Most people probably understand the difference between a tree and a shrub, but it might prove more difficult to explain. Trees and shrubs are both woody plants, as distinct from the herbaceous, fleshy-stemmed plants that comprise the other half of the plant world. We all think of a shrub as being smaller than a tree, but there are more differences than size alone.
According to renowned British garden designer David Domoney, a shrub is defined as a woody plant that is smaller than a tree and generally has a rounded shape. The main difference between the two is that a shrub has several main stems growing from ground level, rather than one trunk.
But it's not as clear cut as that: sometimes trees are grown as shrubs; hemlock, hornbeam, and hawthorn trees are often grown as shrubs in urban environments.
It's worth noting that there is a scientific classification distinction between trees and shrubs, so really, the debate over if something is a tree or shrub is mostly a linguistic and aesthetic issue that matters the most when you're deciding what to plant where and how big something should be. Let's take a closer look at the characteristics of a tree and the characteristics of a shrub.
Characteristics of Trees
First, trees, shrubs, and woody vines are the only plants with woody growth, which makes them similar. Many people think, for example, that so-called banana trees are trees, but in fact, they are considered the world's largest herb.
Once you know that a plant has woody growth, you can determine whether it is a tree, a shrub, or a vine. Vines are obvious for their thin stems and branches and their trailing growth. But the differences between trees and shrubs can be more difficult to pinpoint. The generally acknowledged definition of a tree, according to Utah State University (USU), is a woody plant having one erect perennial stem (trunk) at least 3 inches in diameter at a point 4 1/2 feet above the ground, a definitely formed crown of foliage, and a mature height of at least 13 feet.
Merriam-Webster agrees, defining a tree as a woody perennial plant having a single usually elongate main stem generally with few or no branches on its lower part.
The Colorado State Forest Service explains how trees grow by examining their physiology:
- A tree is a tall plant with woody tissue. Trees gather light for photosynthesis through their leaves; this process creates “food” for the tree.
- Most of a tree trunk is dead tissue and serves only to support the weight of the tree crown. The outside layers of the tree trunk are the only living portion. The cambium produces new wood and new bark.
- The band of tissue outside of the cambium is the phloem. Phloem transports new materials (the sugars created from photosynthesis) from the crown to the roots. Dead phloem tissue becomes the bark of a tree.
- The band of tissue just inside of the cambium is the xylem, which transports water from the roots to the crown. Dead xylem tissue forms the heartwood, or the wood we use for many different purposes.
- Every year, trees grow two annual rings. In the spring, usually a wider and thinner-walled layer called springwood forms. In the summer, a thicker-walled layer, called summerwood, develops. Annual rings are typical in temperate forest trees.
Some examples of common trees found in the United States include red maple, loblolly pine, sweetgum, Douglas fir, quaking aspen, sugar maple, balsam fir, flowering dogwood, lodgepole pine, and white oak.
Characteristics of Shrubs
Shrubs are defined as woody plants with several perennial stems that may be erect or may lie close to the ground. Shrubs will usually have a height less than 13 feet and stems no more than about three inches in diameter.
Merriam-Webster calls a shrub "a low, usually several-stemmed woody plant," and "a woody plant that has several stems and is smaller than most trees."
Whether you call them shrubs or bushes, these plants are "important to any landscape," says Jerry Goodspeed, USU Extension horticulturist. He continues, "Perennials and annuals provide color and variety. Trees add shade and perspective, and usually frame our homes and yards. Shrubs are the plants we relate to—they help us feel a part of the landscape because they bring it down to our level....A shrub or bush is a woody plant with a mature height of between one and a half and 10 feet. Anything smaller is ground cover. Anything larger is a tree. Most bushes are also easy to place in the landscape."
Note that there is some disagreement about the maximum height of shrubs. Goodspeed defines the maximum height of a shrub as 10 feet, while others, as noted above, define the boundary as 13 feet. In any case, both heights are generally less than that of mature trees.
Examples of common shrubs found in the United States include witch hazel, forsythia, lilacs, rose of Sharon, Fothergilla, oakleaf hydrangea, red twig dogwood, holly, King's Gold and Gold Mops, Stewartstonian azalea, roses, and hibiscus.
These definitions serve as good starting points for distinguishing between trees and shrubs, but, as with most things, there are exceptions. As long as you follow the general definitions, though, you should be able to decide whether a plant is a tree or a shrub.
Nutty plants such as hazelnuts (filberts) are plants that can be grown as either a shrub or a tree. If left alone, it can become a “trub.” According to Dennis Hinkamp of the USU Extension, a trub is a plant that cannot decide if it is a tree or a shrub. It gets bushy, but it grows to a height of more than 15 feet, which classifies it as a true trub.
Jerry Goodspeed, Utah State University Extension horticulturist, says, "Hazelnuts...should be trained and grown as a tree because they are more productive as a tree and make a mean-spirited, lousy shrub...When trained as a tree, hazelnuts can grow to about 20 feet high, with an equal spread.