Ahhh! It’s invasive!

I often mention a plant in a text and get the response in the comments: “Ahhh! It’s invasive! Sometimes it’s true! There are many plants used in our gardens or sold commercially that are truly invasive. And sometimes it takes years before we realize that a plant introduced by settlers or used as an ornamental is invasive. But, usually, it’s much more complicated to label a plant as invasive than it seems.

Invasive Plants: A Few Definitions

I’ve already touched on the subject in the article Covering Ground With Natives a few weeks ago, but it’s worth exploring it further. The term “invasive” is often misused, and there’s confusion about which plants are actually invasive.

The definition of an invasive plant can vary depending on where or by whom it is defined.

The United States Department of Agriculture (USDA) says that an invasive species is “non-native (or foreign) to the ecosystem under consideration; and [that its] introduction causes or is likely to cause economic or environmental damage or damage to human health.”

In Quebec, we use the term “invasive alien species” (IAS). We also use the expression “priority invasive alien plant species” (PIEFS) to better target our resources in the fight against these most harmful invasive plants.

A Short List of Invasive Plants

Here are a few examples of plants considered invasive exotic species in Quebec:

Acer negundo, Manitoba Maple
Acer platanoides, Norway maple
Aegopodium podagraria, Goutweed

Alliaria petiolata, Garlic mustard.
Anthriscus sylvestris, Wild chervil
Ficaria verna, Lesser celandine, fig buttercup

Frangula alnus, Glossy buckthorn
Galium mollugo, Smooth bedstraw
Helianthus tuberosus, Jerusalem artichoke
Heracleum mantegazzianum, Giant hogweed
Heracleum sphondylium, Meadow cow-parsnip
Impatiens glandulifera, Himalayan balsam
Miscanthus sacchariflorus, Amur silvergrass
Pastinaca sativa, Wild parsnip
Petasites japonicus, Japanese sweet coltsfoot

Reynoutria japonica, Japanese knotweed
Reynoutria sachalinensis, Giant knotweed
Reynoutria x bohemica, Bohemian knotweed
Rhamnus carthartica, European buckthorn
Symphytum officinale, Common comfrey
Ulmus pumila, Siberian elm
Valeriana officinalis, Common valerian
Vincetoxicum nigrum, Black swallowwort
Vincetoxicum rossicum, European swallowwort

Therefore, an invasive plant is:

• Non-native
• Dominant in its new environment
• Causes damage to the environment, the economy or human health.

Native, but Harmful

The horsetail (Equisetum arvense) you’ve been fighting for decades in your vegetable garden is therefore not invasive, because it’s indigenous. Poison ivy, which spreads by seed and rhizomes, is also native to North America, although 85% of the population is allergic to it and will suffer skin inflammation on contact with its leaves. The Mayapple (Podophyllum peltatum) is native to Quebec and is even considered endangered. In a garden, it will spread rapidly by underground rhizomes and smother its competitors, but it’s not invasive!

Once again, the term native is relative. A plant that is native to one country is not necessarily native to a specific region. How precise should we be? At the same time, it’s normal for a plant to spread without human intervention, especially when there has been disturbance in an environment.

In short, native plants are not invasives, even if they spread aggressively to displace your ornamental plants or vegetables, cause economic harm by affecting agricultural crops, or cause health problems. In such cases, they should be referred to as noxious weeds.

Naturalized Plants

There’s another category to consider: naturalized plants. These are exotic plants that have adapted to a new environment and spread easily. But, unlike invasive exotics, they don’t damage their new habitat by taking the place of native plants. An example of this is the dandelion (Taraxacum officinale). While many hate it because it soils their perfect lawn, last time I heard, it doesn’t take the place of our native plants or destroy existing ecosystems.

Grey Zone

I say “last I heard”, because dandelions have not been shown to be damaging to native ecosystems (more to the aesthetic sense of some!), but it’s possible that in the future, after more in-depth studies on the subject, we’ll change our minds. However, the dandelion is said to cause economic damage to farmers who lose crops to it or incur costs to eliminate it. So, invasive or not?

There are several ornamental plants that spread by stolons, rhizomes or seeds that are not on the list of invasive alien species in Quebec, but could eventually be. If the periwinkle (Vinca minor) is common in our gardens, it’s precisely because it spreads so easily, even in the shade or under other plants. It is not currently considered invasive in Quebec, but its ability to spread easily could put it on the list eventually… or not!

Fly honeysuckle (Lonicera caerulea), native to much of the northern hemisphere, is cultivated for its sweet-tart berries. Despite its hardiness, it doesn’t grow very fast. Its cousin, the Tartarian honeysuckle (Lonicera tatarica), produces beautiful, fragrant white or pink flowers in spring. It resists being pruned, so is often used as a hedge. It’s also much more vigorous than fly honeysuckle, to the point where it’s considered invasive in many places, including Ontario, but not yet Quebec.

Japanese barberry (Berberis thunbergii) has become very popular in our gardens in recent years, but has the potential to become invasive. Just like Chinese silver grass (Miscanthus sinensis), dead-nettle (Lamium spp.) or rugosa rose (Rosa rugosa).

Don’t get me wrong, I’m not suggesting you eliminate these species from your beds immediately. You could, however, keep an eye on them or avoid planting any more, especially if you live near wild or protected environments, where these plants could spread and cause damage.

Stay Alert!

But be careful! Just because a plant spreads through your garden doesn’t mean it’s invasive. Plants that spread easily even have a certain appeal, as they fill the empty spaces in our flower beds. As long as they don’t prevent your other plants from thriving.

Unfortunately, there’s no centralized list of plants with invasive potential. To make matters more complicated, a plant is not necessarily invasive everywhere. Pampas grass is illegal to grow in many places, but it won’t survive the winter in northern countries and is used as an annual only. There is no danger of it escaping from cultivation.

We must therefore rely on the vigilance of gardeners in this matter, as there is no official procedure to follow at the moment. When selecting plants for your garden, check whether your choices have invasive potential in your region.