Why cucurbits and tomatoes fail to produce fruit

If your tomatoes have not set fruit, even though the plants have blooms and are healthy it is because the process of pollination has been interrupted.

My tomatoes are not producing fruit. What am I doing wrong? Many gardeners find that their cucumbers, squash, pumpkins and tomatoes have not set fruit, even though the plants have blooms and the plants are healthy. These plants are not producing fruit because the process of pollination has been interrupted.

Some information about the flower is needed, in order to better understand pollination and fertility of plants. Some plants produce bisexual flowers. These are flowers that are considered “perfect” (male and female part on same plant) flowers that can pollinate themselves. A good example of perfect flowers is the tomato plant.

Other plants develop unisexual flowers or “imperfect” flowers (male or female part on another plant). This group of plants has only one sex per bloom. The female flowers develop fruit and the male flower yields the pollen. Imperfect flowers are the kind found in members of the cucurbit family, such as squash, cucumbers, melons, pumpkin and gourds.

Many gardeners have observed on the cucumbers and squash a bloom forming at the end of the small fruit. They also may have seen other blooms that had no fruit connected. The first is the female flower and the second is the male.

Often cucurbits fail to produce fruit because there are no female flowers present.

Male flowers begin to appear two or three weeks before female flowers. Finally, the female flower, with the tiny fruit attached, begins to appear.

Fruitlessness in cucurbits is a sign of the absence of bees. Unlike the tomato, cucurbits need some agent or carrier to transport the pollen from the female flower. Many insects can carrier the pollen, but it is usually either honeybees or bumblebees.

A good symptom that indicates bees are not doing their job is the cucumber and squash fruit shrivel while young, turning dark brown and dropping off. Improper development of fruit, even though it remains on the vine, is an indication of incomplete or poor pollination.

Female flowers are only susceptible to pollination the morning they are open until the afternoon of that day. Cold, rainy or windy weather that reduces bee activity can cause poor cucumber production due to inadequate pollination.

One method to determine if enough bees are present is to observe several cucumber flowers during the mid-morning and note the number of bee visits. Flowers being observed should average at least one bee visit during a 10-minute period. If fewer visits were noted, more bees would be needed for good pollination.

If the gardener determines that fruit is not setting because of the lack of bee activity, there is another way of pollination called hand pollination.

The technique is easy, although it does require a little knowledge of botany.

On a warm, dry day, use a small, soft brush to transfer the pollen from the anthers of the male flower to the stigmas of the female flower. This probably should be done to several flowers to make sure of good pollination.

Unlike cucurbits, fruitlessness in the tomato is usually the result of defective pollen caused by temperature extremes. Day temperatures above 90 degrees and night temperatures above 70 degrees result in reduced fruit setting. There is considerable evidence that night temperature (59 to 68 degrees) is the critical factor in setting tomato fruit.

Low temperature reduces the production and viability of pollen. High temperature, especially if accompanied by low humidity and moisture, hinders fruit setting through failure in pollination and fertilization.

Fruitlessness or low fruit numbers can be cause by over fertilization of a nitrogen source. When large amounts of nitrogen are applied to these vegetables, the plant grows heavy foliage but little fruit. Light fertilization should be applied at planting and just as the plant start producing fruit.

Jim Coe lives in Lawton.

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