It’s not always easy to know what the names native, exotic, naturalized and invasive mean when it comes to plants. Here’s a short Botany 101 course that should help clear up some of the confusion.
Native or exotic plant?
A plant is considered native when it grows naturally in a given region, with no human intervention. It is said to be exotic when it is introduced, intentionally or otherwise, into an area outside its natural distribution area. To show what this means, let’s take the sugar maple (Acer saccharum), a tree native to Québec, because it has always grown here. The Norway maple (Acer platanoides), on the other hand, is a tree that comes from Europe and was introduced here as an ornamental. That means that it didn’t grow in the wild in Québec before it was planted here and then spread.
The Manitoba Maple (Acer negundo) is a tree native to Canada. Its natural range – the Prairies – expanded with the arrival of the first coureurs des bois, who brought it back with them to Québec. It has been considered an exotic species here ever since. So plants can be introduced from different provinces, but also from other countries and continents.
Exotic plant: naturalized or invasive?
When an exotic plant manages to reproduce naturally in its new environment, it is referred to as a naturalized species. For instance, the oxeye daisy (Leucanthemum vulgare) and the common dandelion (Taraxacum officinale) are naturalized in Québec because they reproduce here in abundance, whereas the common lilac (Syringa vulgaris) and the tiger daylily (Hemerocallis fulva) are not, because they do not reproduce from seed.
This means that if you come across a lilac deep in the woods, you can assume that the site was inhabited at one time and you may even be able to find some house foundations. Lilacs survive long after people are gone, but cannot spread very far. So they remain confined to the place where they were planted. In the case of the daylily, the plant’s rhizomes spread slowly, but the plant requires human intervention in order to propagate and, especially, to move.
A plant that spreads and alters the composition, structure and workings of the natural ecosystems in which it spreads is called an invasive exotic species. The Norway maple, for example, is considered an invasive exotic species because there are a large number of them growing on Mount Royal and they are gradually pushing out the sugar maples there. Many of the plants in this category have been recognized for years. To name just a few, think of purple loosestrife (Lythrum salicaria), Japanese knotweed (Reynoutria japonica var.japonica, syn. Fallopia japonica), cow parsley (Anthriscus sylvestris), buckthorn (Rhamnus cathartica) and, finally, giant hogweed (Heracleum mantegazzianum).
To find out whether a plant is native to Québec or exotic (introduced), consult the Database of Vascular Plants of Canada.
Adapted from an article by Frédéric Coursol, published in Journal Métro on October 13, 2010.