Why vegetables drop blossoms

Last Updated: 
July 30, 2010

CORVALLIS, Ore. – Sometimes the large, yellow flowers that squash seem so eager to produce drop off without setting fruit, or other times they don't appear at all. The same can happen with green beans, peas, cucumbers, melons, tomatoes, eggplant, corn or peppers.

Deborah Kean, a researcher at Oregon State University's vegetable research farm in Corvallis, gives some of the reasons for blossom drop.

Squash

Squash have separate male and female flowers. The males always drop off after they shed their pollen, while female flowers may fall off for lack of pollination. Blossom drop is often associated with cool temperatures, as bees and other pollinators aren't out collecting and transferring pollen as much when temperatures are down.

"Typically, a few female flowers will bloom before there are any male flowers, so of course they don't get pollinated," Kean explained. "If you want to help Mother Nature along, you can hand-pollinate squash by removing a male flower, then rubbing the pollen-producing anthers (top of the stamen) onto the stigma of the female flower."

Tomatoes, peppers and eggplants

In areas of Oregon where late spring and summer nights are cooler, blossom drop in tomatoes and peppers can occur. Too much nitrogen and water can stimulate a lot of leafy growth, but few blossoms. Heat can cause blossom drop as well. Tomato blossoms drop off without setting fruit when night temperatures fall below 55 degrees or day temperatures exceed 90 degrees for extended periods.

Cherry tomatoes and the OSU-bred parthenocarpic tomatoes, including Gold Nugget, Oregon Spring, Oregon Star, Siletz and Legend, are the exception, as they will set fruit over a wider temperature range than most large-fruited types. Parthenocarpic and cherry tomatoes will fruit throughout the heat of summer.

Blossoms drop off if tomato, pepper or eggplant plants are water-stressed. Deep and infrequent watering is best. Make sure your plants are getting enough sun. If they receive less than six hours of sunlight a day, they may not bloom. Also, windy conditions can interfere with fruit set, so if you can create windbreaks in the garden, do so.

Pepper blossoms are even more sensitive to temperature fluctuations during pollination. Normal pollination and fruit set don't occur when night temperatures fall below 58 degrees and daytime temperatures rise above 85 degrees. Under these temperature conditions, the blossoms fall off, often before pollination.

Beans and peas

Intense heat (generally higher than 95 degrees) or water stress (under-watering) can cause beans and peas to drop their flowers. Snap beans produce way more blossoms than pods because bean breeders have selected for concentrated pod set.

Cucumbers and melons

Cooler weather keeps pollinators from flying, and if female flowers are not pollinated, they will abort. Male flowers always drop because they only provide the pollen.

Corn

Corn is another vegetable adversely affected by high temperatures. The corn tassel (pollen producing part of the corn flower) is often killed when temperatures are above 100 degrees. Injury to the tassel will prevent development of kernels in the ear. Fortunately for most parts of the state, summer temperatures don't stay above 100 degrees for long.

Vegetable varieties vary in their sensitivity to blossom drop. If you have trouble with one variety, try a different one next year. By planting varieties adapted to the growing conditions in your region, you can avoid many of the blossom-drop and non-pollination problems.

"Some flower drop is normal," Kean said, "Plants generally produce far more flowers than the amount of fruit or vegetables they can support."

Author: Judy Scott
Source: Deborah Kean