Is it a tree or a shrub?

Melanie Mathieson
Gardening Guru

Many gardeners can be confused by the large number of trees and shrubs on the market today and all of the different varieties that seem to be in one family. You can even have a tree family with some hybridized species that fall under the shrub category. Because this is a hybridized species that has been genetically altered from a wild specie, you usually you do not find versions of these shrub species in the wild.
From a scientific viewpoint, a tree is defined as a plant with a single woody trunk that supports a leafy crown of leaves. A shrub can be defined as a woody plant that has multiple woody stems usually at ground level. In general terms trees almost always live longer than shrubs. Generally in the forest the definition of tree and shrub is quite obvious but you sometimes find evidence of a tree species having characteristics of a shrub or a shrub looking just like a tree. Examples of this include the black cherry tree, which is found frequently in southern Ontario and is prized for making furniture. In the Rainy River District, the black cherry is really quite common but because it is growing at the very upper limits of its range, the tree will never become the prized beauty in the lower part of its growing range. In fact black cherry in its ‘shrubby’ form looks just like pin or choke cherry and is often mistaken for it because all of these species can be found growing together. Working as a forester in the District I have also come across mountain maple shrubs growing along side red maple trees that were the same size. It was difficult to tell the two species apart without checking multiple characteristics unique to each specie. Now a trip to the nursery can create even more confusion because the science of hybridization has created trees with shrub-like form and shrubs with tree-like characteristics. Some tree species have been hybridized into dwarf versions of themselves, which only reach a full-grown height of sometimes only 2-3 feet, so things can become quite confusing. Another example is the cedar; a tree in the forest but has been hybridized into many varieties of shrub-like plants with unique growth habits, shapes and sizes.
Whether the tree or shrub you are looking at is wild or a hybridized specie used for urban horticulture there are some standard definitions that apply. Basically trees and shrubs are divided into two categories; evergreen, those that have green foliage throughout the year and do not shed all of their leaves (needles) at one time and deciduous, those that lose their leaves all at once at the end of the growing season. Contrary to popular belief, not all evergreens are coniferous trees or shrubs. Coniferous is the term given to trees or shrubs that produce their seeds within a cone and most species have needles (pine, spruce, fir) instead of leaves but species like cedar and many junipers have scale-like leaves. Some evergreens have broad leaves such as rhododendrons, azaleas, Ginko biloba, wintergreen, bear berry and boxwood, just to name a few. Most deciduous trees and shrubs have broad leaves but the larch (tamarack) tree is a coniferous deciduous tree. The larch turns a brilliant gold colour in the fall before shedding all of its needles at the end of each growing season.
The life span can vary greatly on trees and shrubs. A general rule of thumb is that the life span on most shrubs is generally shorter than most trees.
Life Span
Tree
Shrub
Short
60 - 100 yrs.
10 - 30 yrs.
Medium
100 - 200 yrs.
30 - 50 yrs.
Long
200 + yrs.
50 - 80 yrs.
Trees such as maples, oaks, white and red pine, white and blue spruce, ashes and elms can easily live past 200 years in this region, longer in more favourable growing regions in Canada and the United States. Make sure when you are choosing a tree or shrub for your landscape that you are aware of how long the tree or shrub will live and that you are prepared for that tree or shrub to occupy that space for its lifespan.
With the introduction of hybridization there have been many new varieties of trees and shrubs on the market since the 1960’s. Trees and shrubs are usually genetically altered (hybridized) by using a multiple of varieties within the same species or family and breed a final tree or shrub with the very best or most desirable characteristics. One of the most intriguing results of hybridization is the ‘dwarfing’ of an usually large tree or shrub species into perfect miniatures of themselves. By definition dwarf means ‘smaller than the usual size for the species’, but in horticulture terms this usually means a species that is slow growing and will only reach 2-3 feet maximum in size. Again be very careful when selecting dwarf species that you read the hang tag each time because plants that look similar can be totally different varieties. Plants are roughly the same size in the nursery setting so choose your varieties carefully and always double check what the hangtag mentions about its mature size. You would hate to plant a dwarf species that will get ten feet tall in the space that you left in the garden for a three-foot variety. Other varieties whether dwarf or not can be very fast or very slow growing so make sure you know what you are purchasing. Also always make sure the plant that you are choosing is hardy to our growing zone (Zone 4). You would hate to go through the expense and trouble of planting a tree or shrub and then it not survive the winter a few years down the road. Our reputable local nurseries always carry species that are hardy in our zone but be extra cautious if you are purchasing from one of the box-store garden centres because those plants can be provided from large green houses in the Toronto area.
I hope this had solved some of the confusion about trying to select a tree or shrub for your landscape. There will be more detailed columns in the future focusing on certain aspects of trees and shrubs and adding them to your landscape. In the meantime always remember the tag that is attached to the tree or shrub at the nursery often provides the best information about the species you are choosing so make sure you read each tag carefully before making your final decision.