If your tomatoes dropped their blossoms instead of setting eagerly awaited fruits, you’re not alone. The weather for the last month or so has been quite inhospitable for tomato fruit set.

Most plants have experienced colder temperatures than what is needed for pollination to be successful. In stark contrast, we had a brief period of unseasonably warm weather.

Warm-season crops, which include tomatoes, peppers, beans, squash, cucumbers and melons, require temperatures above 55 F in order to successfully set fruit. Excessive heat also reduces fruit set, especially as temperatures climb into the upper 80s and above. The optimum temperature for tomato pollination is 70-82 F. Much of the state was outside this range for much of the time after tomatoes were planted this year!

Relative humidity is another important weather factor for pollination. When air is too humid, pollen grains tend to stick together, making dispersal more difficult. When air is too dry, the stigma - the part of the flower that receives the pollen - may dry out.

Looking at the weather data archived by Purdue University’s Indiana State Climate Office, we’ve experienced many days of unseasonably chilly temperatures when daily lows dropped under the minimum required and very few days when the daily highs were favorable for pollination.

In the 28 days between May 11 and June 7, Indianapolis had 25 days when temperatures were outside the optimum range for tomato pollination. Of these, 19 days had a daily low below 55 F and 15 days with measurable rainfall.

During that same period, Vincennes had 24 days outside the optimum range for pollination, 14 days when the temperature dropped below 55 and 16 days of measurable rainfall. In fact, there was only one day during this period that had optimal temperatures with no rain in either city.

No wonder the tomato blossoms are dropping! In addition, temperatures above or below the optimal range result in poor vegetative growth, so root, stem and leaf development were likely to be slow.

The good news is that plants should recover quickly as weather conditions become more favorable and the lack of fruit set encourages plants to keep flowering. Plants can play catch up, to a certain extent, if we get more optimum conditions, but I expect a lower total and a later-than-”normal” harvest for this year. I hope I’ll be eating those words later this summer!

This farm news was published in the June 28, 2006 issue of Farm World, serving Indiana, Ohio, Illinois, Kentucky, Michigan and Tennessee.