MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

Some athletes have been consumed by scandal for taking hormones or other substances to improve their performance. Plants also produce hormones or plant growth regulators that can potentially be used to improve their performance as well. Gibberellins are one group of plant growth regulators that are also synthesized and sold as commercial products (ProGibb, ProVide). In blueberries, gibberellins can increase fruit set or decrease flower numbers, depending on when they are applied.

When bees are numerous and weather is warm and calm, 80-95 percent of blueberry flowers may set fruit. However, cold, rainy weather during bloom restricts honey bee activity and pollination, resulting in lower fruit set and often reduced berry size. Flowers that are not pollinated within three to five days after opening are unlikely to set fruit. After normal pollination, berry growth is dependent on the production of gibberellin and perhaps other growth promoters in the ovary tissues and viable seeds. If flowers are not pollinated, they abort. If only a few ovules are fertilized, the fruit may set, but not contain enough seeds to grow to full size.

When pollination is limited by poor weather, gibberellin (GA) sometimes improves percentage set and berry size. Several GA products (ProGibb, GibGro) are labeled for highbush blueberries. GA may result in retention of some seedless (parthenocarpic) fruit that normally drop, and increases the size of berries with low seed numbers. GA can be applied in a single spray during bloom (80 gram active ingredient per acre) or two 40 g sprays, one during bloom and the second 10-14 days later. Higher spray volumes (40 to 100 gallons per acre) may improve coverage and effects. Slow-drying conditions also increase absorption. Also make sure your spray water pH is not above 7.5.

Since GA is costly, it is important to know when it will help. If weather has been favorable for bee activity and the white corollas fall easily from the bushes, pollination is probably adequate. Keep in mind that blueberries can bloom over a long time, and often only a few days on good conditions are enough for adequate pollination. Consistently cold, rainy or windy weather through bloom causes pollination problems. If the corollas hang on bushes longer than usual and turn red or purple before eventually dropping, pollination may have been inadequate. 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. Make sure to leave non-treated check rows to tell if your money was well spent.

Recent work indicates that GA may have the opposite effect of inhibiting blueberry flower bud formation. ProGibb has been commercially used to limit flowering on sour cherry for some time. Inhibiting blueberry flowering would be of great value in establishing new plants. We now recommend pruning or stripping off flower buds during the first two growing seasons to encourage vegetative growth and greater fruiting in subsequent years. This is particularly useful on more precocious varieties like Elliott. Manual flower bud removal is not always accomplished because it is very time consuming.
In earlier work on potted blueberries, we observed only a slight inhibition of flowering when plants were treated with ProGibb in June or July, when we guessed flower initiation was occuring. However, Brent Black at Utah State University recently reported that flower initiation on ‘Bluecrop’ was reduced by nearly 90 percent by treating plants in August or September with ProVide (GA4+7). This suggested that flower bud initiation or differentiation occurs later in the season. In 2006 through 2007, we tested ProVide and ProGibb on the late varieties ‘Elliott’, ‘Liberty,’ ‘Aurora.’ Timings ranged from early July to mid-October. The only timing that reduced flower bud numbers was the middle timing (August 27 plus September 3), but reductions were small. No differences between the two GA products were observed.

These results do not clarify the important question of when blueberry flower bud initiation and differentiation occurs. If we knew this, we could better time sprays for best results. Additional work is underway that will hopefully assist us in developing a useful treatment to reduce flowering on young bushes.