Pollination of Tomatoes
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 glower 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 and 70 F. When tomato plants experience night temperatures lower than 55 or above 75 degrees, interference with the growth of the pollen tubes prevents normal fertilization. The pollen may even become sterile, thus causing the blossoms to drop.
High daytime temperatures, rain, or prolonged humid condtioind also hanmper 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 nonexistant 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 demand more complete fertilization. This is not a disease but a physiological disorder.
From The Gardener, Vol. 6 No. 2, Summer 1995, a publication of Western Washington University Extension.