10 Neat Things About Invasive Plants
by Shauna Dobbie
by Dorothy Dobbie

The Local Gardener magazines, Ontario Gardener, Manitoba Gardener and Alberta Gardener, are published by Pegasus Publications Inc.

Drawing on her 30 years' experience as a senior executive in the magazine publishing industry, Dorothy launched Manitoba Gardener in 1998, initially running the business out of her home. Two years later, Dorothy's daughter Shauna, living in Ontario, jumped into the fray with Ontario Gardener. And two years after that, they started Alberta Gardener. Visit us at and register for our "Ten Neat things" newsletter. Watch Shaw TV for garden tips and Listen to CJOB for the Gardener Sundays at 9:08

May 26, 2019

1. How many?

Researchers did a study in 2002 that came to the conclusion that at least 1,442 invasive species were taking over Canada's forests, waterways and farmlands. These include animals, insects, fish and plants that have been introduced from overseas and have found little stopping them from spreading like wildfire. When their numbers are high, they tend to out-compete native species for food and space.

2. Garlic mustard.

Garlic mustard was brought to North America in the 1800s as a culinary herb. It has no significant predators here and in fact is toxic to larvae of the mustard white butterfly, which think it is mustard. And it is left alone by white-tailed deer, which rough up the areas surrounding it while foraging, making it easier for the seed to take root. Furthermore, it produces chemicals underground that suppress mycorrhizal fungi, the good fungi that help most plants grow.

3. Purple loosestrife.

This is a lovely invasive. It was introduced as an ornamental from Europe, with its pretty spikes of purple flowers and brilliant red foliage in fall. The problem is that it grows so quickly in waterways here that disrupts waterflow and chokes out native cattails. This interrupts the life cycle of birds, amphibians and algae. Fortunately, there are two species of beetle and three species of weevil that eat purple loosestrife only, and they are used as a biological control.

4. Canada thistle.

The name notwithstanding, Canada has nothing to do with the origin of this species. Canada thistle comes to North America from Europe and Asia. It reproduces both by seeds and by underground runners. They aren't all bad, though; they are an excellent source of nectar for pollinators and the foliage is eaten by painted lady butterfly larvae.

5. Goutweed.

This is still sold by a number of garden centres and many people say it's a fine plant if you put it in the right place. The problem is, if you plant it in the right place it doesn't stay there, not if it is in the ground. If you pull it, the brittle roots will break off and grow entirely new plants. As someone who battled the variegated form of goutweed, I beg of you not to plant it.

6. Vinca minor.

Where it is hardy, it is a little too hardy. It's a lovely, glossy green groundcover with the bonus of purple flowers in the spring. It spreads readily and thrives in dry, dappled shade. My garden was full of it until I saw it covering the forest floor in a natural area in Ontario. It doesn't multiply by seed much but creeps rapidly if left unchecked. It grows in a very dense mat and won't let trees or anything else grow from seed. Go ahead and grow it in your garden, but please keep it away from the forest.

7. Yellow flag iris.

One cultivar of this flower has been granted the Award of Garden Merit by the Royal Horticulture Society of Great Britain. In Scotland, it is important as a breeding ground for the bird, the corn crake. In North America, though, it chokes out native species. It is a wetland plant. Growing it in your backyard pond won't matter, but when it gets into natural water systems it is a problem.

8. Giant hogweed.

The name isn't kidding; this plant is giant. It looks like Queen Anne's lace but it is 18 feet high. The problem with this plant, aside from producing thousands of seeds that live for up to 15 years in the soil, comes from the clear sap that it exudes. Get this sap on your skin and, when exposed to sunlight, painful blisters will appear.

9. Tamarisk.

I have one of these trees in my garden and people are always impressed by it. I was shocked to see it on the invasive species list; it hasn't spread at all in my urban garden, to my neighbours' gardens or to the park across the street. Apparently, though, tamarisks form thickets to choke out native species and they consume salt, which they distribute around them by dropping their needley leaves, discouraging other plants from growing. At the base of my tamarisk is a forsythia that I dug up years ago, coming back with a vengeance. Apparently, my tamarisk doesn't know it's supposed to spread and choke out other plants.

10. Kudzu.

This is the mother of all invasive plants. Fortunately, it dies at -20 degrees Celsius. It was found in Essex County, Ontario in 2009 but doesn't seem to have spread since then. The vine covers areas of the southern US, growing up to one foot per day. Yikes. The good news is, it is edible and it seems to combat alcoholism.

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