By Pamela Sleightholm
Invasive plants can come from the horticultural trade
Take a trip to a conventional nursery and you’re bound to found dozens of plants that are considered invasive species in Ontario. In the horticulture trade, some of these plants are considered desirable because of their pest resistance, colours, bloom period and flower shape. Unfortunately, these same characteristics can be detrimental to native insect and plant species.
When we think of invasive plants, purple loosestrife, garlic mustard, dog-strangling vine and goutweed come to mind. Would you be surprised if we added garden favourites like lily of the valley, forsythia, Japanese barberry, periwinkle and English ivy to the list?
These photos were taken either in Rattray Marsh Conservation Area or the University of Toronto Mississauga nature trail. All species are invasives that originated from gardens. Clockwise from top left: garlic mustard, lily of the valley, English ivy, forsythia, Siberian squill (the small blue flower) and periwinkle
How do they get out of a residential garden and into wild spaces?
Many gardeners mistake invasives for plants that spread aggressively from where they’re planted – they assume that if the plants stay where they’re wanted in the garden that means they’re not invasive. But if you take a walk through a local park or conservation area, you will see the evidence of that spread far from any residential garden.
Some plants spread by seeds travelling in the wind (examples include Norway maple and phragmites) while others are eaten by birds or other animals and deposited in wild areas (such as garlic mustard, privet and white mulberry). Some may hitch a ride on people's clothes or animals' fur. Broken rhizomes can be spread by water or even construction equipment (examples of this are phragmites, again, and goutweed). Even the planters that some buy in the fall and winter can include invasive species like pampas grass, whose seeds will set sail in the wind.
And unfortunately, there's still evidence of humans dumping invasive plants in forests - sometimes unknowingly - by discarding soil or emptying planters in wild areas.
What's the real harm?
Some invasives, like the Norway maple, grow tall, dense canopies that shade everything beneath it. They also hog water with their outward-spreading roots, so even plants adapted to deep shade cannot thrive beneath them. Their samaras, like all other maples, spread by wind. But their seedlings grow profusely, and if left unchecked can cause seedlings to sprout within a wide radius of a single tree. Norway maples used to be planted by municipalities in boulevard spaces because of their hardiness and shade. The very characteristics that made it desirable are the same that make it harmful to the ecosystem.
Some invasive plants' seeds can remain viable for decades, making their recurrence inevitable and requiring continued management. Others are allelopathic, meaning that they release chemicals into the soil that inhibit the growth of other plants.
Most invasive plants spread quickly and can overtake native populations. But even worse than that, they don’t perform the same ecosystem services as native plants – they don’t provide pollen and nectar needed by specialist pollinators, they don’t absorb water as efficiently as native plants and aren’t larval host plants for pollinators. The plants that do that are native plants - those that have evolved with our climate, flora and fauna.
The Invasive Species Centre estimates that municipalities and conservation authorities spend over $50 million on invasive species in Ontario per year. This total includes invasive plants, insects, fish and animals.
Why are invasive species still for sale at my local nursery?
Our tax dollars pay for invasive species management and removal, yet conventional nurseries still profit from selling invasive plants. Right now, only 15 plants are included in the Ontario Invasive Species Act, which regulates their sale, propagation, ownership and release.
If you’re unsure what plants are invasive in Mississauga, Credit Valley Conservation has an excellent resource to help in plant selections. If you want to take action on this, consider joining the Canadian Coalition for Invasive Plant Regulations. They provide a great deal of information and lobby government to prevent the sale of invasive plant species.
If you've learned that you have invasive species in your garden, get them out, then enjoy planning new space that will be beautiful and supportive of our ecosystem as well!
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