How to Control Invasive Plants?

By Julie Boudreau

I don’t know any garden that isn’t struggling with at least one plant that’s a little more expressive than the others! One day or another, everyone makes the mistake of introducing a plant into their garden that turns out to be very invasive and difficult to control. We’re not talking about plants that get bigger from year to year and are easy to contain with a little kick on the shovel or push on the trowel. We’re talking about these plants that are getting out of control! We pull it out… and it grows back. We prevent it from going left and there she appears in the lawn.

In my garden, the main culprits are peach-leaved bellflower (Campanula persicifolia), glossy scabious (Scabiosa lucida) and a white-flowered ironweed (Vernonia arkansana ‘Alba’), three varieties that reseed abundantly in my garden (and my lawn). And unfortunately, seedlings from the white variety of ironweed give plants with purple flowers. Still, I also cultivate the original species, Vernonia arkansana, which does not reseed in my garden.

Enemy Plant Invasion Strategies

Why a plant becomes invasive in one garden and not in another is a bit of a mystery. In some cases, the invasion is complete and total and is expressed uniformly in all gardens. Examples include gout weed (Aegopodium podagraria) or Canada anemone (Anemone canadensis).

For other plants, it all depends on the growing conditions. If the conditions are optimal, the plant opens the floodgates of freedom and sets off to conquer the territory! If the soil or sunlight conditions are less perfect, the plant obediently lives its existence without ever exceeding the limits imposed on it. In short, it’s a great topic of conversation among gardeners. Some complain about the invasive nature of a plant while others praise its merits!

Invasive plants have different strategies to ensure their survival and colony growth. The first is to produce thousands of seeds… which germinate easily! So the plant appears ever further and further away!

The majority of invasive plants that are very difficult to eradicate propagate vegetatively using rhizomes (underground stems that resemble roots). All you need to do is forget a tiny piece of rhizome in the soil for the plant to grow again.

Finally, they are also, in general, large and very fast-growing plants.

As in any fight, it is important to know your enemy well. By knowing how a plant spreads, it’s easier to apply the correct control methods.

Prevent it From Going to Seed

For plants that propagate by seed, the main priority will be to stop the bleeding at the source: we must prevent the plant from flowering and above all, we must prevent it from going to seed. This is the start of everything, a quick pruning in early summer can delay flowering or completely prevent it. You have to stay alert and make sure you don’t forget any plants. Then concentrate on pulling out the plants, without fearing the appearance of new colonies.

That said, some seeds can live for several years in the soil before germinating. Even if you do a good job of controlling the production of new seeds, you may need to continue monitoring and control for several years.

Be More Persistent Than the Invasive Plant

One of the only ecological methods to control an invasive plant is uprooting. It is arduous work and must be done regularly, over several years. Using a trowel, dig up the unwanted plants, trying to get all the rhizomes. Repeat the same exercise several times during the summer because it will grow back. Each little fragment forgotten in the soil will give birth to a new plant. You must prevent its regrowth from gaining momentum. Thus, the plant tells us where the small pieces of forgotten rhizomes are located.

It is realistic to say that this technique can require three or four years of hard work in order to definitively get rid of an invasive plant. But you will always have to keep an alert eye and eradicate the slightest regrowth.

Exhaust the Plant

Even if it is a technique that helps slow the development of an invasion, you will not be able to completely get rid of an invasive plant. Exhaustion involves cutting the plant back to the ground as soon as it grows. This approach is based on the fact that a plant absolutely needs leaves to carry out photosynthesis and accumulate energy. If we deprive the plant of its leaves, it weakens. It therefore becomes less vigorous.

If you play this little game with a non-invasive plant, like a rhubarb plant, for example, you will end up killing the plant. This is why you should not harvest all the petioles from a rhubarb plant! Unfortunately, the invasive plant, due to its vigor, is rarely killed by this type of practice.

It is therefore a perfect strategy if you lack time to do the uprooting. It also makes uprooting work easier, because the plants are less vigorous. But this is only a temporary solution.

Start From Scratch

Another eco-friendly approach for desperate situations is tarping. This method consists of applying a thick, very opaque black canvas to the ground. This tarp covers the entire soil. It completely blocks light, preventing plants from photosynthesizing. Anything under the web will eventually die. It is very important not to step on these tarps, because the slightest hole becomes an opportunity for the invader to show up. You have to wait an average of two full summers before removing the tarp.

Then you have to quickly work the soil and plant the new garden!

In conclusion, it should not be forgotten that up till now, gardeners and their passion for ornamental plants are the source of more than 30% of the introductions of invasive species on Quebec territory. This should always be kept in mind when introducing a new plant. Because prevention is THE best way to not be bothered by invasive plants. Sometimes, a simple search of what’s going on in other areas quickly confirms the invasive nature of a plant. As eco-responsible gardeners, let’s try to do better and be more forward-looking.