Why do flowers drop off veggie plants?

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Last Updated: 
May 14, 2004

CORVALLIS - Do the flowers of your green beans, cucumbers, melons, squash, tomatoes, eggplant or peppers bloom, and then drop off without setting fruit? Or do they not set bloom at all? Some of the reasons behind blossom drop or too few blossoms on your vegetable plants may include too much heat, lack of pollination, too cold and water stress.

Deborah Kean, researcher at Oregon State University’s Vegetable Research Farm in Corvallis, gives home gardeners some background on blossom drop in major groups of vegetables.

SQUASH – Gardeners tend to notice blossom drop in squash because squash flowers are so huge and noticeable. Squash have separate male and female flowers. The males always drop off after they shed their pollen. Female flowers may fall off due to 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.

Sometimes female flowers will bloom before there are any male flowers out.

"Typically, a few female flowers will bloom before there are any male flowers, so, of course, they don’t get pollinated," explained Kean. "Because female flowers already have what looks like a baby fruit on the end (inferior ovary, as botanists call it), people think the fruit is aborting when, in fact, it is just unpollinated flowers drying up and falling off."

If you want to help Mother Nature along, you can hand pollinate squash by removing a male flower, then rub the pollen producing anthers (top of the stamen) onto the stigma of the female flower.

TOMATOES AND PEPPERS, 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, even in Tucson, according to information from the University of Arizona.

Blossoms drop off if your tomato, pepper or eggplant plants are water stressed. Deep and infrequent watering is best. Also, make sure your plants are getting enough sun. If they receive less than 6 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 and 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 (e.g. under watering) can cause beans and peas to drop their flowers. Snap beans produce way more blossoms than they can set pods, because bean breeders have selected for concentrated pod set.

Sometimes, during a heat wave, beans will drop blossoms, sometimes resulting in what growers call a "split set," or two distinct crops of beans in one set, explained Kean.

"This drives vegetable processors crazy, as they do a one time harvest," she said. "But this may not bother home gardeners."

CUCUMBERS, MELONS – Cooler weather keeps pollinators from flying. If female flowers are not pollinated, they will abort. Male flowers always drop, as they don’t form the fruit; they 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. And remember, some flower drop on these garden plants is normal. Plants generally produce way more flowers than the number of fruits than they can support.

Author: Carol Savonen
Source: Deborah Kean