Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Several growers have commented that fruit size in strawberries is smaller than normal in 2017. This poses the question “what affects strawberry fruit size?”
In plasticulture strawberries, one critical factor with varieties such as Chandler is the number of branch crowns that develop in the fall. Early planting or extended warm weather in the fall may cause plants to produce excess crowns leading to too many buds, flowers and fruits per plant in the spring and, consequently, small berries. This is also a common problem with carry-over plasticulture strawberries where crown thinning was not done or was inadequate.
Another cause of smaller sized strawberries is related to pollination. Strawberries are aggregate fruits. That is, 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. 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 honey bees or bumblebees are usually necessary to allow for complete pollination. Some flowers actually produce bigger berries with cross pollination 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 honey bees or bumble bees 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.
This additional pollination by insects is limited when row covers are placed over fields for extended periods during flowering, by poor weather for honey bee flights (rainy, windy, cold), or by other actions affecting pollinator performance.