Do you have tomato plants that are blooming but not setting fruit? There are a number of reasons why this might be happening. One obvious reason could be that there is an excessive amount of nitrogen (i.e. fertilizer) the plant has access to. Nitrogen causes plants to emphasize vegetative growth, often to the detriment of flower production, hence no fruit. Moreover, over-fertilization can lead to a delay in flower production and/or a decrease in fruit-set among the flowers produced.
Another possible problem with no fruit-set is the lack of pollination of the tomato flowers. Tomatoes are typically wind pollinated, therefore are not dependent on pollinators. Having said that though, tomatoes normally won’t set fruit if the night temperature is below 50 F due to sparse production of pollen. Temperatures that reach up to and remain above 75 F at night and day temperatures above 95 F coupled with dry, hot winds will cause poor fruit set on tomatoes, (cherry tomatoes, however, seem to be much more heat tolerant than slicers). It is these high temperatures that interfere with pollen viability and/or cause excessive style growth leading to the lack of pollination, ergo low to no fruit set during the extreme summer heat.
If the temperatures are ideal, and a tomato flower becomes pollinated, it usually takes about 3 weeks for it to develop into fruit about the size of golf balls. Growth then becomes more rapid with the mature size being reached in an additional three to six weeks. A few more days are then needed to change color.
Although there are “heat-set” slicing tomato varieties, such as Florida 91, Sun Leaper and Sun Master that will set fruit at “higher temperatures”, the difference is normally only 2 to 3 degrees...which is not really that much. It is the cooler temperatures that will allow flowers to resume fruit-set.
Furthermore, the extremely hot weather we have here in this area of Kansas not only interferes with flower pollination, it can also affect how quickly fruit matures. The best temperature for tomato growth and fruit development is 85 to 90 F. When temperatures exceed 100 F, the plant goes into survival mode and concentrates on moving water to the leaves. Fruit development slows to a crawl. When temperatures moderate, even to the low to mid 90s, the fruit will then continue to ripen more quickly.
Tomato color can also be affected by heat. When temperatures rise above 95 F, red pigments don’t form properly though the orange and yellow pigments do. This results in orange fruit. This doesn’t affect the edibility of the tomato at all, just its presentation.
So, can we do anything to help our tomatoes ripen and have good color during extreme heat? You bet there is. One can pick tomatoes when they are in the “breaker” stage. Breaker stage tomatoes are those that have started to turn color. At this point, the tomato has cut itself off from the vine and nothing will be gained by keeping it on the plant. If tomatoes are picked at this stage and brought into an air-conditioned house, they will ripen more quickly and develop a good, red color. Indoor temperatures ranging around 75 to 85 F will work best.

Rip Winkel is the horticulture agent in the Cottonwood District (Barton and Ellis Counties) for K-State Research and Extension. Contact him at rwinkel@ksu.edu or 785-682-9430 or 620-793-1910.