Is it a tree or shrub? Is the answer determined by the number of trunks? The height of the plant? The shape? Where it is planted? According to the Morton Arboretum, biologically there is no difference. Both trees and shrubs are woody, comprised of branches and stems with woody cells that don’t die in our winters. Also, they both have bark. So, which is which? Although it isn’t an unbreakable rule, height determines the difference. When a woody plant reaches 15 feet tall, it is considered a tree.
There are many exceptions. Often the homeowner grows the plant as a shrub because that is what they have room for and fits into their design. Left alone and growing in full sun, lilacs can reach 20 feet tall, but we consider them shrubs. In the Midwest, we grow yews as a shrubby hedge, but in England there are yews hundreds of years old and 30 to 40 feet tall. Conversely, there are species of oak that only reach a few feet at their highest.
Often, we think of trees as one shape and shrubs as a different configuration. Ask a child to draw a tree and the picture will be a single straight trunk with a circle on top for the canopy. Shrubs, on the other hand, we visualize with multistemmed trunks emerging from the soil. However, the trees redbud and river birch may have one trunk or up to five.
Usually, multi-trunked trees have one central trunk emerging from the ground, then branch into multiples above the soil. In fact, if a tree has all its trunks coming up from the soil, it’s planted too deep and will not live up to its potential. Bark must be allowed to dry out in between rains. When it is covered with soil or even mulch, it remains too moist, ideal conditions for disease and insect infestation.
So, forget the labels. Grow the woody plants in the manner that works for your landscape. The only requirements are planting where it has the correct conditions for the species. Sun lovers in full sun. Understory trees in partial shade. Plant with the root flare above ground. Keep watered until established or during a drought. Perhaps choose a species that Japanese beetles avoid. And enjoy your woody plant for years to come.
Call 217-465-8585 or email sarahp@illinois.edu with any gardening questions, and an Extension Master Gardener will be happy to help.
Illinois Extension leads public outreach for University of Illinois by translating research into action plans that allow Illinois families, businesses, and community leaders to solve problems, make informed decisions, and adapt to changes and opportunities. Illinois Extension is part of the University of Illinois Urbana-Champaign College of Agricultural, Consumer and Environmental Sciences.