May 3, 2022

Successful pollination key to strawberry, blueberry production

Strawberries are aggregate fruits. They have multiple ovules per receptacle where the fruit is formed. The strawberry receptacle may have up to 500 ovules per berry. You will see these as “seeds” on the outside of the strawberry fruit which are called achenes.

To have the largest berry possible, you need as many of these ovules to be successfully pollinated as possible. To avoid misshapen fruits the achenes need to be pollinated evenly and fully. With pollination, the receptacle tissue around the achenes will develop to form the strawberry fruit.

Strawberries have both male and female flower parts on the same flower and can self-pollinate. Wind and rain can move pollen within the flower. However, this usually does not allow for full pollination of all the ovules. Bees, such as honeybees or bumblebees, are usually necessary to allow for complete pollination. Some flowers produce bigger berries when cross pollinated with pollen from other flowers. Incomplete pollination will often result in smaller or misshapen berries.

Strawberry flowers are not heavy nectar producers. However, bees do visit the flowers and studies have shown that where native bees are limited, adding hives of honeybees or bumblebees increased productivity. It is recommended that each flower receive 16-25 bee visits. This is particularly true of the king berries, which form from the first flower to open on a fruiting truss.

Northern highbush blueberry bushes can produce berries even when there is no or limited pollen movement by bees. Some of the flowers can turn into berries, even if there are poor pollination conditions or low bee activity during bloom. However, often these berries will be small, slow to ripen, and may drop off early. For maximum potential yield, it is important that the flowers are visited by bees during bloom to transfer sufficient pollen while the flower is still viable so that fertilization can occur, leading to seed set, berry expansion, and larger berries.

In addition, some varieties benefit from cross pollination. Fields should be planted with a combination of varieties that bloom around the same time and that are compatible. For cultivars dependent on having cross-pollination for full yields, this can provide a 10-20% increase in yield from the improved fruit set and berry size.

Flowers of blueberries are generally less attractive to honeybees than other flowers due to the relatively low nectar. Because of this, move bees into blueberry fields after 5% bloom but before 25% percent of full bloom to avoid movement to more preferred flowering plants. Placement near to the blueberry field can also help to keep them focused on the crop.

Research has shown variation across northern highbush cultivars in their needs for bee pollination due to the relative attractiveness of different cultivars and their degree of self-compatibility. Experience shows that a minimum of 2 hives per acre are needed. In some cases, five hives per acre are recommended (such as for Jersey and Earliblue). Some growers are using up to eight colonies per acre to ensure good pollination if spring weather is cool and there are only a few good days for honeybee activity. A rule of thumb is that you’ll need four to eight honeybees per bush in the warmest part of the day during bloom to get blueberries pollinated.

Bumblebees are very efficient at pollinating blueberry, with activity at lower temperatures than honey bees, faster visits to flowers, and higher rates of pollen transfer per flower visit. A single visit of a bumble bee to a blueberry flower can deposit sufficient pollen to get full pollination, whereas three visits are needed by honeybees.

Blueberry information was adapted from https://bee-health.extension.org/pollinating-highbush-blueberries/

– Gordon Johnson, Extension Vegetable & Fruit Specialist, University of Delaware