Prime among them are the occurence of extreme temperatures. When tomato plants experience extreme temperatures, either below 55 or above 90 F, for an extended period of time it may cause developing tomato blossoms to drop off. These blossoms drop off before the tomato fruits can set and therefore the tomato fruits do not develop. Another cause of poor fruit set in tomato plants is excessive nitrogen composition in the surrounding soil of the tomato garden. When the soil around tomato plants has a high nitrogen concentration, this promotes lush leaf growth to the detriment of tomato blossom and tomato fruit formation.
Excessive nitrogen is the most common cause of lush foliage with no tomato fruits in the garden.
Shading may also account for poor tomato fruit set. Be sure to locate your tomato garden where it will receive no less than six hours of sunlight per day. If the number of hours your tomato plants receive direct sunlight drops below six, it could be the cause of poor fruit set. The last, and often overlooked, cause of poor fruit set in tomato plants is very simply, dry soil. If your tomato plants are not receiving adequate water for their nourishment needs, tomato blossoms will dry and fall off plants.