Gibberellin to enhance blueberry fruitset

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.

Compared to most fruit crops, a high percentage of blueberry flowers normally produce fruit. When bees are numerous and weather is warm and calm, 80-95 % of 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 3-5 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 % 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 the cost of 80 g of GA is over $100, it is important to know when to use GA. If weather has been reasonable good 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 is enough to provide adequate pollination. Consistently cold, rainy and/or windy weather through bloom causes pollination problems. If the corollas stay on the bushes longer than usual and turn red/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. A key to learning about GA benefits is to leave non-treated check rows. This is the only way to tell if your money was well spent.

2006 Trial. Because weather during bloom was poor, we treated Jersey bushes at the Southwest Michigan Research and Extension Center with ProGibb applied twice at 40 g a.i./acre (17, 27 May) or applied once (17 May) to a single application of 80 g/acre on. On 17 May, 30% of Jersey petals had fallen. Treatments were applied in 50 gallons spray per acre.

Results were fairly typical for years when there is a response to ProGibb. The 80 g rate applied once increased yield by about 30% over control plots, and there was a trend towards a response from the 40 g treatment as well (Table 1). The higher yield appeared to result from more berries rather than an increase in average berry weight. During the week prior to the first ProGibb spray, measurable rain occurred every day, and maximum daily temperature exceeded 60oF only on one day. These are the type of conditions when growers are likely to see benefits from ProGibb.

Table 3. Effect of ProGibb applications of average berry weight and yield of ‘Jersey’ blueberries, SWMREC, 2006.


Average berry weight (g)

Yield (lb/bush)

1st pick

2nd pick

1st pick

2nd pick



1.22 a

0.78 a

4.1 a

0.9 a

5.0 a

ProGibb 40 g twice

1.18 a

0.82 a

5.0 a

1.1 a

6.1 ab

ProGibb 80 g once

1.27 a

0.85 a

5.0 a

 1.5 b

6.5 b

Dr. Hanson's work is funded in part by MSU's AgBioResearch.

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