Tomato flowers come complete with both male and female organs and are self-fertilizing. Pollen is shed with great abundance between 10:00 a.m. and 4:00 p.m. on dry, sunny days. Normally, the wind will pollinate the flower sufficiently. To ensure better pollination, gently shake or vibrate the entire tomato plant. The best time to do this is midday when it's warm, and the humidity is low.
Optimum fruit set occurs within a very narrow night temperature range of between 60° F and
70° F. When tomato plants experience night temperatures lower than 55° F or above 75° F, interference with the growth of pollen tubes prevents normal fertilization. The pollen may even become sterile, thus causing the blossoms to drop. High daytime temperatures, rain, or prolonged humid conditions also hamper good fruit set. If the humidity is too low, the pollen will be too dry and will not adhere to the stigma. If the humidity is too high, the pollen will not shed readily. Pollen grains may then stick together, resulting in poor or nonexistent pollination.
Have you ever wondered why large-fruited tomato cultivars are sometimes "catfaced" but not the small-fruited ones? This is due to a failure of complete fertilization of the ovule. The larger fruit demands more complete fertilization. This is not a disease but a physiological disorder.
For further infomation contact your local WSU Extension Office.
From The Gardener, Vol. 6 No. 2, Summer 1995
Holly Kennell, WSU Extension agent, King County