When faced with long spells of daytime temperatures over 85°F and  nights above 72°F, tomatoes may fail to set fruit. The plants may look dark green and vigorous, evidence that all other conditions are favorable. But blossoms just dry up and fall off.

If the heat spell lasts a week or less, plants will quickly recover. However, during long spells of warm nighttime temperatures, plants will stop setting and there will be a subsequent gap in tomato production that can have serious financial consequences for commercial growers.

In recent years, there has been a flood of new varieties that are bred to have greater heat tolerance. Known as “heat set” or “hot set” tomatoes, some of the commonly grown hybrids are BHN 216, Solar Fire, Sun Leaper, Sunpride, Sunchaser, Sunmaster, Solar Chaser, Summer Set, Talladega, Heat Wave, Floraset and Florida 91. According to the University of Alabama Cooperative Extension, many heat-set varieties also perform well under the opposite conditions of extended cool, rainy weather.

Some heirlooms are also known to be heat tolerant. Hazelfield Farm, Homestead 24, Illinois Beauty, Neptune, Tropic, Arkansas Traveler, Ozark Pink and Eva Purple Ball. Additionally, some early (cold-set) varieties are also able to function in hot weather: Stupice is one. Some cherry tomatoes also do well: Lollipop and Yellow Pear are two.

Southern tomato growers often choose heat set varieties for summer and fall production — a strategy growers farther north might want to emulate if summer temperatures get higher. Other recommended practices for growing tomatoes in heat include planting deeper, where the soil is cooler, providing afternoon shade, watering in the morning, and using white plastic mulch to keep soil cooler.

It should be noted, however, that heat set tomatoes can tolerate only slightly hotter than optimum temperatures. In extremely hot weather, they won’t produce as well.

If you’ve experienced poor tomato yields because of the heat in summers past, you might want to trial a few heat set varieties. Here are two additional resources for learning more about heat set tomatoes:
Alabama Extension: http://www.aces.edu/department/com_veg/blossom_drop.pdf
Louisiana Extension: http://www.lsuagcenter.com/en/crops_livestock/crops/vegetables/Performance+of+Hot+Set+Tomato+Varieties+in+Louisiana+Summer+Fall+2006.htm