Invasive plants

Guide to Invasive Plants

I can’t get away from it anymore, too many of you are asking me what to do with your [insert invasive plant name here]. So here goes: what are invasive plants, what can we do to control them and, finally, what is the future of these plants in our ecosystem?

Let me warn you right away: this is not a happy subject. I may offend, discourage, and perhaps alienate a few nature lovers towards the end…!

European buckthorn (Rhamnus cathartica). Photo: zeenaturalista

Invasive Plants vs. Invasive Alien Species

There are many interpretations of the question, “What is an invasive plant?”

For some, if it’s a plant that establishes itself and takes over the whole area, despite our efforts to control it, it’s invasive. This is true even if it’s a native plant in its natural habitat and has its rightful place, but its spread into flowerbeds is criticized.

Canada Anemone (Anemonastrum canadense). Photo: krisskinou

For others, when we speak of a species taking over the world, we necessarily mean an invasive alien species (IAS). When we say “alien”, we mean that the species is outside its normal area of distribution: it comes from elsewhere and has no place in our ecosystems.

To me, it’s simply two different attributes; many bramble species are native and invasive; lawn (grass) is an alien, generally non-invasive (much to the chagrin of some!); dandelions, alien and invasive. How’s that for jargon? Let’s start again.

Invasive Alien Species

In my humble opinion, IAS are generally more problematic, for three main reasons: they have no predators to control their spread, they have a good capacity to adapt and use resources, and they reproduce at breakneck speed.

No animal in Quebec feeds on Phragmites. They grow extremely fast, stealing all the space and light from the cattails (and other plants) in spring. They reproduce as much by their roots as by the thousands of seeds on their pretty feathers. Three conditions in three for this invasive alien species.

Common reed (Phragmites australis). Photo: chasseurdeplantes

Although this category is generally more dominant in your questions than the first, I understand that you only want to protect your land, regardless of whether the species that disturbs you is Asian, European, African, or indeed native. In short, while this article focuses more on IAS, you should know that the methods for getting rid of them are the same for our intrepid natives.

Where Does an Alien Species Come From?

Before going any further, we need to understand where these plants and animals come from and why they are here. One word: globalization. And that’s all there is to it.

Voluntary Introductions

Oh, there have been a few genius ideas like “let’s introduce Asian ladybugs into our fields! They’ll be excellent predators, but they won’t survive the winter, no problem.” Of course, they’re everywhere now and, surprise, they’re pretty winter hardy. These voluntary introductions often end up being problematic for one reason or another. It’s impossible to calculate all the parameters with nature. Life finds ways of thwarting predictions.

Lack of Knowledge

Other species are introduced through lack of knowledge. It starts with a children’s show about turtles. Parents buy turtles as pets: they’re cute, cost only a few dollars, and require little maintenance. Except that these red-eared sliders grow as big as a dinner plate, and have claws and beaks that can hurt children’s fragile little hands. Then we learn that it needs a bigger aquarium, that it lives 40 years, and we don’t want it anymore. So, we release it in a lake and give it a nice little farewell ceremony. And now it’s feeding on the eggs of our own turtles, as well as introducing into its new environment residues of the Eurasian water milfoil it had in its aquarium. Two invasive species for the price of one!

Eurasian watermilfoil (Myriophyllum spicatum). Photo: phil_crimson

Les coups de coeur

You can also fall in love with an innocent Japanese knotweed plant at the garden center. Plants are harmless, aren’t they? But of course, your beautiful bamboo-looking bush quickly becomes a forest, a living hell! And you learn that for the initiated, this plant has another little name… Demon’s Knotweed.

Accidental Introductions

Finally, there are accidental introductions, which are very difficult to see and prevent. We’re lucky enough to have fairly harsh winters in Quebec, so tree frogs that arrive in plant deliveries die and we avoid tragedy, but some species survive. Earthworms in plant substrate, algae on the hulls of cargo ships, insects in fruit…

If it weren’t for all these exchanges with other countries and cultures, there would certainly be fewer imbalances in nature. Just think how quickly COVID spread! It’s not realistic to think that we ONLY import bananas. There are also microbes, insects, dust and a whole other world.

So… do we close all borders?

Hey, don’t try depriving me of sushi! Like everyone else, I’ve got my phone in my pocket, I dream of traveling and I order online. The reality is that our lifestyle is based on trade, and that’s okay: it gives us plenty of other advantages: medicine, technology, coffee…

Photo: Sincerely Media

So What Do We Do?

As an aside, I’d like to tell you a school anecdote. You know that teacher you really don’t have a good memory of? The one who made you hate a subject, or feel really ridiculous because of a power trip? I’ve had a few like that, including one in college who taught a course on the history of science.

My work team and I finish a great oral on invasive species in Australia. It’s time for questions…

Mr. X, who loves to listen to himself talk and has the one indisputable truth about everything: “Why didn’t you choose Quebec’s invasive species?”

Me, young, and not too patient with this dear teacher: “We wanted to work on a different subject and as Australia is full of Islands, the issues are different, too…”

Mr. X: “OK, OK… Name me some invasive species from here.”

A brave member of my team: “There’s the blue mussel that…”

Mr. X, who cuts us off, “Another one.”

Me, starting to warm up in earnest: “The house sparrow, the dandelion, the Asian ladybug, there’s plenty here too, but that wasn’t our topic today…”

Mr. X, clearly trying to ask us about what he knows, rather than admitting he knows nothing about Australia: “OK, OK, so tell me how we can deal with the problem of invasive alien species?”

Knowing full well that there is no magic, easy or general solution, we do our best to respond: population control, introduction of predators, sterilization, occultation…

But Mr. X insists: “Yes, but it doesn’t work. So, tell me what’s the only good way to solve the IAS problem.”

We all look at each other, biting our tongues, at a loss for ideas, and doubtful because we’ve never seen anything about a miracle solution in our research. He cuts us off, questions us on a subject other than our own, and then he knows something we don’t, and that can save the world?

“We don’t know.”

Mr. X, really, REALLY proud of himself: “The best way to solve the problem of introduced alien species is to NOT introduce any species.”

Ooh… well done Sherlock. Are we going to solve the problem by going back in time to intercept the colonists? I’m pretty sure they didn’t know what they were doing when they accidentally introduced their earthworms, so we’d better tell them to be more careful. Frankly, you’re talking about a miracle solution…!

Anyway, thanks for the therapy, I feel better having shared this with you. On a positive note, I’ve become a biologist with an open mind. Thank you, Mr. X.

So let’s get back to a realistic solution: what to do with your invasive plants on a garden scale?

These species are very, very difficult to get rid of. As you’ve probably gathered by now, these species are resilient, highly adaptable and very comfortable in our environment. That’s the secret of their success! So to get rid of them, there’s no magic solution: it’s going to be a marathon, not a sprint, and you’ll have to be even more stubborn than Mr. X.

1. Forget Herbicides

Invasive plants are often resistant to herbicides. You’ll only contaminate and kill other plants around you.

Photo: Etienne Girardet

2. Manual Removal

Hand-pull every little seedling of the unwanted plant BEFORE it sets seed. For most plants, this is a job that has to be repeated every week for most of the summer.

It’s very important not to leave your corpses where they are; those pesky plants can rise like zombies! Spread them out in the sun on a concrete slab or patio, or put them in a well-sealed, sturdy garbage bag in the sun for a few days. This kills the plants and ensures that they don’t reseed in your city’s compost. Can you imagine a compost full of unwanted seeds? The horror!

I tried to control a population of Garlic mustard at my place for two years in this way. It worked, the bunch was less and less dense… and then I discovered another one at the bottom of my field and gave up. It’s too much work, it’s in my forest (so it’s not really as easy as in a flowerbed), they don’t really bother me (apart from the fact that I KNOW they have no business there), and above all: I’ve played in it so much that I’ve become intolerant to the smell of Garlic mustard. Just thinking about it makes me nauseous!

Garlic mustard: 1, Audrey: 0.

Garlic mustard (Alliaria petiolata). Photo: dreabreso

3. Repeated Mowing

If your invasives are in your lawn or are too big to pull out (like Phragmites, for example), you can always mow/cut the green parts of the plant. Doing so will prevent the plant from feeding itself through photosynthesis. I insist: the GREEN parts. If you’re fed up with dandelions, unless you plough your poor soil, the leaves won’t go through the mower.

Once again, it’s a marathon: these green parts will grow back relentlessly until the plant has used up ALL its accumulated energy, and invasives are generally very good energy stores. In fact, they often have fleshy roots (shaped like carrots or potatoes) that store enormous reserves of energy.

In the case of staghorn sumac (Rhus typhina), it takes an ax to grind! Native here in Quebec, it’s an invasive alien in Europe. Photo: pantherophis

4. Occultation

This is undoubtedly the easiest solution for us, but it’s also the most expensive and the least attractive… And not necessarily the quickest! It involves covering the invaded area with a large tarpaulin to block out all the sun’s rays. I repeat: ALL RAYS. A garbage bag lets the light through. So does an old sweater.

It’s imperative to cover more than the affected area: we’re talking about up to a meter (3 feet) and sometimes even more! This is because invasive plants can often produce shoots along their roots. If you don’t want the plant to start growing around the tarpaulin, or even through cracks and small snags, you need to cover a large area and monitor it regularly.

Ah… that dear Japanese knotweed! (Reynoutria japonica). Photo: galcozack

If you cut off the light supply for a few YEARS, the plants will eventually wear out and die. This solution also kills all the other plants under the tarpaulin. This technique is best used in fields or conservation areas. It’s hard to imagine a grower manually weeding an area of several square meters every three days!

The Consequences of Invasive Plants in Nature

Many people here won’t like me. Especially Mr. X. You’re about to be confronted with a terrible reality.

You can’t control an invasive plant on a territory-wide scale.

There, I’ve said it. There’s no magic solution: once a plant has invaded an environment, it’s nearly impossible to get rid of it 100%. Yes, a little control is possible with a lot of effort (and money), but the reality is that invasive plants live up to their name. There will always be a handful of seeds somewhere, a piece of root ready to wake up when its time comes, an irresponsible garden center selling problematic species. That is not to say we should give up our efforts to control invasive species.

We can try to control the appearance of new invasive species: don’t buy mystery seeds on the Internet, wash your equipment well when you move from one nature spot to another (boots, tools, boats, etc.). There’s always a way to be aware and responsible. But unfortunately for those who are already there, it’s TOO LATE.

Nature Is Changing

Do we curl up in a corner and mourn our dying nature? Of course not! Nature doesn’t die, it changes. Yes, we’re losing some of our biodiversity with these plants taking the place of our natives, and we may even witness the extinction of certain species. But that’s the way life is: it moves, it changes, it evolves.

In a few thousand years’ time, these now invasive species may give rise to a whole new family of diverse plants. Or an animal might discover that it actually enjoys eating this plant, and become the predator that regulates its populations (let’s go, deer, eat my Garlic mustard!).

Fear not, nature will recover. The human ego… that’s another story!

Photo: Nika Benedictova

Audrey Martel is a biologist who graduated from the University of Montreal. After more than ten years in the field of scientific animation, notably for Parks Canada and the Granby Zoo, she joined Nature Conservancy of Canada to take up new challenges in scientific writing. She then moved into marketing and joined Leo Studio. Full of life and always up for a giggle, or the discovery of a new edible plant, she never abandoned her love for nature and writes articles for both Nature sauvage and the Laidback Gardener.

12 comments on “Guide to Invasive Plants

  1. patcappelli

    I got rid of my garlic mustard so did my sister, she eats it in early spring or rather juices it. It is garlic and can be cooked and used in dishes. We won so anyone out there that has garlic mustard its easy to pull out and repetitive pulling works. I also got rid of vinca which once pulled out does not come back, a easy job. I have solarized and moved dog strangling vine and it is almost all gone. I also have that horrible false bamboo, it is almost all gone. Whe I go walking in the woods I tear out and destroy any invasive plant. I can make a difference. I am not a defeatist. Everyone should complain to garden centres when they see invasive plants and tell them it should not be sold. Complain, complain and lets make it illegal to sell, own or plant invasive species.

  2. I agree , having trained and certified professionals available to gardeners would be a good solution. I recognize how different it is for individuals to contain these invasive species and having all options available would be helpful.

  3. It is SO easy to offend with this topic. I avoid it also. Some readers are very protective of all species, no matter how aggressively invasive or how exotic. Some readers insist that native species can not be invasive because they were here first. I have been told that I am invasive and that I should go back to where I came from. (I, as well as my ancestors, are native.)

    • Mathieu Hodgson

      It is a touchy subject, but an important one. I wish it were to possible to discuss it calmly, which I believe is more productive. Honestly, whenever an article on invasive plants comes out, I feel sick to my stomach thinking about nasty comments I’ll receive.

      • Catherine

        I think it is possible to discuss it calmly. In order to do that the facts need to be given. Not opinions as this article does. As a Master Gardener in Canada we are inundated with people asking how to remove invasive plants from their gardens and mournful at the loss of habitat in natural areas. They are dumbfounded and angry that these known invasive plants are still sold and promoted. Had they known at the time they never would have planted goutweed, vinca, lily of the valley etc. Many members of the public are very aware of the need for regulation prohibiting the sale of invasive plants. This newsletter can chose to be a leader and help educate and start by not promoting or downplaying invasive plants as has been done in this newsletter and the one now removed from May 17 promoting goutweed among others. I urge you to join the Canadian Coalition for Invasive Plant Regulation https://ccipr.ca sign the pledge, make a difference for the environment. At least highlight the work they are doing and that of many other organizations in North America to combat invasive plants.

      • Mathieu Hodgson

        Thank you! I have contacted the CCIPR and intend to develop a more robust editorial line concerning Invasive species. I am also working for a method to automatically add warnings to past articles which include invasive plants, until I can manually correct those.

  4. My quack grass has won the war!

  5. cooperdg

    Kodiak, Alaska has hawkweed – lots of it. We’ve managed to control it in our own yard (I think!) but it has spread wantonly everywhere. We have pulled, cut, rooted up the darn plant in public areas and it is a losing battle. The best I can do is try to not let it get in my yard but unfortunately the neighbors don’t think the same way. Good article! I feel understood.

  6. Lots of thought and reason in this post. Thank you!

  7. I realize you are trying to address this issue but I wonder how much research you do . For example saying because you can stand the smell of garlic mustard you aren’t worried about it, ignoring the damage garlic mustard does to native plants and fungi. Also to say not to use herbicides is leaving out a very effective tool. Herbicides are part of an invasive weed control strategy, very effective on phragmites, dog straggling vine, Japanese knotweed etc.

    • You misunderstood what she said about garlic mustard…
      “ I’ve played in it so much that I’ve become intolerant to the smell of Garlic mustard. Just thinking about it makes me nauseous!“

      She said “intolerant” not tolerant

    • Mathieu Hodgson

      In my opinion, herbicides should be used by trained and certified professionals. Here in Quebec, more and more cities have banned the sale of pesticides without proper permits. Ideally, municipalities would have services their citizens could contact to notify them of the presence of an invasive plant on their territory and direct them to the proper resources. Unfortunately, gardeners are mostly left to themselves to deal with alien invasive species. There are many groups across North America working on the issue but I am find there is a lack of coordination between them, which is in no way their fault, but rather that of different levels of government.

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