One of the most common questions home fruit growers ask is, "Why won't my plants set fruit?" There are many possible reasons for poor fruit set, including
Pollination and subsequent seed development are prerequisites for fruit set. With most fruits, flowers that appear in early spring begin as buds that form in the axils of the leaves during the previous year. Flowers of many fruits bloom during early spring and can be damaged by frost. If temperatures fall below 30 degrees F when the flowers are vulnerable, some or all may be killed, reducing or eliminating fruit set.
Pollination occurs after the flowers have opened. Some fruits, such as grapes and peaches, shed pollen from their anthers (the male part of the flower), which falls by gravity or is carried by wind currents to the pistil (female part of the flower).
With strawberries, blueberries, apples, plums, and sweet cherries, insects carry the pollen from flower to flower. Heavy rains during bloom can interfere with pollen distribution or insect activity. Seed formation will be poor if pollination is inadequate, and seed formation is essential for the growth and development of most fruits. For example, apples with only a few seeds will fall off the tree in June or remain small and misshapen.
Some fruits, such as strawberries, raspberries, peaches, tart cherries, and grapes, are self-fruitful. Each plant can set fruit with just its own pollen.
Other fruits, such as apples, sweet cherries, pears, plums, apricots, and elderberries, are not self-fruitful. They require cross-pollination from another cultivar for fruit to set. Blueberry plants are self-fruitful, but berry size is larger with cross-pollination from another cultivar. Most nursery catalogs provide information about which cultivars are good for pollinating each other—for blueberries and other fruit crops as well.
Some fruits require cross-pollination from another cultivar.
Certain apple cultivars, such as Jonagold and Rhode Island Greening, produce pollen that is ineffective in setting fruit on other cultivars. To be sure of adequate cross-pollination, plant at least three different apple cultivars.
With groups of sweet cherry cultivars, the pollen of some cultivars is not compatible with others within the group. Yet the cultivar Stella is cross-compatible with most sweet cherry cultivars and provides a good source of pollen for other cultivars. Stella is also self-fruitful.
Many European plums (often called prune plums because of their high sugar content) are partially self-fruitful. But you can improve their fruit set by planting two or more cultivars. You will need to plant two or more cultivars of Asian plums because most are not self-fruitful.
Plant all fruit trees used as "pollenizers" within 100 feet of the cultivar to be pollinated. You may need fewer plants if you rely on neighbors' trees as pollenizers, but you could have a major problem if those trees are destroyed.
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