Although tomatoes need sunshine and warm temperatures to grow, temperatures above 95 degrees Fahrenheit and intense sun stress the plants. Heat-stressed tomatoes drop blossoms, fruit stops developing and the leaves may turn dry and brittle as the heat leads to moisture stress. However, proper care during the heat wave and immediately following it can help tomato plants rebound from any damage.
An application of mulch before temperatures rise can prevent many heat-related tomato problems. Applying mulch even after heat damage occurs can still help the plants regain their health. A 2- to 4-inch layer of straw mulch helps trap soil moisture, which prevents the soil from drying out too quickly. The mulch also reflects some of the intense sun rays so the soil doesn't heat up as much. Straw mulch still allows moisture to seep through from irrigation, so there are no special watering concerns.
Fruit damage sometimes occurs when tomatoes don't receive consistent moisture during a heat wave. Attempt to keep the top 6 inches of soil moist throughout the intense heat so the fruit doesn't crack once it resumes development. Consistent soil moisture also helps prevent blossom end rot. Deep watering, which moistens at least the top 6 inches of soil, is preferable over daily light irrigation. Water deeply one to three times a week, both during and after the heat wave, to maintain even soil moisture and prevent the worst damage.
If many leaves dry out and die during the heat wave you may need to lightly prune the plant once temperatures cool. Leave the dead foliage on the plant until the heat begins to subside, as the dead leaves can still shade developing fruit and minimize solar damage. Once temperatures cool, trim off any completely dead and brittle leaves and remove them from the bed.
Plants with sparse foliage or those that experience a lot of leaf death are more likely to develop cracked fruit, because there are no leaves to shade the tomatoes from the sun's intensity. Erect a simple cloth sunshade near the plants during the heat of the day to provide some shade until the plant begins to grow new leaves to replace those that died. A length of thin fabric stretched between two stakes works well.
Heavy fruit or leaf growth immediately following heat and sun damage can stress the tomato plant and prevent recovery. Harvest any fruits that are full size and beginning to change color from green to yellow or orange. Allow these to ripen indoors. You can leave completely unripened green fruit on the plant. Avoid any fertilizer applications until after temperatures cool and the plant begins to put on new growth. Fertilizing too soon causes heavy growth that only stresses a recovering plant.
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