To manage a tree’s size effectively, especially in a confined garden or urban setting, intervention is necessary because trees are genetically programmed to reach their full, natural height and spread. Size control maintains a desired aesthetic, prevents the tree from overwhelming its space, and avoids future conflicts with buildings, pavement, or utility lines. A combination of careful canopy modification and subterranean root management can keep a specimen healthy and proportionally small. Successfully maintaining a tree at a reduced size requires a consistent, multi-year strategy rather than a single, drastic action.
Structural Pruning for Size Control
Managing a tree’s canopy involves selective cuts designed to slow overall growth and maintain a strong structure. The most effective technique for height and spread reduction is reduction pruning, which shortens a branch back to a smaller, healthy lateral branch. This cut directs the tree’s growth energy into the smaller side branch, slowing the growth of the whole limb. To perform a proper reduction cut, the remaining lateral branch should be at least one-third the diameter of the branch being removed, which ensures the wound closes efficiently and the remaining limb can support the flow of water and nutrients.
Thinning cuts involve the complete removal of a branch back to its point of origin on the trunk or a parent branch. This method reduces canopy density, allowing more light and air penetration, but it does not reduce the overall height or width of the tree. Indiscriminately cutting branches back to stubs, known as “topping,” should be avoided entirely because it creates large wounds and stimulates a flush of weak, upright growth called “water sprouts.” To manage stress, remove no more than one-third of the live foliage in a single pruning session. The best time for most structural pruning is during the dormant season, typically late winter or early spring, before new growth begins.
Managing Root Systems
To balance the reduced size of the canopy, the root system must also be managed, as above-ground growth is directly proportional to the root mass. Root pruning is the process of severing roots to limit their spread and restrict the tree’s vigor. This procedure is best performed during the tree’s dormant season to minimize stress before the growing season begins.
When root pruning, sever roots at a distance sufficient to protect the tree’s structural stability. A recommended minimum distance for cutting structural roots is five to six times the tree’s diameter at breast height (DBH). For instance, a tree with a 12-inch DBH should have its roots cut no closer than five to six feet from the trunk. For trees planted near pavement or foundations, physical root barriers made of high-density plastic or metal can be installed vertically in the soil to redirect root growth. Barriers should be placed between the tree and the structure to force roots deeper or in a different direction, rather than completely encircling the tree, which can lead to stability issues.
Choosing the Right Tree and Location
The most straightforward way to keep a tree small is by selecting a species or variety that naturally stays within the desired size parameters. Choosing dwarf or semi-dwarf cultivars minimizes the need for aggressive pruning later on. For example, the size of many fruit trees is determined by the rootstock onto which the desired variety is grafted, allowing growers to select for a naturally smaller tree.
Growing a tree in a large container or a restricted raised bed is another technique for size restriction. This method naturally limits the tree’s growth by constricting its root mass, creating a proportional reduction in the canopy, often referred to as the “bonsai effect.” Specialized training methods like pollarding and espalier can also be employed to intentionally manage a tree’s size and shape. Pollarding involves making annual cuts to the same point on a branch to create a dense knob of regrowth, while espalier trains branches to grow flat against a support structure.