Pruning is an essential practice to achieve consistent production of high-quality fruit. When trees are pruned, their natural growth and fruiting habits can be altered. Left unpruned, seedling apple trees can grow 40 feet high and bear little fruit. With strategic pruning, however, one can manipulate growth and maintain trees to a manageable shape and height, reduce pest and disease incidence and improve productivity and fruit quality.
Why prune?
—Achieve sustainable production of high-quality fruit.
—Manipulate tree vigor: where and how much vegetative growth.
—Reduce pest and disease pressure.
—Improve light interception and distribution.
Pruning is a subjective science, and good pruning requires practice and experience. Pruning styles and goals will differ among growers, depending on key factors that include cultivar, rootstock, training system, vigor of the site and fruit yield and quality goals.
Here, we cover basic concepts and techniques, general to all apple trees.
Identify fruiting wood
To manage fruit quantity and quality with pruning, it is fundamental to understand where fruiting sites are found. In apple, reproductive buds (future flower clusters) are found in both terminal and lateral (axillary) positions. These buds form the year before flowering and fruiting. Buds are either vegetative, reproductive or mixed (containing both vegetative and reproductive meristems), and it is often difficult to distinguish among these bud types by visual inspection alone.
Several factors determine whether a bud is vegetative or reproductive, including light, tree vigor and fruit density. Floral buds can be in a terminal position (apical fruiting bud) on 1-year-old shoots or on spurs located in shoots 2 years old or older. The best quality fruit tends to come from young spurs. This is one reason growers will constantly renew wood and spurs with pruning. Vegetative buds will develop into extension shoots or spurs (shoots with short internodes) and are located in both terminal and lateral positions. There are other buds, called adventitious buds, that are hidden in the wood and can break if induced by pruning or scoring.
Well-watered, unstressed apple trees with no light or nutrient limitations tend to produce excessive vegetative growth. Mild stress can shift the balance toward fruiting, but excessive stress can reduce fruit quality — resulting in small fruit with poor color — and induce biennial bearing. With strategic pruning, growers can achieve a balance between vegetative and fruiting wood to maximize production of high-quality fruit year after year.
Apical dominance
Several factors regulate tree growth and development, and the influence of internal hormone balance is key among them. The signaling for a bud to break and start to grow is influenced strongly by a balance between auxin and cytokinin, two classes of plant hormones.
Auxins are produced in the apex, or tip, of growing shoots. These hormones are distributed basipetally — by gravity to the tree below. High levels of auxin will inhibit lateral bud break, leading to apical dominance characterized by strong upright growth. Given that auxin moves down with gravity, upright shoots exhibit more inhibition of lateral bud break than horizontal shoots.
Cytokinins, by contrast, are produced in the roots and move throughout the tree via the vascular system. The ratio between these two hormone types is critical in determining whether buds will break.
When limbs are pruned with heading cuts, the source of auxin is removed. This reduces the ratio of auxin to cytokinin and typically leads to bud break in lateral, axillary buds.
Similarly, when a branch is bent away from vertical, broken or is naturally in a horizontal position, the inhibition will be localized to a shorter area and in the base of the shoots, and upper buds will then break.
Understanding this simple concept will help predict the tree’s response to pruning and expected differences between vigorous upright shoots versus horizontal, less vigorous branches.
Types of pruning cuts
There are a few different types of pruning cuts — classified by where the cut is made. The tree’s response to each type of cut will vary, and their use will depend on the goal, the training system, the vigor of the block and the preference of the orchardist. The two main types of pruning cuts are thinning and heading.
—Thinning cuts: These are cuts made close to the point of origin and designed to remove the entire shoot or branch. Thinning is used when you want to minimize the potential for regrowth. Generally, thinning cuts are made when there are too many branches, crowded areas, or the tree is too vigorous. Thinning cuts can improve light distribution in a canopy. They are also used to remove diseased or damaged limbs.
—Heading cuts: These are cuts made at some point along the length of a branch, leaving a section of the pruned limb on the tree. These can be made to direct future growth when used to prune back to a specific bud or lateral shoot. Heading cuts are often done to manage crop load of a branch, induce lateral growth to fill space, prevent hanging fruit and promote light interception.
——Tipping cuts: Within heading cuts there is a distinction between general heading cuts and tipping — a heading cut that is made within a few inches of the terminal end of the shoot. These cuts generally promote growth of the buds immediately below the cut via a reduction in apical dominance. If the cut is done to an existing lateral or spur, there is generally no regrowth and the existing shoot becomes more vigorous.
——Renewal cuts: These cuts may be considered a type of heading cut, but shorter — between 1 and 2 inches from the origin. These cuts are designed to promote regrowth and ensure renewal. Generally, there are adventitious buds that will break, leading to a new growth, or one can cut back to a live basal bud.
Quick considerations for pruning an apple tree:
Start by identifying where the fruit and fruiting buds are in your trees, as these may vary among cultivars. For example, in WA 38 apple trees, a tip-bearing type of tree, most of the fruit grows on the outside of the tree, in terminal shoots and young spurs. Then:
—Thin out the vigorous vertical wood, wood in the wrong position or where there are too many branches.
—Use heading cuts to balance the number of spurs per branch and to maintain strong wood that can support the fruit load.
—Use tipping cuts to induce growth below the point of the cut, or on branches that are too long (more than 12 inches) and those that will bend and hang if there is a fruit in the terminal bud.
—Do renewal cuts (short stubs) to renew older wood in areas where a branch is becoming too vigorous (has a diameter similar to the main leader).
Timing of pruning
Pruning is most commonly accomplished during the winter, but pruning in spring, summer or fall can be beneficial and can achieve different goals. Trees store carbohydrates and nutrients in their wood and root systems during the winter. When temperatures start to rise in spring, carbohydrates and nutrients will be remobilized to new growing points: the buds. When trees are pruned in the winter, the number of potential growing points is reduced, thus more of the stored energy is available to the remaining buds. Consequently, winter pruning promotes vigor and vegetative growth.
In contrast, spring pruning — until about 4–6 weeks after bloom in apples — removes already remobilized reserves, reducing vigor. When the growth stops during the summer, the carbohydrates will be distributed to fruit and new root growth, starting to build the reserves for the following year.
The further we go into the summer, the greater the impact on vigor from removing the source of the current season’s energy. The regrowth promoted by summer pruning is less vigorous than the original branch. Summer or fall pruning can enhance fruit color development while reducing vigor for the following season. Summer pruning is generally recommended in orchards that are excessively vigorous.
—by Bernardita Sallato and Matthew Whiting
Bernardita Sallato is a tree fruit extension specialist and Matthew Whiting is a professor of tree fruit physiology, both at Washington State University. Sallato can be reached at: b.sallato@wsu.edu.
Watch: A new series on apple pruning basics and crop load management for WA 38 is available in English and Spanish from Washington State University Extension specialist Bernardita Sallato.
In Spanish/en español
Good Fruit Grower y la Universidad Estatal de Washington se han asociado para ofrecer dos vídeos, disponibles en inglés y español, sobre como podar y la administración de la carga de cultivo para la variedad de manzana WA 38.
In English/en inglés
Bernardita Sallato, Washington State University Extension specialist, discusses fruiting and vegetative buds during a video on pruning strategies she produced in English and Spanish in partnership with Good Fruit Grower.
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