Tall shrubs are often thought of as small trees when not adhering to technical definitions. It's no surprise as some large bushes can be quite tree-like in appearance. In some cases, the difference between trees and shrubs is so obvious as to require no explanation. In turn, there are many instances when the distinction is not always clear.
It's not easy classifying the following five woody plants as either tall shrubs or small trees:
Additionally, examples that are more clear-cut can sometimes give us pause. Take oak trees—if you've ever cut down a small oak, you know that it can send multiple shoots up from the ground later and eventually begin to look rather bushy. Even expert reports that define the difference between trees and shrubs recognize many notable exceptions.
Pruning and Training
Further complicating the difference between trees and shrubs is the inveterate debate of whether to classify things based on their intrinsic characteristics or on how they're utilized by humans. For example, consider how we use Rose of Sharon (8 to 10 feet in height)—we sometimes use it en masse in a hedge along a property border, to form a living-wall privacy fence. Conversely, we use it all by itself, as a specimen plant.
You may wish to prune off the lower branches of your Rose of Sharon (Hibiscus syriacus) if you're treating it as a specimen plant, promoting the development of a discernible "trunk," thereby making it more tree-like. But if you're using Rose of Sharon in a privacy hedge, you will 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 nine and a half inches around at chest level, therefore making Smooth Sumac 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's 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 to 30 feet. But if grown in a planting zone in which they're 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 to 25 feet in height) 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 and can cut an impressive figure in your landscape. They also do not require much shrub care once established, which is why they're so often seen in old cemeteries, looking none the worse for having endured the onslaughts of Father Time without much pampering from people.
Harry Lauder's Walking Stick (Corylus avellana Contorta), a curious and 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 to 10 feet in height) that looks better after it has dropped its leaves in fall as then you can fully appreciate its mystifying branching pattern. This feature makes it a good choice for creating winter interest in northern landscapes.
Dealing with the distinction between tall shrubs and small trees naturally leads to the practice of creating standards. In a horticultural context, 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 with Harry Lauder's Walking Stick), as is often done with rose bushes to make a so-called "tree rose."