Nov 19, 2015

Pruning mature trees in a semi-dwarf apple orchard

This includes the production of “standard” varieties, such as Delicious, Golden Delicious, York, Rome, etc. Productive semi-dwarf blocks can play an important role in maintaining annual cash flow while older, less profitable blocks are being replaced.

Establishing new orchards requires multiple years of planning to obtain the right trees, establish the canopy, come into production, and generate a positive return on investment. While medium density orchards are no longer recommended, existing semi-dwarf orchards will be in use for some time and must be properly managed for optimal yields of marketable fruit.

Purpose of pruning

Pruning is an important management practice for fruit size and quality. Pruning is defined as the cutting away of a portion of a plant for horticultural purposes. In fruit production these purposes include influencing plant growth to modify canopy shape, renewal of the fruit bearing surface, and to improve light and spray distribution within the canopy. In semi-dwarf apple orchards, most pruning is done to improve light distribution to increase fruit quality.

Annual pruning and timing

Pruning for light distribution maintenance is an ongoing and cumulative practice that should be done annually. Healthy trees will produce considerable regrowth in a single season, recreating shade. Pruning trees annually is required to avoid corrective pruning, which can result in severe adjustments to the canopy volume.

Most maintenance pruning should be done while trees are dormant. Dormant pruning allows for better visibility of the branches, increasing the speed and accuracy of the work, and doesn’t compete for limited labor supplies with other seasonal tasks. Delayed dormant pruning, shortly after bloom, removes photosynthetic leaf surface at a time when carbon reserves are at a seasonal low. Pruning at this time has a strong inhibition of vigor, and can be used to help manage excessively vigorous trees.

Pruning can also be done in late autumn after harvest. In late autumn and early winter, the trees are not yet fully acclimated to tolerate cold temperatures. Pruning trees at this time can make them more sensitive to low temperature injury. It is best to wait until the trees have been exposed to freezing temperatures and until the leaves have begun to turn yellow before beginning early dormant pruning. Keep a watchful eye on the long range forecast and suspend pruning when a severe drop in temperature is forecast. Recently pruned trees can be damaged when temperatures suddenly drop 50-60 degrees to 0°F or below. This increased sensitivity is greatest within 48 hours after pruning and gradually declines over a two week period.

The central leader blueprint

Over the past 40 years, the central leader (CL) became the predominant training system for semi-dwarf trees (Figure 1). Central leader training favored good light interception and light distribution, leading to high yields of quality fruit. Two other factors led to the widespread adoption of central leader tree training: it was relatively easy to teach, and was compatible with the natural growth habit of apple tree canopies.

Figure 1. Young Golden Delicious/ M.7 apple tree trained to central leader.

Central leader trees have a permanent tier of scaffold branches starting 24 to 35 inches from the graft union and three or four additional tiers of semi-permanent scaffold branches at three foot intervals going up the leader. Each tier of scaffolds going up the leader should be smaller and shorter than the branches in the previous tier. There should be no more than five scaffolds in the bottom tier, with four being the norm. There should be no more than four scaffolds in the successive tiers, and three is often adequate.

The following are generalized guidelines for pruning to maintain the mature CL apple tree canopy.

Make mostly thinning cuts

A mature semi-dwarf apple tree has greater canopy volume, and more branching than is optimal for fruit color and quality, therefore most pruning cuts should be thinning cuts, which remove the entire branch at its origin (Figure 2). Pruning branches results in a localized stimulation of vegetative regrowth. If a portion of the branch is left, this regrowth is likely to be stronger, because such heading cuts leave buds in close proximity to the localized stimulation.

Figure 2. Thinning cuts remove the entire limb at its origin.

One particularly unproductive form of this cut is the bench cut, in which a large, upright scaffold limb is stubbed back to a flatter side branch on the bottom of the limb (Figure 3). The bench cut is made in the misplaced hope that the tree will conform to the flatter angle of the branch to which it is “benched”.

Figure 3. A bench cut is a stubbing cut made on a vigorous upright limb.

The actual growth response from bench cuts is a flush of several strong shoots near the cut, which often serve to shade the rest of the canopy as effectively as the section of limb that was removed, and without the compensation of the fruits that the original limb would have produced (Figures 4 & 5).

Figure 4. Local invigoration of growth resulting from a bench cut.

Figure 5. Excessive growth from a bench cut to vigorous Gala/ M.7 apple trees after two years.

The second most frequent type of cut is referred to as stubbing back, in which horizontal branches in the upper canopy are shortened to a weaker side branch (Figure 6). This cut is useful to maintain the conic shape and allow sunlight to reach the lower canopy section. Regrowth from this cut is restricted by the flat branch angle and the presence of fruiting spurs on such limbs. At the base of the branch, there is a slight ridge, called the collar. The thinning cut should be made to retain the collar, so as to result in the smallest wound and rapid healing of the cut surface.

Figure 6. Stubbing back can be done on wide angle limbs to maintain the cone shape of canopy.

Make large cuts first

Large upright limbs compete with the leader. If not removed (Figure 7), the cone shape of the canopy is lost, and with that is lost any hope of good light distribution. Remove limbs that are greater than ¹⁄₃ the diameter of the leader. Match the tools to the size cuts that are being made. This ensures that one large cut is made when necessary, as opposed to several smaller insufficient cuts to thin the canopy.

Figure 7. Thin vigorous limbs in the top to maintain conical canopy shape.

Select branches based upon branching angle

Leave branches with an angle between 45 and 65 degrees from vertical to achieve a desirable balance of vegetative growth and fruiting. Upright branches are too vegetative and vigorous, while pendant branches are weak and tend to produce small fruit.

Start in the top

Sunlight first hits the top of the canopy, and then filters down through. The premise of the CL tree is to allow sufficient sunlight to pass through to illuminate the lowest branches. It follows that if the top is over-grown the rest of the canopy will have insufficient sunlight. One way to focus pruning to address the causes of shade is to start pruning operations in the top.

While pruning the upper canopy, adjust the tree height to the desired level by pruning to a weaker side branch. Tree height should be limited to twice the free alley width. If eight feet of free alley is needed to operate equipment, then 16 feet is the maximum height for that orchard. Should the trees be left taller than 16 feet in this example, cross-row shading will result.

Thin out the remaining branches to create an even distribution of light. This entails removal of otherwise acceptable limbs to eliminate dense areas in the canopy.

Pruning severity

Sometimes the need for corrective pruning arises, due to crop failure or management error. Severe pruning can reduce flowering and yield (by 30% or more), and can stimulate excessive vegetative growth and increase bitter pit. If severe corrective pruning cuts are called for, no more than ¹⁄₃ of the canopy volume should be removed in a given year. Initial corrective pruning should focus on making a few large cuts, and the remainder of the tree should be thinned out lightly, if at all.

The vigorous regrowth that follows heavy pruning can be managed with two or more applications of prohexadione calcium (Apogee/ Kudos), at moderate-to-high rates. Root pruning is an alternative technique to get long-term control of vigor from severe limb pruning (call or write for details). Summer pruning can be used to remove some of the vigorous shoots that sprout following heavy dormant pruning. Summer pruning can improve red fruit coloration, but will not correct the invigoration of growth or the reduction in flowering that result from such cuts.

Pruning the upside down tree

Some older blocks of large apple trees may not conform to the central leader shape. Some of these were not trained to CL, but instead formed a multi-leader vase- or globe-shaped canopy. Others began as CL trees but large upright scaffolds were not removed. The leader has lost dominance and the result is an inverted cone, or “upside down” tree (Figure 8).

Figure 8. Upside down canopy shape needs corrective pruning.

Should the grower correct such trees or not? To correct such a block will require severe pruning, which will have negative consequences, as previously described. Suffering a yield reduction in a block that has been retained to maintain cash flow may be counter-productive. On the other hand, renovating the block may restore marketable fruit size and fruit quality in just two or three years. The health of the block, the variety, and its place in the portfolio of the enterprise are factors that weigh on the decision to renovate.

It may not pay to renovate varieties that don’t require 50% red fruit color to sell. Grannie Smith does well in a multi-leader tree, as do varieties destined for processing. Trees that have only a short-term future needn’t be renovated either. For these situations, make thinning cuts to open the canopy up to sunlight and spray penetration and leave the canopy shape in its present state. One strategy for this type of tree is to prune each large scaffold as if it were a leader.

If the decision is made to renovate the orchard, remove about a third of the excess canopy the first year, focusing on making a few large cuts and with an eye toward restoring the cone shape (Figure 9). Plan on using prohexadione calcium to manage excessive vigor, and on summer pruning to reduce shading from regrowth. The process of recovering the cone shape is continued in the second and third years by thinning out and shaping the canopy with large (saw) and medium (lopper) cuts.

Figure 9. Same tree as in Figure 8 with first year corrective pruning. Several smaller limbs have been left for now and will be thinned out next season.

Fruit on trees undergoing renovation should receive multiple calcium chloride sprays and should not be placed in long-term storage until the vigorous response to severe pruning is over. Nitrogen applications to trees being renovated should be sharply restricted unless leaf analysis reveals a deficiency.

Another useful system for renovating upside down trees is to convert them to Central Leader-Palmette. In this system the bottom tier of scaffolds is retained, and all major limbs growing perpendicular to the row direction above the bottom tier are removed. This system provides the pruning crew with a quick and unambiguous rule for which branches to remove. It also results in a more favorable distribution of light by creating a narrow tree wall canopy in the top of the tree.

In conclusion

Prune to thin the canopy with as few cuts as possible, remove hangers and risers, stub back to a cone and don’t get carried away!

– James Schupp, Penn State Extension