Helpful Tips to Keep Your Blueberries Healthy

by DOUG PHILLIPS

Central Florida growers have recently reported occurrences of undeveloped fruit dropping, in particular on the cultivar ‘Meadowlark.’  Although insect activity such as heavy flower thrip feeding can sometimes damage the female flower organs and result in fruit drop, a more common cause may be poor pollination.

Southern highbush blueberries (SHB) are capable of setting fruit through self-pollination; however, studies have shown that cross-pollination of at least two cultivars has the potential to increase fruit set, decrease time to harvest, and increase viable seed number and fruit size to some degree in many cultivars.  In a 2012-2013 Florida study, cross-pollination generally resulted in similar or greater fruit set than self-pollination, although this varied by cultivar and some cultivars had higher fruit set with self-pollination in some years. ‘Meadowlark’ had lower percentages of fruit set with self-pollination in that study.

Cross-pollination can be facilitated by interplanting with other cultivars, and by increasing pollinator activity.  Current recommendations are to interplant at least two cultivars sharing a similar flowering period. Some growers have planted SHB in solid blocks.  These types of plantings will set fruit but, depending on the cultivar and the level of pollinator activity, may not perform up to their full potential.  Planting a single cultivar in large blocks has not been thoroughly investigated, and therefore is not currently recommended.

Sufficient pollinator activity is essential to attain good fruit set.  Many Florida growers use a honey bee hive density of 2 to 3 hives per acre.  However, when there is competition for the pollinators (such as from wild plants or orange groves in adjacent areas) during the fertile time when a blueberry cultivar is blooming, insufficient pollination may occur.  Weather can also impact the level of pollinator activity. Flight activity of the honey bee begins at temperatures between 53° and 57°F and decreases with an increase in wind speed. If cooler and/or windy weather predominates during the time when flowers are receptive to pollen (around four days for highbush blueberries), there could be a lower level of bee activity and thus a lower level of pollination.  Increasing bee hive density may result in better pollination and fruit set.

An alternative to increasing the number of honey bee hives per acre would be to add bumble bees as an additional pollinator. Bumblebees have been shown to be very effective pollinators of blueberry flowers, thought to be due to their buzz pollination or sonication of the pollen.

In summary, the current recommendations from the University of Florida/IFAS are to interplant SHB with at least two different cultivars with a similar flowering period, and if poor pollination has been observed either increase honey bee hive density to 3-5 hives per acre or consider adding bumblebees as a pollinator.

Doug Phillips

University of Florida/IFAS

Blueberry Extension Coordinator