Size-reduction pruning, sometimes called containment pruning, is a specialized technique used to manage the growth of a tree that is genetically programmed to be much larger. This practice aims to keep a tree healthy and structurally sound while maintaining a predetermined, smaller size. Unlike standard aesthetic pruning, which focuses on removing dead or diseased wood, size reduction deliberately limits the tree’s natural growth habit. This is an ongoing commitment to ensure the tree remains safe and does not outgrow its intended space.
Timing and Preparation for Size Reduction
The optimal time to perform aggressive size-reduction pruning is during the tree’s dormant season, typically from late fall through late winter before new buds swell. Pruning during this period minimizes stress because the tree is not actively expending energy on leaf production or growth. With the leaves absent on deciduous trees, the structure of the canopy is fully visible, allowing for more precise cuts that maintain the tree’s natural form.
Dormant pruning also gives the tree maximum time to begin the wound-closure process before the rapid growth of the spring season. Proper preparation includes gathering sharp tools, such as hand pruners for small branches, loppers for medium-sized limbs, and a pruning saw for larger cuts. It is important to sanitize cutting tools with a 70% isopropyl alcohol solution or a 10% bleach solution between trees to prevent the transmission of diseases.
The Biological Response to Pruning
Size control relies on manipulating the tree’s natural growth hormones by overcoming apical dominance. This is the biological process where the terminal bud at the tip of a branch produces auxins, hormones that travel down the stem and suppress the growth of lower, lateral buds. This suppression causes the tree to grow upward and outward from its tips.
When a branch tip or leader is removed through a reduction cut, the source of the growth-suppressing hormone is eliminated. This redirection of energy stimulates the lateral buds closest to the cut to begin growing. By cutting back to a smaller, outward-facing lateral branch, the new growth is directed away from the center and is less vigorous than the original terminal growth. This process encourages a denser, more compact canopy structure, effectively keeping the tree smaller.
Specific Cutting Techniques for Size Control
Effective size reduction depends almost entirely on using a technique called a reduction cut, which shortens a branch back to a lateral branch or bud. This technique is distinct from a thinning cut, which removes an entire branch back to the trunk or parent limb to open up the canopy. Reduction cuts are designed to shift the growth to a smaller, subordinate branch that will take over as the new terminal.
The key rule for a successful reduction cut is the “one-third rule.” This states that the remaining lateral branch must be at least one-third the diameter of the branch being removed. Cutting back to a lateral that is too small will prevent the wound from healing properly. The tree will likely respond by producing a flush of weak, upright shoots called water sprouts, which require more frequent pruning and create a less stable canopy.
When making a thinning cut to remove a branch entirely, the cut must be made just outside the branch collar. The branch collar is the slightly swollen area where the branch meets the trunk or parent limb, and it contains specialized cells that facilitate wound sealing. For larger limbs, a three-cut method—an undercut, a top cut to remove the weight, and a final cut outside the collar—should be used to prevent the weight of the branch from tearing the bark down the trunk. Using these precise techniques prevents the heavy re-sprouting and structural weakness that result from indiscriminate topping cuts.
Annual Maintenance for Consistent Size
Size control is not a single pruning event but a long-term commitment requiring regular, careful maintenance to prevent the tree from rapidly regaining its size. Once the initial reduction has been performed, subsequent annual pruning focuses on reinforcing the established, smaller framework. Pruning should be performed annually or biennially to manage the new growth that has emerged since the last cut.
To maintain tree health and vigor, it is recommended that no more than 20% to 25% of the tree’s live foliage be removed in any single year. Removing too much foliage at once can stress the tree, leading to decline or excessive compensatory growth in the following season. Maintenance pruning involves making small reduction cuts on the most vigorous new growth, especially on the dominant leaders, to ensure the canopy remains dense and within the desired size parameters.