Did you know that temperature, humidity and plant vibration are all important factors in tomato pollination? And if tomatoes do not pollinate successfully, there will be few if any tomatoes?
I learned this lesson the hard way. One season, I decided to keep my tomato beds covered all day and night. I thought that by keeping the plants consistently as warm as possible, inside their plastic tents, they would be more productive. Instead, the majority of the blossoms quickly turned brown soon after blooming and fell off. The browned blossoms did not result in tomatoes. The few blossoms that stayed on the plant did produce tomatoes but they were few. I also noticed this happening with the tomato plants I was growing in my greenhouse where there is only one small vent. I kept the door closed to the greenhouse, again to keep the plants nice and warm.
I couldn't figure out what caused this problem. Tomato pollination may have crossed my mind but I didn't seriously consider that as the problem. I spent the entire season asking every gardener I knew or met but no one had a solid answer until late Fall when local master gardener, Christina Mone, spoke at our garden club. She directed me to a specific page on the Washington State University (WSU) Extension website and there I had my "aha"!
WSU Extension Agent, Holly Kennell has published a very helpful report on tomato pollination (see link below).
According to Holly, tomato flowers come complete with both male and female organs and therefore they are considered self-fertilizing. The pollen is released between 10:00 a.m. and 4:00 p.m. on dry, sunny days and the wind will pollinate the flower sufficiently. However, if there is no breeze at all or the plant is sheltered in a greenhouse or under plastic, the plants will need some help. Holly says that you can gently vibrate the entire tomato plant to distribute the pollen yourself--best time to do this is midday when it's warm, and the humidity is low.
My solution is go out to the garden every morning and fold or roll up the plastic on the sides of the beds about halfway and clip them there, allowing the breeze to do it's thing. In the evening, I roll the plastic back down. For the greenhouse, I tried leaving the door open all day but it didn't help very much because it isn't set up for wind to flow through so I no longer plant tomatoes there. I stick to plants that are easy for me to pollinate easily myself with a small paintbrush, such as peppers and watermelons.
Temperature and humidity
In Holly's report, she says that optimum fruit set occurs within a very narrow night temperature range of between 60° F and 70° F. So, 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. AHA!! Also, 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. My guess is that by keeping my plants covered day and night never allowed the temperature to cool down at night from high daytime temperatures and probably held in too much humidity for the pollen to work right.
Watch the Weather
If temperatures are above 80 degrees during the day and are expected to stay high during the night, I will leave the plastic up all night. On the other hand, if daytime temperatures are below 70 degrees and/or it's raining sideways I will leave the plastic down during the day. I have to admit that I'm much more in tune with my local weather during tomato growing season!
I have never experienced this phenomenon but Holly mentions in her report that this may occur in large tomatoes due to a failure of complete fertilization of the ovule. The larger fruit demands more complete fertilization. She adds that this is not a disease but a physiological disorder.
Kennell, Holly (1995) The Gardener. This report is available online through the
Washington State University Extension Service, Gardening in Western Washington--Library--Tomato Pollination