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Dr. Terence Robinson speaks at the 2013 Washington Hort Show. (File photo, TJ Mullinax/Good Fruit Grower)

Dr. Terence Robinson speaks at the 2013 Washington Hort Show. (File photo, TJ Mullinax/Good Fruit Grower)

Because of Honeycrisp’s tendency to fruit prolifically, growers are advised to start adjusting the crop load during winter pruning.

“This variety has too many flower buds in the on year,” Dr. Terence Robinson told participants at a Washington State University Honeycrisp Fruit School last December.

Anything that can be done to reduce the bud load will help avoid biennial bearing and improve fruit quality, he said. When the crop load is too high, the tree cannot pump enough carbohydrates into the fruit, and it will lack color and the ability to resist storage rots.

“Fruit from heavily cropped Honeycrisp trees are definitely smaller, greener, and poor tasting,” Robinson said. “You’d think if you left them on the tree they’d eventually get ripe, but they don’t have enough resources put into them, so delaying harvest does nothing. What you get is crummy little fruit that goes into the bin without any quality. Crop load management has to start with pruning.”

Robinson has developed a four-step precision pruning concept:

1 Do rough pruning, removing the largest two or three limbs from each tree, leaving the smaller-diameter fruiting wood.

2 Estimate the number of buds.

3 Decide how many fruit you want. Five to six fruit per square centimeter of trunk cross-sectional area would be a moderate crop, and Robinson does not recommend more than eight. He has developed a plastic gauge that fits around the trunk and shows the appropriate fruit number for the tree size. It is available from Valent USA.

4 Use detail pruning to reduce the number of spurs. In New York, where there’s the risk of bud damage from spring frosts, growers might want to leave 1.5 times the target fruit number, but some people take a more aggressive approach and prune down to the actual target fruit number, he said.

The closer you can prune to the actual number of fruit you want, the better the physiological response of the tree.


Robinson said he’s found that Honeycrisp is easy to chemically thin when the trees are young and still have some vigor, and can be overthinned with MaxCel (6-benzyladenine).

But the variety becomes harder to thin as the trees mature. Even after aggressively pruning to reduce the number of buds, growers might still need to apply successive chemical thinning sprays.

For example, supposing the grower wants 100 fruit per tree. Robinson would recommend pruning the tree down to 150 buds, each of which have the potential to produce five flowers, making 750 potential fruit.

He would then recommend a bloom thinner of ATS (ammonium thiosulfate) or lime sulfur and fish oil, with the goal of having 300 flowers remaining. A petal fall spray of NAA (naphthaleneacetic acid) plus Sevin (carbaryl) might reduce the flower number to 200.

With another NAA and Sevin application when the fruit are 10 to 12 millimeters in diameter, the number could be reduced to about 120. It would then take just a little touchup hand thinning to remove the remaining 20 fruit.

To promote return bloom, he recommends four summer sprays of NAA every ten days, but avoids applying it after July 10 for fear of increasing fruit drop.

Robinson said some rootstocks, such as Budagovsky 9 and Geneva 41, promote return bloom. “Over the course of ten years, it’s made a huge different in yields by not having off years,” he said.

Plan ahead

Dan Griffith, horticulturist with G.S. Long Company in Yakima, Washington, emphasized the importance of planning ahead. He starts working with growers in January through March to plan their chemical thinning programs.

Otherwise, he said, decisions are made too quickly in April when the grower might be sleep deprived from fighting frost.

He asks growers for their spray records, production figures, and hand-thinning costs for each block for the past three years so he can understand what the thinning program has been, what the grower has been spending, and what’s been working and what hasn’t. He enters the information into a spreadsheet.

“It’s a template that you can modify and tweak in the spring when you’re looking at bloom so you can make a ­better, informed decision,” he said.

Griffith finds bloom sprays to be more effective in promoting return bloom than the petal fall sprays.

For Honeycrisp, he recommends two to three applications of lime sulfur during bloom, followed by NAA, Sevin, and Ethrel (ethephon) at petal fall and again when the fruitlets are 10 to 15 millimeters in diameter. He also recommends an Ethrel application 45 to 50 days after bloom to promote return bloom.

Kevin Larson of Roche Fruit, Yakima, said he has no pollinizers in his Honeycrisp blocks, which makes them easier to thin, and the fruit still has good seed counts. He applies lime sulfur two to three times as a bloom thinner.

High stakes

Tory Schmidt, research associate with the Washington Tree Fruit Research Commission, said Honeycrisp is no harder to chemically thin than Red Delicious or Golden Delicious, though not as easy to thin as Braeburn or Granny Smith.

However, the stakes in terms of not thinning the crop load properly are much higher in Honeycrisp.

In trials, bloom-time applications of lime sulfur or fish oil and lime sulfur clearly improved fruit size and fruit quality.

Schmidt reported on trials he has done, using GA3 (gibberellins) to try to break the biennial bearing cycle. The concept is that if bloom is light this year, it is likely to be heavy next year, and GA3 can be applied to inhibit next year’s bloom.

With Fuji, regardless of this year’s crop load, there was a nice reduction in the following year’s bloom from the GA3 treatment. But with Honeycrisp, even when all the flowers were removed, there was no response from the GA3 at all the following year.

“What that suggests is that Honeycrisp might be even more stubborn than Fuji in terms of the biennial bearing cycle and we might have an even tougher time trying to manage it with growth regulators than we would other varieties,” he said. •

(The photo caption above was modified to correct the spelling of Dr. Terence Robinson’s name.)