Developing seeds inside apples send strong signals to apple shoots telling them not to set flower buds for the next year. Thinning reduces this signal.

Developing seeds inside apples send strong signals to apple shoots telling them not to set flower buds for the next year. Thinning reduces this signal.

Photo courtesy of Steven McArtney

Biennial bearers are the bane of apple growers, but the industry is growing more of them. Many newer varieties, like Honeycrisp, Pacific Rose, Fuji, Cameo, and Braeburn, are prone to alternate light and heavy cropping.

Biennial bearing can have a pronounced negative economic effect on orchard productivity and fruit quality, says Dr. Steven McArtney, southeast apple specialist at North Carolina State University. Cumulative yields are lower, and fruit on light-cropping trees often develops calcium-related storage disorders like bitter pit.

Even the less-prone varieties can be shocked into biennial bearing patterns by such events as major freezes that eliminate apples for a year. “Frost damage is a tremendous trigger for biennial bearing,” he said.

McArtney has made a career of searching for ways to settle biennial-tending varieties into annual bearing patterns. He’s been the apple specialist for the four southeastern states of North and South Carolina, Georgia, and Tennessee since 2003, after spending 16 years with HortResearch in New Zealand.

He described Pacific Rose as “the beast that got me started. This is a great apple to eat, but a terrible variety to grow because of biennial bearing. When I was at Hort­Research in New Zealand, Pacific Rose was referred to by many in the industry as ‘Pacific dog’ because of its severe biennial bearing tendency.”

A successful chemical thinning program will often result in returning an orchard to consistent cropping, he said, noting that after chemical thinners came to the market about 1950, the national apple crop showed much less year-to-year size fluctuation.

Studies show that the seeds in developing apples produce gibberellins that suppress bloom the following year by inhibiting flower bud formation. Lots of fruit mean lots of seeds, and that means few flowers the following year.

“Chemical thinners take off a lot of fruit—and also a lot of seeds,” McArtney said. “Thinning takes away the gibberellin signal. Without the signal, you’ll more likely get a flower.”

But with the really sensitive varieties, like Honeycrisp, Pacific Rose, and Fuji, a good chemical thinning job is not enough to guarantee a good return bloom, he said.

The following are the steps apple growers might want to try if they’re hoping to tame a problem orchard’s ­biennial bearing.

Thinning: Do a good job of chemical thinning using combinations of products like Sevin (carbaryl), NAA (naphthalene­acetic acid), and MaxCel (6-benzyladenine).

Summer sprays: Use NAA or Ethrel (ethephon) summer sprays in the heavy cropping year to increase flower bud formation the next year.

NAA may be applied in four applications of 5 parts per million two weeks apart in June and July, or it may be applied in four weekly sprays starting one month before harvest. The preharvest spray somehow changes vegetative buds to flower buds, even after the usual period when the tree makes these decisions, McArtney said.

If Ethrel is used, it should be applied once six weeks after bloom. The rate of Ethrel depends upon the variety, he said. Severely biennial varieties like Fuji might need rates as high as 72 ounces per acre, while varieties that are only slightly biennial will only need 16 ounces per acre.  A rate of 48 ounces per acre is recommended for Golden Delicious. Ethrel should not be used on Honeycrisp as it might worsen preharvest drop.

McArtney finds NAA easier to use in the orchard because it is one rate for all varieties and can be included with summer cover sprays.

Bud analysis: Dissect some buds after harvest and see how many are vegetative and how many will throw blooms next spring. What you find will help you decide how heavily to prune next winter. When cut open, the meristem appears either flat or domed—and the domes are the first sign of next year’s flower buds.

“If your orchard is a mixture of heavy- and light-­cropping trees within a block, then you might want to delay pruning until you can see what you’ve got,” he said. “Don’t bother pruning the light-­flowering trees, and prune the heavy-flowering trees more aggressively.”

Hand thinning: Follow up with hand thinning where necessary, setting a target number of fruit per tree and taking off the smallest fruit, and paying no attention to fruit spacing.

McArtney spoke to growers on this topic during the Mid-Atlantic Fruit and Vegetable Convention in Hershey, Pennsylvania, last winter.