Did you know that temperature, humidity and plant vibration are all important factors in tomato pollination? If tomato blossoms are not pollinated successfully, there will be few if any tomatoes.
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 and fell off. No fruit was produced from these blossoms. I also noticed this happening with the tomato plants I was growing in my small greenhouse where there is only one vent in the ceiling.
Tomato flowers come complete with both male and female organs and therefore they are considered self-fertilizing. However, that doesn't mean they can do this without help. The pollen grains are released via motion from wind or insects from the anthers (male organs) between 10:00 a.m. and 4:00 p.m. on dry, sunny days. The pollen grains then fall onto the sticky stigma (female organ), in the blossoms, and then pollen tubes form to fertilize the undeveloped seeds. If enough pollen tubes are formed on a given blossom, a fully-formed fruit will result.
If plants are sheltered in a way that prevents breezes and insects from reaching them, tomato blossom pollen cannot be distributed. Adding a fan to the greenhouse, or using your hands to gently vibrate tomato plants at midday when it's warm but hopefully not too humid can assist this phase of pollination.
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.
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.
So if tomato plants are grown under a shelter, as mine are, some consideration needs to be made to optimize the conditions for pollination once blossoms appear on plants.
The mechanics of tomato plant pollination taught me that keeping my plants completely covered day and night all through blossoming kept out the breezes and insects while preventing the nighttime temperature to cool down and probably held in too much humidity which resulted in brown blossoms and virtually no fruit.
My plastic-covered tomato shelters have been re-worked to feature sides that can be rolled up and secured as the weather and blossoming dictate. I keep the sides down at planting time which allows me to plant them a month sooner than is typical in my climate zone. When blossoms appear, I open the sides during the day to let in the wind and insects. In the evening, I check the temperature forecast and bring the sides down if necessary. I have found that around the beginning of June, I can leave the sides up for the summer unless we have a significant temperature drop. That just requires that I keep tabs on the forecast.
My solution works in my maritime climate in the Pacific Northwest, which is not always great for growing tomatoes. However, a very hot and dry climate will offer challenges to pollination that cannot be solved with my methods. You can vibrate the plants to release the pollen but if it's too hot and dry at night, the pollination tubes will not reliably form and you will end up with limited fruit production. I would suggest seeking out plant varieties that are bred to perform best in your type of climate and grow them without any covering. Also check with your local garden experts to learn specific strategies for your area.
Kennell, Holly (1995) The Gardener. Washington State University Extension Service.
Blog: Old Drone. Bees, pollination and more. A simple explanation of tomato pollination, please?