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Pruning
Apple Trees
This booklet is provided, with great
thanks, from author Renae Moran. and the University of Maine Co-Op. There
have been several modifications to this publication to better fit this
website. A complete version can be acquired through the University Co-Op.
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This booklet has a simple pruning lesson to show you how to prune a semidwarf apple tree, the most common tree size in Maine’s orchards. It begins with a discussion of why fruit trees should be pruned annually, and follows with an explanation of tree training or what shape a fruit tree should have. A detailed description of some pruning principles is included to help you understand some of the pruning methods that are used. This lesson finishes with a simplified method of pruning and a step by step description of the steps involved.
Pruning can either improve fruit production or hurt it depending on whether it is done correctly. Too much pruning can invigorate the tree and inhibit flowering. Too little pruning can lead to a dense canopy of shaded unproductive branches. Careful pruning requires a knowledge of what to prune, how much to prune, and the tree’s natural growth habit.
Why Prune Apple Trees?
Pruning correctly improves fruit quality and increases the value of the crop. This is the primary reason for pruning fruit trees. A pruned tree has more apples with a greater degree of red color and larger fruit size. As a result, a greater proportion of the crop meets the minimum size and color requirements for Fancy grades.
Pruned trees are also easier to manage. A pruned tree has a more open canopy allowing sprays to contact all parts, so better disease control is achieved. A pruned tree is also easier to harvest. Pruning reduces tree size and the density of the canopy making it easier to reach the fruit.
Apple trees are full sun plants. They need a lot of light to form fruit buds and to set fruit. Semidwarf trees, because of their large size, can have parts of the canopy that are heavily shaded. A tree casts a shadow on itself, so part of its canopy can be in full shade. Full shade is bad for fruit production. Pruning increases flowering and enhances red coloring because it eliminates some if this shading.
In
the diagram of a pruned and unpruned tree (Figure 1), the part of the tree
that is in black is the part of the tree that does not get enough light to
form fruit buds. The part in gray gets enough for flowering, but fruit
in this section have very little red color. The unpruned tree on the
left has a greater proportion of the canopy not getting enough light.
The unpruned tree, even though it is larger, is not more productive than the
smaller, pruned tree. On a per acre basis, unpruned trees are less
productive because of the space they waste.
Pruning increases fruit size because it reduces the number of fruit buds on the tree. This will have an effect similar to thinning the crop. The top branch in figure 2 has fewer fruit buds compared to the one on the bottom, which has an excess. Too many fruit buds on a tree is similar to overpopulation. The fruit will be starved for energy and stunted even though they are in full sun.
Pruning also increases fruit size because of the replacement of old branches with new ones. It has a renewal effect. Renewal is the removal of old branches and allowing a new one to grow in its place. As a branch ages, it is less capable of sizing fruit. The branch in the lower photo of figure 2 has very little leafy shoot growth. There will not be enough shoot growth to support all the fruit on this branch. Because pruning stimulates the growth of new shoots which will develop new spurs, it leads to production of larger fruit.
Pruning Trees of Different Vigor
There are two different ways to prune a tree. The correct way depends on tree vigor. Since most orchards have semidwarf trees that are high in vigor, they should be pruned to discourage regrowth or the growth of vigorous leafy shoots. This is the same as pruning to reduce shading. When a branch is pruned, new shoots will develop from latent buds in close proximity to the pruning cut. The shoot growth that occurs as a result of pruning is called regrowth. Less common is the need to encourage vigorous growth. This would be done when trees are stunted, and is more likely to occur with spur types and dwarf trees.
Spur type trees differ from standard types in the number of spurs they form. Spurs are simply short shoots. On an apple tree, fruit buds are located at the tips of spurs and the tips of some shoots. In rare cases, fruit buds form laterally on the shoot or along the length of it. Lateral flowering is more common in places with a longer growing season. Spurs stop growing soon after bloom and begin to form a flower bud. Shoots, on the other hand, continue to grow in length for about one month following bloom and longer for more vigorous shoots. The longer they grow, the less likely they are to form a fruit bud.
Semidwarf trees with vigorous shoot growth, as in figure 3, should be pruned to discourage regrowth. The vigorous shoots in the top of the tree cast a shadow on the rest of the tree. Because of this, fruit that occur on the lower limbs will not have enough red color to make the fancy grades.
The dwarf tree in figure 4 does not have shading of the lower limbs. Instead, it is low in vigor and has too many fruit buds. In this case, the tree should be pruned to encourage modest shoot growth.
To accomplish good pruning requires being familiar with tree vigor, in other words, the number and length of the shoots. This will largely be affected by its rootstock, whether it is standard, semidwarf or dwarf. Standard rootstocks produce the largest and most vigorous trees. Dwarf rootstocks result in very small trees that are low in vigor. Semidwarf trees are in between in size.
Tree Training Systems
Good
pruning also requires a knowledge of the training system that was imposed on
the tree. The training system determines the shape of the tree and the
orientation of the limbs. Fruit trees are trained so that the least
amount of shading occurs inside the tree canopy. Pruning and shaping a
fruit tree is not a random process. For apple, there are two commonly
used training systems. The best one to use depends on mature tree size.
The open center training system (Figure 5) is more traditional and is used for very large trees. This system controls tree height, but at the expense of light inside the tree canopy. Full sized open center trees are becoming less and less common.
The central leader training system (Figure 5) is ideal for semidwarf trees. The cone shape of this tree allows more light to reach the inside the canopy. To achieve the cone shape, the upper limbs must be shorter than the lower limbs. They cause heavy shading of the lower limbs when they are too long. Another important aspect of this system is the wide spacing between the upper and lower limbs. Ideally, they should be separated by a space of at least three feet. Spacing them four to five feet is better for larger trees.
The
Two Types of Pruning Cuts
There are two types of pruning cuts, thinning and heading. Thinning is the removal of the entire branch or limb at its base. Heading is the removal of part of the branch or limb. With a heading cut, you leave behind part of the branch. Heading and thinning cuts differ in how they affect subsequent regrowth. In general, thinning cuts result in less regrowth than heading cuts. But, the effect of a heading cut will vary depending on the severity of the cut and age of the wood that is pruned.
A branch has sections that differ in age as shown in figure 6. One-year-old shoots near the tip of the branch are considered “new wood”. They do not have lateral spurs or shoots yet. Instead, they have lateral buds that will develop into spurs or shoots in the following season. “Old wood” is the part of the branch that is two-years-old or older. The two-year-old and older sections have lateral spurs and shoots. The amount and vigor of the regrowth that occurs after pruning will vary with the age of the branch that is
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pruned.
A heading cut into one-year-old shoots causes bushy regrowth (Figure 7). Removal of the shoot tip stimulates the growth of the lateral buds. This type of pruning is rarely done to fruit trees because it creates more shading of the rest of the tree. If left unpruned, the one-year-old shoot develops fruit buds that bear in the following year.
Making a heading cut into the two-year-old section of the branch causes a moderate invigoration of existing shoots and spurs, rather than the development of new shoots. This type of pruning is done to shorten a branch as in the shaping of the central leader tree. This type of pruning is also done to shorten limbs when they grow too far into neighboring trees or into the tractor alley.
With a heading cut made in the three-year-old or older section, there is more vigor in the subsequent growth of the remaining spurs and shoots. This is a more severe cut than heading two-year-old wood. This type of pruning is done to renew or replace an old branch. The new branch that takes its place will develop spurs that are more capable of bearing large-sized fruit than older spurs.
There is a big difference between heading and thinning on the subsequent regrowth of one-year-old shoots. In the case of heading, many new shoots develop with few fruit buds forming. This type of growth will shade the rest of the tree. When shoots are removed with a thinning cut, there is less regrowth and less shading.
In pruning an old branch, both heading and thinning cuts are frequently done. Thinning cuts are useful for removing drooping branches and water sprouts. Branches that droop cause a lot of shading of the fruit below them. Heading cuts are useful for removing the end of the limb if it is drooping or if the limb has become too long. Head back the limb to just above a side branch or spur so that a stub is not left at the tip.
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Even with thinning cuts, there can be new shoots that grow back. This is because there are buds at the base of each shoot that grow into new shoots after pruning. Leaving a small stub encourages this kind of regrowth. So make clean cuts, leaving behind only the collar of the shoot or limb. The collar is the ring of ridges that encircle the base of the shoot.
Thinning and heading cuts can be done to encourage growth. The unpruned branch in figure 11 will not have enough shoot growth to support all the fruits that these spurs will bear. On the average, there was less than an inch of growth on each spur. Six to 12 inches of shoot growth is needed for good fruit size. This condition is brought about by lack of spur removal and lack of limb renewal. Growth can be encouraged by eliminating some of the spurs. The remaining spurs will be invigorated and hopefully some will have more than an inch of growth. The pruned branch in figure 11 has far fewer fruit buds. But this was accomplished with many thinning cuts, which is very time consuming. Since spur pruning is very labor intensive, another option is to make a severe heading cut in this limb. Severe heading cuts will reduce yield because of the loss of bearing limbs, but they take a lot less time.
Experienced pruners know how to prune a tree with the least number of cuts. Pruning is time consuming and expensive, so prune out as many spurs as possible with the least amount of cuts to save money.
Recognizing
Fruit Buds
Besides knowing what types of pruning cuts to make, an expert pruner can distinguish fruit buds from leafy buds. Fruit buds are easily recognized by their shape. They are larger and more rounded than leafy buds as shown in figure 12. The tips of spurs and short shoots are where they can be found. Rarely are they located laterally on a shoot. Leafy buds are smaller and triangular shaped. They are located laterally on shoots and spurs in addition to shoot tips.
Figure 12. Fruit buds are located at the tips of spurs. They are larger and more rounded than leafy buds.
The
Growth Habits of Apple Trees
Apple varieties vary in growth habit, or more specifically where they bear fruit buds. There are three general growth habits, and these are spur types, tip bearers and the intermediate type. To a small extent, the growth habit will affect how a tree should be pruned.
Spur types form most of their fruit buds at the tips of spurs (Figure 14). Most strains of Delicious have a spur type growth habit. A spur is simply a shoot that grew less than two inches. Since spurs occur on the part of the limb that is two-years-old or older, most of the fruit bearing surface is on older wood which is located on the inside of the tree canopy where shading occurs. They also have an upright growth habit. Vigorous shoots will be oriented vertically which can also cause some shading. As a result, this type can have excessive shading of the fruit. When pruning a spur type, keep the top of the canopy thinned out. Tall limbs can be headed back to weak shoots.
When tree vigor is low, this type can have too many spurs and will become stunted or spur bound if not pruned annually. A combination of spur pruning and limb renewal will maintain the balance between spur and shoot growth. Remove some spurs every year to encourage some shoot growth, and head back older limbs to encourage the growth of renewal shoots.
With tip bearing types, most of the fruit buds occur at the tips of shoots so the bearing surface will tend to be mostly on the periphery of the canopy. Tip bearers form few or no spurs. Cortland is a tip bearer. Because fruit occur at the tips of shoots, the tree has a drooping growth habit.
When pruning a tip bearer, pay attention to how many fruit buds are removed since it is easy to prune out too many with this type. There is also a tendency for excessive shading created by limbs that droop. These should to be headed back to a horizontal shoot. Unfortunately, heading back limbs can result in a lot of fruit removal, so limit the number of heading cuts to those that eliminate the most shading.
The intermediate type is balanced in growth habit. These trees form fruit buds on both spurs and shoot tips. Honeycrisp and McIntosh have an intermediate growth habit. With these varieties, the tree tends to have a spreading habit rather than upright or drooping. Since this type is balanced in growth, there are no special considerations. Prune to discourage vigorous growth and to reduce shading.
Knowledge of the underlying principles is essential to predicting what happens to a tree when it is pruned. A tree can be pruned to encourage growth or to discourage growth. In doing either, both heading and thinning cuts will be involved. The severity of the pruning cuts and the number of fruit buds removed will be much less when pruning to discourage growth. When pruning to discourage growth, it is important to retain as many fruit buds as possible. A lightly cropped tree is likely to have more vigor than a heavily cropped tree. In trying to encourage growth, eliminating fruit buds is the key. So, knowing what they look like becomes essential to good pruning. But in each case, getting the best fruit quality is the ultimate goal.
A
Method for Pruning a Semidwarf Tree
This last section provides guidance for pruning central leader apple trees. Pruning open center, pear and stone fruit trees will vary somewhat.
When pruning trees, it helps to break up the job into simplified steps. This method has five steps to pruning a central leader tree. Start by making the large cuts first and finish with any detailed pruning that may be needed. Removing major limbs first will make it easier to see what else need to be pruned.
1. Prune out dead and dying limbs. These are recognized by their lack of new growth and by their lighter color.
2. Select the leader and prune other limbs that compete with it. These are branches that grow in close proximity to the leader. When the leader becomes too tall to harvest, shorten it with a head cut into two-year-old wood. If the leader bends over with the weight of the fruit, select another shoot and thin out any in close proximity to it.
3. Remove limbs that cause too much shading. The upper limbs should be at least three feet above the lower ones on semidwarf trees. On dwarf trees, limbs can be spaced closer together. There should be no more than five lower limbs in a vigorous central leader tree. In some cases, a tree will have the right number of limbs, so none should be removed.
4. Maintain the cone shape. The essential shape of a central leader is easily lost when the upper limbs become as long as the lower ones. These upper limbs should be headed back so that they are shorter. A rule of thumb is to head back the upper limbs so that they are about 2/3 the length of the lower limbs.
5. Finish with the detailed pruning. Remove shoots that are not productive and that cause shading. These are water sprouts and shoots that droop. In many cases, the top of the canopy has too many vigorous upright shoots. Some of these should be removed with thinning cuts.
In figure 17a, a drawing of an unpruned tree, there are two branches that are growing like a leader. There should only be one since these branches cause of a lot of shading. One of them should be removed and it is usually the more vigorous of the two. The remaining leader can be headed back into older wood if it is too tall.
The next step is to remove excess limbs. In figure 17b, there are two. Both are located too close to the limbs below them so that the lower limbs are being shaded. These excess limbs are also being shaded by limbs directly above them. Removing them will allow more light to reach the limbs that remain. Excess limbs should be removed with a clean thinning cut. In the following year, there may be shoots that grow back from these cuts. These should be thinned out. With good annual pruning, there may not be any limbs that need to be removed.
The next step is to maintain the cone shape, as shown in figure 17c. This tree has some long limbs in the upper canopy that make the tree more globular than cone shaped. And, they cut out a lot of light that should be going to the lower limbs. These upper limbs should be shortened by making head cuts in old wood, as indicated by the lines. Exactly how far back to prune is not critical as long as these upper limbs are shorter than the lower limbs. It is not necessary to shorten the lower limbs unless they grow into the tractor space or into neighboring trees. When they grow into neighboring trees or into the herbicide zone, they need to be cut back.
The last step is to remove excess shoots (Figure 17d). The ones that should be removed are vigorous water sprouts, vigorous upright shoots in the top of the tree, and drooping branches. At this point, be careful not to prune out too many. Over pruning will cause the tree to rebound with vigorous regrowth in the following season.
It is important to know when to quit. Pruning too much invigorates the tree and increases shading. The tree in figure 17e could be considered a moderately to heavily pruned tree. No more pruning should be done to this tree.
Figure 17. Pruning a central leader tree in five steps. a) Maintain one leader. b) Remove excess limbs. c) Maintain the cone shape. d) Remove excess shoots. e) Know when to quit.
The
tree in figure 18 is a heavily pruned tree.
As a result of the severe pruning, this tree may have vigorous
regrowth and large green apples. This
tree has the classic shape of a central leader tree, one leader, a cone shape
and upper limbs that are spaced at least three feet above the lower limbs.
The tree in figure 16 was lightly pruned.
Figure 18. A heavily pruned tree.
The tree in figure 3 is an unpruned tree with vigorous regrowth. This tree has only one leader and does not have an excess number of limbs. The pruning that this tree needs is shortening of the leader, shortening the upper limbs and thinning of excess shoots. There are too many shoots in the top of the tree and just enough on the lower limbs. So a lot of thinning cuts should be done in the top and few in the lower part of the tree.
The tree in figure 19 has very low in vigor, an indication that is could be sick and dying. When vigor is poor all over the tree, the trunk is probably injured or partly dead. Do not spend much time on these trees because once part of the trunk dies, there is nothing you can do to fix the tree. If the low vigor is cause by lack of pruning, some renewal pruning is needed. Start by making severe heading cuts into some of the limbs. If there is no vigorous regrowth the tree is probably sick, and much time was not wasted trying to reinvigorate it. Spur pruning may be too time consuming and should only be done to bring back high value varieties that are in good health.
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Figure 19. A very low vigor tree that may be sick or dying.
The tree in figure 20 lost its leader several years ago. A new shoot, probably a watersprout was retained and allowed to grow as a new central leader.
The tree in figure 21 photo has not been pruned sufficiently for several years. To bring the tree back to its optimum, the job needs to be spread out over a period of three years. No more than one third of the total canopy should be removed in any year to avoid overly invigorating the tree. Removing more than one third of the tree causes too much regrowth, and will create more pruning work. The same five steps for annual pruning can be followed, but are done in different years. The following method is suggested:
Year 1. Reduce tree height. Prune out branches that are too tall for spraying and picking. The tree may no longer have a central leader, but instead could have several as in figure 21. This is a modification of the central leader that can be left as is.
Year 2. Remove excess limbs. In some cases, the right limb to remove may be almost as much as one third of the canopy. If this is so, then remove only one. Thin out regrowth in the tree top.
Year
3. Shorten the upper limbs of the tree. Continue to thin out vigorous regrowth in the top of the
canopy and any shoots the grew back from the point where limbs were removed.
Begin detailed pruning.
Pruning
a Nonbearing Tree
Pruning nonbearing trees is different from pruning mature trees. They should be pruned right after planting if they have no side branches. The tree on the left in figure 22 was unbranched at planting. This tree was headed at 30-36 inches above the ground to encourage the growth of side branches that will become limbs. The tree on the right in figure 22 already had side branches at planting. These were kept and will become the limbs. The leader was headed to two feet above these side branches in order to stimulate more shoot growth since it is a dwarf tree. Semidwarf trees that come with good side branches should not be headed. In the following years, shoots that compete with the leader should be removed. These shoots are oriented vertically and grow in close proximity to the leader. That should be all the pruning that is needed until the tree is approaching full production. Nonbearing trees should be pruned as little as possible since pruning delays fruit production.
Pruning can seem very complicated at first. Understanding the effect that pruning will have on the subsequent growth will eliminate some of this. But, the best teacher is actual experience. Expert pruners learned what they know by pruning a lot of trees.