By Eric Hanson
Blueberries are in bloom around the state. Gibberellin can improve fruit set when weather prevents good bee activity and pollination.
Gibberellin (GA) is naturally produced by blueberry seeds. In well-pollinated berries, GA is abundant and promotes normal berry growth. Without pollination and seed production, berries abort or do not size fully. When bee activity and pollination are limited by extended periods of cold, rainy, windy weather, GA applications can sometimes increase the average size and number of berries. When GA is beneficial, yield increases are usually in the modest 10-20 percent range. These yield effects are hard to recognize by looking at bushes.
GA products labeled for highbush blueberries, including ProGibb 40%, ProGibb 4%, ProGibb 4LV Plus 5.7% (Valent BioSciences) and several formulations of GibGro 4% (NuFarm Americas), and possibly others. Instructions for different products are similar. Apply GA in a single spray during bloom (80 gram active ingredient per acre) or two 40 g sprays, one during bloom and the second 10 to 14 days later. Higher spray volumes (40 to 100 gallons per acre) may improve coverage and effects. Slow-drying conditions such as at night also increase absorption. Spray water pH needs to be between 4.0 and 8.0.
GA treatments are relatively expensive, so it is important to know when benefits are likely. If weather has been reasonably good for bee activity and the white corollas (petals) fall easily from the bushes, pollination is probably adequate. Keep in mind that blueberries can bloom over a long time, and often only a couple days of good conditions is enough for adequate pollination. Cold, rainy and windy weather through bloom limits pollination. A clue that pollination was inadequate is that some corollas (petals) hang on the bush longer and turn reddish-purple before falling. The corollas of pollinated flowers drop readily while still white. Varieties with fruit set problems (Jersey, Coville, Earliblue, Berkeley, Blueray) are most likely to benefit from GA. Jersey, for example, is relatively unattractive to honeybees, and berry numbers and size are often limited by inadequate pollination. GA does not always provide a benefit and effects can be subtle, so always leave non-treated check rows. This is the only way to tell if your money was well spent.
Work in the southeast U.S. indicates that GA also can help overcome partial freeze damage to Rabbiteye blueberry flowers. The rabbiteye recommendation is to apply 24 to 32 g GA just after a damaging frost event and again in 10 to 18 days. Scott NeSmith at the University of Georgia suggests waiting a few days after the freeze to allow for some pollination to occur. GA tends to suppress the normal pollination process somewhat, so allowing for natural pollination to occur before treating can help set seeded fruit on undamaged flowers. GA appears to trick partly injured flowers into setting berries, but cannot compensate for severe injury. If the ovary is brown, no fruit can grow. This technique has not been studied on northern highbush blueberries, so it is not known whether benefits can be achieved in Michigan. It may be useful to try on partly damaged fields, particularly if pollination conditions have also been poor. If you use GA after a frost event, please leave untreated checks.