(News article for June 11, 2022; edited)
Gardeners sometimes find that their plants are flowering but not producing fruit. (“Fruit” is used here in the botanical sense and includes the consumed part of vegetable plants such as cucumbers, tomatoes, and snap beans.) This lack of production can occur for a variety of reasons.
Most squash, cucumber, and melon plants (all of which are “cucurbits”) produce separate male and female flowers on the same plant. In some cucurbits, the first flowers that appear are generally male, while in others female flowers tend to develop first. Both must be present for pollination to occur.
Female squash, cucumber, and melon flowers can be identified by the presence of a swollen area under the petals that looks like a miniature fruit. This is the ovary of the flower. Male flowers, on the other hand, have a narrow stem. If only male or only female flowers are present, wait, and the others will likely develop soon.
While many cucurbits have male and female flowers on the same plants, there are exceptions. Some cucumber varieties, like Dasher II, are gynoecious, meaning that they only produce female flowers. When these are grown, another variety must be planted to provide pollen. Likewise, a seeded or diploid watermelon variety must be present to provide pollen for seedless or triploid watermelon plants.
If both female and male flowers are present but no fruits form, a lack of pollination is one possibility. If there are few bees in the area, or if they are just not active, this could be part of the problem.
If you use insecticides, wait until late afternoon or early evening - when bees are no longer active - to apply them. To encourage more bee activity, you can also plant flowering plants like zinnias nearby.
Weather can affect bee activity. Honeybees typically do not fly during rain, so prolonged rainy weather can result in reduced fruit production.
If pollination by bees is insufficient, one option is to hand-pollinate cucurbit plants. You can either touch the pollen-bearing anther of the male flower directly to the stigma of the female flower, or you can use a paintbrush to transfer the pollen. It is a good idea to do this in the morning.
Besides a lack of pollen transfer from male to female flowers, there are other reasons that vegetables sometimes produce flowers but not fruit.
In tomatoes, nighttime temperatures below 55 degrees F or above 75 degrees F can interfere with pollination. Daytime temperatures above 90 degrees F can cause blossoms to drop. If you plant tomatoes after April in Louisiana, choose “heat-set” (heat tolerant) varieties such as Florida 91, Phoenix, or Solar Fire.
High temperatures can also cause blossom drop in beans, and planting snap beans between mid-May and mid-August is not advised. “Yardlong” or asparagus beans, on the other hand, can tolerate high temperatures. These are closely related to the heat-tolerant southern peas (black-eyed, purple hull, crowder, and cream peas).
Excessively dry or wet soil can cause blossom drop in some vegetables, as well. Provide adequate but not excessive water. If soil is poorly drained, consider planting in a raised bed or in containers with drainage holes.
Pollination problems in sweet corn can result from how the planting is arranged. Corn is wind pollinated. For good pollination, plant corn in several rows next to each other rather than one or two long rows.
Let me know if you have questions.
Contact Mary Helen Ferguson.
In watermelon and other cucurbits, pollen must be transferred from male flowers to female flowers for pollination to occur. The ovary of the female watermelon flower (shown here) looks like a miniature watermelon. (Photo by S. Ausmus, USDA Agricultural Research Service)