Shading tomatoes is a common practice among gardeners in very hot climates (for instance, in Florida, Texas, Arizona, Alabama, Georgia, and southern California in the U.S.). Providing shade is a helpful way to keep plants producing during the summer heat or at least help reduce their stress. This keeps tomato plants in better condition so that they perform better when temperatures level out or in the fall when cooler temperatures return.
Sometimes it just gets too hot and stays that way for too long. Your plants may need a little help to get through a heat wave or a long summer. Gardeners in the south (northern hemisphere) and north (southern hemisphere) have long known this. It’s not uncommon for those in hotter climates to shade tomatoes during the part of the day when sunlight is most intense – from about 10 AM to 2 PM.
You can shade early season tomatoes and seedlings during a sudden heat wave simply by setting a folding chair over them.
Another trick to use, this one as plants grow and if you cage them, is to tuck tomato leaves inside cages rather than allowing them to grow outside the structures. This creates dense, protective layers of foliage, which reduce transpiration (loss of fluid) during a heat wave. Thicker foliage also prevents fruit from sunscald. You’ll want to tuck new branches and leaves every couple of days, as leaf stems can easily snap off once they've grown too long.
But most often, you’ll need a system for shading tomatoes during a heat wave, or regularly during midday (10 AM – 2 PM) in the height of mid-season and thereafter, because that’s when the sun is most intense and temperatures are highest. By mid-season tomato plants have been in the ground for a couple of months and are pretty big. To protect plants properly from the sun, you need both a shade cloth and a way to support it. The support mechanism will need to reach up high enough to allow the cloth to cover the plant top to bottom.
You can use many materials to shade plants. Some gardeners drape old sheets or burlap over supports. Some set lattice panels around tomato plants to diffuse light. These materials can keep some light off plants.
But material constructed specifically for shading tomatoes keeps a larger amount of UV rays at bay and works to keep plants cooler. Shade cloth is specially-developed fabric that is used in greenhouses, on patios, or in the garden to reduce the impact of light. It is either knitted or woven. Shade cloth is rated by a percentage figure, which indicates the amount of reduction in light it achieves. For instance, 50% shade cloth reduces the amount of light the plants receive by 50%. The cloth also achieves a reduction in temperature, usually about half the amount of light. 50% shade cloth will reduce heat by about 25%.
More on growing tomatoes in extreme sun and heat