Like other fruiting garden plants, tomato plants produce flowers before they set fruit. These small yellow blossoms must be pollinated by hand, wind or insects to produce tomatoes. Improper growing conditions or poor handling may cause the flowers of a tomato plant to fall off before fruit set. Good cultural practices encourage the plants to produce large numbers of heavy, quality fruit.
Depending on the type of tomatoes you grow, your plants may flower all at once in late spring or early summer, or continuously throughout the season. Determinate tomato varieties produce just one crop per year, while indeterminate varieties never stop growing and producing. Normal tomato flowers are small, yellow and bell-shaped, with dusty yellow pollen. They appear in clusters and fall off after being fertilised.
All tomato flowers must be pollinated to produce fruit. While many tomatoes are said to be self-pollinating, these plants are actually just self-fertile. They need the help of the wind, insects or a human to produce much fruit. Tomatoes grown indoors or in areas where few insect pollinators are available should be fertilised using an artist's brush or vibrating pollination tool.
After pollination, tomato flowers fall off the plant, leaving behind the fertilised ovule, which swells and turns into a tiny green tomato. Over time, this juvenile tomato grows to full size, then begins to turn red, orange or yellow, depending on tomato variety. Some unusual varieties of tomato plants also produce purple, brown or white fruit.
The wrong cultural conditions can cause tomatoes to lose their flowers before pollination can occur. This phenomenon, called "blossom drop," happens in very high or low temperatures. When daytime temperatures are above 26.7 to 29.4 degrees Celsius or night temperatures rise above 75 degrees, tomato flowers will spontaneously abort without producing fruit. According to the University of Missouri Extension, blossom drop also happens when night temperatures are below 12.8 degrees Celsius, in very humid weather and in very dry weather.
Indeterminate tomato plants sometimes lose their flowers when the plant has already set a large amount of fruit. This is normal and healthy for the plant. If a tomato plant with a large crop of flowers produced fruit from every blossom, the resulting tomatoes would be small and lower in quality. In plants that drop flowers for this reason, normal blooming should resume once you pick the first crop.