Leaf rolling or cupping

Plants will roll or cup leaves to reduce surface area and maintain the moisture in their leaves. Leaf rolling or cupping is a very common behavior in corn and tomato plants in response to heat stress.

Dropping of blossoms or fruit

Many plants including citrus, cucumbers, peppers, squash and numerous ornamentals will drop blossoms or developing fruit if heat stressed for long periods of time. This allows the plant to conserve water and other resources necessary for survival. Most plants will return to normal flower and fruit production once the high temperatures have passed.


Peppers, melons and other fruits may scald on the side where they are exposed to the sun, especially if the plant is wilted. Sunscald will appear as a lathery tan or brown patch on peppers but may look like a watery spot or a blistered patch on other types of fruit.

Blossom end rot

Caused by a lack of calcium in the developing fruit, this is a common problem in tomatoes. Heat stress can cause this problem as calcium only moves in the plant with water so if the plant wilts it cannot move calcium to the fruit.

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When temperatures reach above 90 this causes cool season vegetables such as lettuce, cilantro, broccoli and spinach to send up a flower stalk. This is called bolting. When cool season plants bolt the leaves become bitter and in some plants the leaves dry up.

The Shasta Master Gardeners Program can be reached by phone at 530-242-2219 or email The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.