Note: Pollination requirements are discussed in Varieties for Planting in the Home Garden, linked near the bottom of each crop page, e.g. [PDF] Almond Varieties for Planting in the Home Garden.
With few exceptions, fruit will not form until pollen from male parts are transferred to the female parts of a flower. Without pollination, flowers may bloom abundantly, but will not bear fruit.
Pollination is the transfer of pollen to the stigma from stamens of the same or a different flower. It can be performed by animals, insects, wind, or water, and in the home orchard or garden by hand. The pollen grain on the stigma germinates, grows the pollen tube down to the style and into the embryo sac where fertilization occurs.
Fertilization is the sexual fusion of male and female elements. Fruit set refers to fertilized flowers that have developed fruit. In essence, this is the process following fertilization. Keep in mind, failure can occur in these processes at any point from the formation and development of flowers and viable pollen forward.
Some species of fruit trees have perfect flowers. Both the anthers, which contain pollen, and the pistils, which develop into fruit, are located in the same blossom. Trees that bear fruit from self-pollination or from pollen from another plant of the same kind, or set fruit without pollination are called self-fruitful or self-pollinating or self-fertile.
However, there are many types of fruit trees with perfect flowers that cannot produce fruit from their own pollen. These are self-unfruitful or self-sterile and require pollen from a related cultivar. A plant that will fertilize a self-sterile or self-unfruitful plant is called a pollenizer.
Some species of fruit trees do not fit conveniently into either category and exceptions can usually be found. Many nut trees are monoecious with separate male and female flowers borne on the same plant. Self-unfruitfulness usually results from poor overlap of pollen shed at the time pistils are receptive. Some species (kiwi) are dioecious bearing male and female flowers on separate plants, so that both male and female plants must be present if fruit is desired. Some persimmons (Fuyu, Hachiya) bear only pistillate flowers, but will set fruit (parthenocarpicly) anyway without pollination and bear full crops.
Often, neighbors may have fruit tree varieties that will serve as pollinators. With appropriate bee activity and, if distances are not great, wild trees can also provide a source of pollen.
In planting for pollination, a fruit tree that needs a pollinator needs it close by. A maximum distance of 100 feet is suggested, but the closer the better. Bees that carry pollen are unlikely to fly back and forth if distance between trees is greater.
In addition to planting pollinators, other alternatives exist. Graft in another cultivar that will serve as a pollinator branch, or place a bouquet of flowers from a pollinator in a jug or basket and lodge or hang the container with bouquet in the branches of the self-unfruitful tree. Do this early in the morning when temperatures are fairly low and when bee activity gets underway.
Numerous popular publications such as Sunset books, commercial nursery literature, University and other publications are good references on pollination requirements. Because of the many variables among cultivars, climate and regional conditions, it is difficult to present absolute rules for pollination requirements. In fact, the only rule is the rule that exceptions will be found!
Some cultivars may be self-fruitful or partially self-fruitful when planted alone; in other locales, they may not be. For example, ‘Bartlett’ and ‘Comice’ pears are usually self-fruitful in California, although they are considered self-sterile in the Northwest. In general, self-fruitful types include:
- Some apples, pears, Asian pears, plums
- Most peaches (nectarines), apricots, citrus, figs
- All sour cherries and persimmons
In general, self-sterile or unfruitful types, whether caused by pollen compatibility, the timing of pollen shed, dioeciousness, or other problems, include:
- Many apples
- Sweet cherries
- Nuts (almonds, walnuts, chestnuts, pecans, filberts, etc.)
- Some pears, Asian pears, peaches, apricots and plums