A common question in horticulture is how to accurately distinguish a tree from a shrub. Both are classified as woody, perennial plants, meaning their stems persist and grow year after year. The confusion arises because this distinction is not a strict botanical classification but rather a descriptive label based on physical form and size. Understanding the difference relies on observing the plant’s structure and its growth potential at full maturity.
The Defining Structural Difference
The most reliable way to separate a tree from a shrub is by examining the structure of its stem. A tree typically exhibits a single, dominant, woody trunk, often with clear apical dominance, meaning growth is concentrated at the tip of this main stem. This central trunk remains unbranched for a significant height above the ground before developing a canopy of foliage. The single trunk provides the necessary structural support to reach considerable heights.
In contrast, a shrub is defined by a multi-stemmed structure, where several woody stems arise directly from the base of the plant or very near the soil line. This growth pattern results in a dense, bushy, and compact appearance, with branching occurring much closer to the ground. Shrubs lack the single, dominant trunk that elevates the foliage, which allows them to fill out lower layers of a landscape.
How Height and Maturity Separate Them
While structure is a defining feature, size is the most visually apparent difference, determined by the plant’s mature height. Trees are generally recognized as exceeding a minimum height, often cited in horticulture as 13 feet (about 4 meters) or more at maturity. Some definitions suggest a tree will reach 20 feet or higher.
Shrubs, conversely, are typically smaller, rarely exceeding 15 to 20 feet in height, and often staying within the range of 3 to 10 feet. The concept of maturity is important because a young tree sapling may appear to be a shrub, but its genetic programming dictates its future growth into a single-trunked, taller form. Therefore, classification depends on the plant’s potential and typical growth habit, rather than its current size.
When Definitions Become Ambiguous
The distinction between a tree and a shrub is not always absolute, as environmental factors and human intervention can blur the lines. In harsh, high-altitude or subarctic environments, trees can be stunted into a low, dense, shrub-like form known as krummholz, a German term meaning “crooked wood.” This occurs when freezing winds and ice kill the apical buds, forcing the tree to grow horizontally along the ground where it is sheltered by snow. These naturally dwarfed plants, such as certain species of spruce or pine, may be centuries old but stand only a few feet tall.
Human practices also create exceptions, such as pruning or coppicing, which is cutting a tree back to the base to encourage multiple stems, effectively training a tree species to grow as a shrub. Conversely, large shrubs like lilacs or certain viburnums can be pruned to remove basal stems and maintain a single, tree-like trunk. These transitional forms and environmental adaptations demonstrate that the tree-shrub separation is a practical description of form, not a rigid biological classification.