While most of our garden plants need at least 6 hours of full sun to be productive, too much sun, especially when combined with too much heat, can be too much for plants to bear. Problems arise especially when temperatures are over 85 to 90 degrees F. Plants near light-colored walls can suffer from reflected light and heat, even when the ambient temperature is not all that high. Putting new plants into our gardens isn't the best idea during the hot part of the year, but sometimes we just can't resist doing this.
At high temperatures, tomatoes fail to become pollinated. The flowers fall off without making fruit and the plant is putting its energy into survival, not fruit.
Tomatoes and peppers can develop light brown, leathery patches on the sides exposed to direct sun. http://ipm.ucanr.edu/PMG/GARDEN/VEGES/ENVIRON/tomsunscald.html
Most garden plants exposed to excess sunlight can suffer sunscald on their leaves. Sun scalded leaves develop brown or white patches and eventually dry and crumble. This decreases a plant's vigor and exposes fruit to more sun.
Newly transplanted plants suffer even more because their delicate root hairs have been damaged during the procedure and can't take up as much water as established plants can. You might see major wilting or even plant death when temperatures are too high.
What you can do
Provide partial shade during the time of the most intense sunlight. Erecting a shade structure that gives the plants some relief from the hottest parts of the day (midday and afternoon) can help. At the CoCoMG demonstration garden in Walnut Creek, we use 40% shade cloth over vegetables and fruit trees, but any shade cloth that blocks 30–40% of the sun is good. Shade cloth that blocks more sun (up to 90%) doesn't let enough light through for plants and is best used to keep people cool.
You need a structure to hold the shade cloth in place. It need not be complicated but must be able to withstand possible windy days. Use hoops or bamboo supports to hold the shade cloth above the plants. It's handy to make it portable so you can move it to different spots in the garden where it's needed and reuse it each year. Position the structure so at least the west side of the plants are shaded. In the hottest areas, shade cloth over the top of the plant is also good.
There are other cultural tips for mitigating the effects of excessive sun and heat. These include providing sufficient water, laying down organic mulch to retain soil moisture and keep the soil cool, and applying appropriate fertilizer to ensure healthy leaf growth which can protect fruit from sun scald.
Homemade PVC structure to shade cabbage growing late into the summer season by Sara H.
Temporary shade for tomatoes using a plastic plant flat by Sara H.
Photo of shade cloth over chard by Terry L.
Help Desk of the UC Master Gardener Program of Contra Costa County (SEH)