Tall shrubs are often thought of as small "trees" when folks in everyday life are not adhering to technical definitions. As a garden writer, I permit myself the pedantry involved in pointing out the difference. But in this article, I offer some examples of large bushes that, admittedly, can be quite tree-like in appearance.
In some cases, the difference between trees and shrubs is so obvious as to require no explanation. Many of us grow up with those woody things with trunks that soar to the sky, which we know as "trees." We are also familiar with those smaller woody things that tend to bush out rather than having tall trunks; we call them "shrubs," for example, weigela.
But try classifying the following five woody plants as either tall shrubs or small trees:
Not so easy, is it? Even cases that are more clear-cut sometimes give us pause. Take oak trees, for example. If you have ever cut down a small oak, you know that it can send multiple shoots up from the ground later and come to look rather bushy.
Surely the experts have definitions that provide us with a clear-cut difference between trees and shrubs, right? But not so fast. Even the experts confess that their definitions of "trees" and "shrubs" admit of exceptions. And if you have ever had to memorize the exceptions to rules such as "i before e except after c," you know how dicey matters can get once exceptions rear their ugly heads.
Further complicating the question, What is the difference between trees and shrubs? is the inveterate debate of whether to classify things based on their intrinsic characteristics or on how they are employed by humans. For example, consider how we use rose of Sharon (8-10 feet in height):
You may wish to prune off the lower branches of your rose of Sharon (Hibiscus syriacus) if you are treating it as a specimen plant, promoting the development of a discernible "trunk," thereby making it more tree-like (see below under "Standards"). But if you are using rose of Sharon in a privacy hedge, you will probably want to grow it as a multi-stemmed, tall shrub, as the presence of the lower branches will make the hedge more difficult to see through.
In the case of wild sumac, no effort on the part of humans is required to give it a discernible trunk. It just grows that way, on its own. The trunk tends to be too skinny on smooth sumac (Rhus glabra) to claim tree status, according to the definition (a "tree" has a trunk at least 9 1/2 inches around at chest level), making it a tall shrub at 10 feet high. But staghorn sumac (Rhus hirta) reaches 18 feet in height and can straddle the line between tall shrubs and small trees.
But it is not only humans who step in and alter the "natural" appearance of plants. The elements can do a pretty good job of it, too. For example, crape myrtles may be omnipresent in tree form in the South, reaching heights of 20-30 feet. But if grown in a planting zone in which they are only borderline-hardy, crape myrtles (Lagerstroemia) may survive (if, indeed, they survive at all) in another form: namely, as tall shrubs.
Another tall shrub (10-25 feet) that has been known to masquerade as a tree is PeeGee hydrangea (Hydrangea paniculata 'Grandiflora'). Like rose of Sharon, it can be sculpted, through judicious pruning, so as to appear tree-like. Mature PeeGee hydrangeas so sculpted cut an impressive figure in your landscape. They also do not require much shrub care once established, which is why they are so often seen, in my area (New England, U.S.), in old cemeteries, looking none the worse for having endured the onslaughts of Father Time without much pampering from human beings.
Harry Lauder's walking stick (Corylus avellana Contorta), that curious tall shrub with curlicues for branches, requires much more care. Created by grafting, you will have to prune out the suckers produced by this tree-like bush, lest the straight branches rise up and overwhelm the curlicues. This is one tall shrub (8-10 feet in height) that looks better after it has dropped its leaves in fall because it is only when its leaves are out of the way that you can fully appreciate its mystifying branching pattern. This feature makes it a good choice for creating winter interest in northern landscapes.
A discussion dealing with the distinction between tall shrubs and small trees naturally leads to the practice of creating standards. In a horticultural context, "standard" has a different meaning than in everyday life. A standard is a shrub or vine (wisteria vine makes for a nice standard) that has been made to look like a tree by pruning out all of the main branches coming up from the root system except for one -- which will serve as a "trunk." Tall shrubs make for better standards than do short bushes if your goal is to create something tree-like; some of the bigger types of rhododendron would be an example. Growers can also "cheat" by using grafting techniques to create a standard (as alluded to above with Harry Lauder's walking stick), as is often done with rose bushes to make a so-called "tree rose."