Manage crop load of young tall spindle apple plantings

Adjust crop load of Honeycrisp and other apple varieties in high density plantings in years two through four to reduce negative impact on canopy vigor and subsequent cropping.

A large number of growers are establishing tall spindle apple plantings at spacings of 3 x 11 or 12 feet. Research has shown these plantings to be extremely productive, which has provided the incentive to give it a try. Michigan State University Extension cautions growers to be aware that developing a narrow profile tall (12-13 feet) tree on dwarfing rootstocks calls for judicious crop load management.

The yield potentials of these trees in the first five years can be impressive. Remember not to get greedy. I know that you may be anxious to begin getting a return on the large investment if you have established one of these plantings. I can remember establishing a vertical axe planting in the 1990s, where in the third growing season I mistakenly cropped the trees at 0.5 to 1 bushel per tree after hand-thinning 50 percent of the crop, thinking I had taken enough off. The Jonagolds in that planting, a strong grower that tends to biennial bear, were over-cropped and cropping in subsequent two years was reduced along with growth. Fortunately, by the sixth leaf, the trees recovered and performed well for the remaining life of the plot. The Galas and Empires in that planting were unaffected as they crop annually with little problem. I remembered my mistake.

These days, growers are planting varieties like Honeycrisp in tall spindle systems on M.9 or B.9 rootstocks. Understand that Honeycrisp is a weak growing variety with high tendency to alternate bear. If you over-crop them in years two or three, don’t expect much of a crop the next year and secondly, expect “stunting” in subsequent years.

The key to productivity in tall spindle plantings is reaching a goal of balanced vegetative growth and cropping. By year four and five, the tops of these trees should be reaching 12-14 feet. Over-cropping in years two and three restricts tree height, but with it also canopy volume which equates to productivity. If your goal is 1,100-1,300 bushels per acre by year five, limit cropping in the earlier years.

Terence Robinson of Cornell University, along with collaborators in the testing project NC 140, found through crop load studies that the optimum crop load ratio in years two through four should be four to six fruits per 1 centimeter squared trunk cross sectional area (trunk measurements at 1 foot height). So basically, you can assess how much crop to leave based on average trunk size. This stands to reason that more crop can be left on more vigorous trees, etc.

Rather than trying to provide growers with the TCSA metric information, I have provided a table converting these values based on trunk diameters in inches. Note that in the table provided, I suggest for Honeycrisp to be in the 4-5 fruit column. Avoid columns headed by 7 or 8 unless your trees are at least six or seven years old or with vigorous canopies. In the table below, a 1-inch diameter trunk tree of Honeycrisp in the second year should have no more that 20-25 fruit.

You can allow cropping in year two at the 4-5 fruit per 1 cm2 TCSA level, which can help settle trees down, especially on vigorous soils. If I can add a little caution, cropping in the lower part of the canopy will have less impact on tree vigor and linear leader development than fruit hanging in the upper portion of the tree.

Recommended crop load associated with trunk diameters of young apple trees in high density systems. Headings for each column are calculated as ratios of numbers of fruit per 1 cm2 TCSA.

Trunk diameter (inches)

Diameter (squared inches)

4

5

6

7

8

0.50

1.3

5

6

8

9

10

0.75

2.8

11

14

17

20

23

1.00

5.1

20

25

30

35

41

1.50

11.4

46

57

68

80

91

2.00

20.3

81

101

122

142

162

2.50

31.7

127

158

190

222

253

3.00

45.6

182

228

273

319

365

3.50

62.0

248

310

372

434

496

4.00

81.0

324

405

486

567

648

* Number of fruits recommended based on research fruits per 1 cm2 TCSA = 0.155 in2 TCSA

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