Q. Three hybrid blueberry bushes I planted several years ago have produced flowers, but no fruit. Why?
A.You probably have a variety of blueberry that requires cross-pollination to bear fruit. You also probably have one of the half-high hybrid blueberries that were bred and introduced by the University of Minnesota. These include 'Northblue,' 'Northcountry,' 'St. Cloud' and the two newest introductions, 'Chippewa' and 'Polaris.' Half-high hybrid blueberries are crosses between the very hardy but small-fruited lowbush blueberries, which grow in northern Minnesota and Wisconsin, and the less hardy but large-fruited highbush blueberries grown for commercial production in states with warmer climates.
These half-high hybrids are great for landscaping as well as for fruit production, but some varieties are not self-fruitful. This means they need to be pollinated by another variety to produce fruit. 'Polaris' and 'St. Cloud' must have another variety nearby for them to bear fruit. 'Chippewa,' 'Northblue' and 'Northcountry' will bear fruit without cross-pollination, but having another variety around may increase their yields, so it's worth putting in a plant or two of another variety.
Blueberries rely on insects, particularly bees, for pollination of their white to pale pink, urn-shaped flowers. If the weather is especially cold, wet or windy when the blueberries are in bloom, the subsequent fruit set may not be great because bees won't fly in poor weather. You can encourage the presence of pollinating bees in your yard by growing a diverse range of plants, including natives. It's also important not to use the pesticide carbaryl (Sevin is a trademark name) on or near plants when bees are present because it will kill them. Commercial blueberry growers rent hives of honeybees to place in their fields to ensure good pollination and fruit production.
If you're interested in growing blueberries or other fruit-producing trees and shrubs, you should know that many of them require cross-pollination. Apples are a prime example. It's also important to know when each apple variety blooms because bloom times must overlap for cross-pollination to occur. If your two varieties both fall in the same category for bloom time, cross-pollination should be no problem. Early and midseason bloomers or mid-season and late bloomers may have enough overlap for sufficient pollination, but an early bloomer and a late bloomer probably won't cross-pollinate.
Some fruit trees that require cross-pollination also require a specific pollinator, not just any other variety of that particular fruit. This often happens because the hybrid varieties have sterile pollen. This means that the tree can bear fruit, if properly pollinated, but it cannot act as a pollinator for other trees. For example, many delicious large-fruited plums -- such as 'Alderman,' 'Superior' and 'Pipestone' -- have sterile pollen and should be planted with or near the plum variety 'Toka,' or the wild American plum (Prunus americana), which is often seen growing along the edges of highways or woods. Several pear varieties also have sterile pollen. 'Gourmet' and 'Luscious' both produce delicious fruit, but can't be used as pollinators. If you had those two pears, you would have to plant a third variety to get fruit from all of them. All of these fruit trees require bees for pollination, so follow the same suggestions as for blueberries.
Some fruit trees and shrubs are self-fertile, while others need cross-pollination.
-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. She spends her spare time gardening, inside or outside, depending on the weather. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.