Pollination is the transfer of pollen from the anther to the stigma inside the flower. After pollination and fertilization, fruit set occurs. On many tree and small fruits, pollen is transferred from flower to flower or plant to plant by an insect pollinator, like bees.
There are two types of pollination. Self-pollination occurs when the pollen is transferred from the anther to the stigma on the same flower, from another flower on the same plant, or from a flower on another plant of the same variety. Self-pollinated plants are said to be self-fruitful.
Many plants cannot produce fruit from their own pollen and are considered self-unfruitful. These plants require cross-pollination for fruit set. Cross-pollination is the transfer of pollen from one plant to the flower of a genetically different plant or variety.
Pollinators play an important role in pollination for both self-fruitful and self-unfruitful plants. For self-unfruitful plants, the transfer of pollen by an insect pollinator is essential. Without it, no fruit is produced. This is especially true for fruit trees, although wind or other factors can also aid cross-pollination in some other plant species. The transfer of pollen from flower to flower in a self-fruitful tree also often relies on bees and other pollinators. Additionally, many self-fruitful plants will set fruit better when cross-pollinated.
Pollination is an important factor when selecting and planting tree and small fruits. A list of pollination requirements for the various fruits is presented below. Home gardeners should consider these fruiting requirements when browsing garden centers or leafing through garden catalogs.
Fruit Trees
Fruit trees that require two different varieties for pollination should be planted within 50 to 100 feet of one another to ensure a good fruit set.
Apples
Apples are considered self-unfruitful. Most apple varieties will set a small crop with their own pollen. For maximum production, however, plant at least two different varieties with overlapping bloom periods to ensure cross-pollination and fruit set.
Apple cultivars can be classified as early, mid, and late-season blooming. The bloom periods of early and mid-season bloomers overlap, permitting adequate cross-pollination and fruit set. Good pollination can also be expected with mid and late-blooming varieties. However, the bloom periods of early and late blooming varieties may not overlap, resulting in poor pollination. Most flowering crabapples will pollinate nearby apple trees if they bloom at the same time.
Additional information on blooming times can be found in the following publications and articles: Tree Fruit Pollination, What cultivar of apple should I plant to ensure good pollination?, and Fruit Cultivars for Iowa.
Apricots
Few apricot cultivars are reliably cold hardy in Iowa. ‘Moongold’ and ‘Sungold’ are hardy throughout Iowa and self-unfruitful. Plant at least one of each for proper pollination. ‘Moorpark’ can be successfully grown in central and southern Iowa. ‘Moorpark’ is self-fruitful.
Sour Cherries
Sour or pie cherries are self-fruitful.
Sweet Cherries
Most sweet cherries are not reliably cold hardy in Iowa. Most cultivars are self-unfruitful. ‘Gold,’ BlackGold™, and WhiteGold™ can be successfully grown in central and southern Iowa. ‘Gold’ is self-unfruitful. BlackGold™ and WhiteGold™ are self-fruitful. (BlackGold™ will pollinate ‘Gold.’)
Peaches
Peaches are not reliably cold hardy in much of Iowa. ‘Reliance’ (yellow flesh, freestone), ‘Contender’ (yellow flesh, freestone), and ‘Polly’ (white flesh, clingstone) are most successfully grown in the southern one-third of Iowa. Most peach cultivars are self-fruitful.
Pears
Most European pears are self-unfruitful. Plant at least two different cultivars for maximum fruit production. Asian pear cultivars are partially self-fruitful. Best yields are obtained when two or more cultivars are planted together. Most Asian pears are not as cold-hardy as European pears. Asian pears perform best in the southern half of Iowa.
Plums
Japanese plums are not reliably cold-hardy in Iowa. However, European and hybrid plums can be successfully grown in the state. European plums are partially to entirely self-fruitful. Hybrid plum cultivars (crosses between American and Japanese plums) are self-unfruitful. Plant 2 or more hybrid plum cultivars to ensure cross-pollination and fruit set. European plums will not pollinate hybrid plums and vice versa.
Small Fruits
Blueberries
Most varieties of blueberries are self-fruitful, but plants produce larger and more berries when two or more different varieties are planted.
Currants
Currants are self-fruitful.
Gooseberries
Gooseberries are self-fruitful.
Elderberries
Elderberries are essentially self-unfruitful. Plant two or more varieties to ensure a good fruit set.
Grapes
Grapes are self-fruitful.
Raspberries
Raspberries are self-fruitful.
Strawberries
Strawberries are self-fruitful.