When tomatoes fail to set fruit or form large fruits.

These are common problems for gardeners from the tropics. If you have a tomato plant that is healthy and flowering but not setting fruit, the reason is likely related to temperature. Both absolute temperatures and the difference between day and night temperatures (diurnal variation) affect tomato fruit set and quality. Realizing that published guidelines vary, we suggest the following requirements:

1. Daytime temperatures below 40°C (104°F) and optimally below 33°C (92°F). Nighttime temperatures below 26°C (79°F). Fruits that do set outside these ranges are often so badly damaged or misshapen due to poor pollination that they are not marketable.

2. Diurnal variation greater than 5.5°C (10°F). Tomato plant growth and fruit set is reduced in areas where the difference between day and night temperatures is less than 5.5°C (10°F). As an exception, a very high diurnal variation, as might occur in a desert or high in the mountains, can apparently overcome some of the above effects of high temperatures.

Heat-tolerant varieties have been developed which can extend the range a bit. Cherry, plum, and other small tomatoes seem to be less adversely affected by temperature extremes, which is why those types are often the ones found in local markets. See ECHO’s seed listing on the web or search the Internet to find them. EDN 53-3.