A Muddy Definition
Shrubs are woody plants that mature to fifteen feet or less in height. If a tall shrub grows to over fifteen feet, it becomes a tree. The reason the height rule is important is related to legislation. For example, many states do not allow trees to be planted in locations that would grow to obscure outdoor advertising signs, but you can plant shrubs. If you are designing plant material on public rights-of-way, you will need to avoid planting trees near areas adjacent to billboards. Local ordinances often require replacement trees and shrubs, with a distinction between the mitigation points allowed for each. Utility companies may have easement restrictions that allow shrubs but do not allow trees to be planted under overhead power lines.
Why fifteen feet? It could be the maximum height maintenance crews can easily reach limbs for pruning, while still standing firmly on the ground. That makes sense. It could be because that height falls between two common growth limits. There are a lot of shrubs that can grow twelve to fifteen feet tall. There are a lot of small trees that mature to twenty to twenty-five feet tall. Not a lot finishes out between those two limits. Some, but not a lot. A single species could have variations in maximum height, depending on where it is grown. There is no clear green line drawn in the air, but fifteen feet is a good number of consensus.
Shrubs with many stems and no distinct central trunk are called bushes. A shrub is a plant branched from the base, without a leafy crown above a trunk. If you think about the definitions for long, you’ll get a headache. I just call them all shrubs.