How to Control Agressive Plants

Agressive plants can quickly become a real headache in your garden. Often plants with trailing rhizomes multiply and invade everything in their path. In this article, Larry Hodgson helps you understand why these plants take over and offers simple solutions to keep them under control. First published on September 12, 2015, this guide is among the most-viewed articles, ranking 18th.

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Sometimes gardeners are faced with a plant with creeping rhizomes, that is, underground stems that produce numerous offshoots or suckers (secondary plants). Obviously, every sucker in turn produces yet more rhizomes and more suckers and soon you have totally lost control.

Often, it’s an exotic invasive plant that is officially monitored and controlled (Japanese knotweed, European Common Reed, etc.), but sometimes it’s a so-called ornamental plant that has proved a formidable invader, such as goutweed (Aegopodium podagraria) or lily of the valley (Convallaria majalis). These are indeed considered exotic invasive species in many regions, although they are sometimes still presented as ornamental and sold in nurseries. Unfortunately, this is not the kind of detail you’ll be told about in the garden centre when you buy the plant. So, caveat emptor (buyer beware)! You might also have to deal with native plants that spread a little too much for your liking, such as field horsetail (Equisetum arvense), which is difficult to completely eliminate.

Hand Weeding is a Waste of Time

One of the worst ways to try to control a rhizomatous plant is by trying to pull it out. That works well enough with annual weeds and perennials without rhizomes, but unfortunately, this method actually helps rhizomatous plants spread, because the slightest piece of rhizome left in the ground will grow into a new plant. When you yank out a plant, you tend to snap off not just one, but several of its rhizomes by accident. Imagine the result: the weed will grow back more thickly than ever!

Hoeing or cultivating to try and control a rhizomatous weed is no better: again, you tend to make things worse by chopping up rhizomes into pieces and leaving most in the soil. And probably the worst way to try and control one of these plants is with a rototiller: it slices and dices rhizomes like nobody’s business, plus spreads the rhizome pieces everywhere! What a mess!

Pull and Sift

You can control spreading weeds by pulling them up, though, if you take the trouble to dig up the entire sector and sift the soil before replacing it. That way you’ll be able to pick out even very small sections of rhizome. It’s a lot of work, but remains quite doable if the invasion is on a small scale.

Shade the Plant to Death

Here’s another way of seeing the situation, though.

Every green plant needs light to live. After all, it’s sunlight that gives plants their energy. They absorb light through  the chlorophyll (a green pigment) found in their leaves, then convert it into sugars and starches they use for their growth. So if you cut off the plant’s source of sunlight…

The easiest method to “cut off the sunlight” is to cover the whole area with a black plastic tarp. Don’t use a geotextile weed barrier: they tend to let some light through. You’ll find a tarp of the right type in any hardware store.

The tarp must cover an area wider than the plant’s original spread, otherwise it will quickly send out rhizomes beyond the exclusion zone.

Leave the tarp in place for an entire growing season, from spring to late fall, or better yet, until the following spring, using bricks or rocks to hold it in place. In the darkness under the tarp, deprived of any sunlight, the plant will still try to grow, but will only produce stringy, pale stems that can’t carry out photosynthesis. Thus the plant is starved of light and will eventually die.

Japanese Knotweed, a Special Case

For some plants, including Japanese knotweed (Reynoutria japonica syn. Fallopia japonica), called the worst weed known to mankind by some experts, a single season of darkness isn’t enough: you have leave the tarp in place for two full years to completely exhaust it. But for most weedy plants, one growing season of utter blackness will do the job.

Obviously, the above method works well for spots where nothing else is growing, but if you apply it to a flowerbed, you will also kill any desirable plants growing there: perennials, bulbs, shrubs, etc. Is it possible to save them before applying the back tarp?

Sure! If you dig carefully, you can plant them elsewhere or else you can grow them in pots for a year, until the original spot is weedfree and ready to be replanted. However, you must make sure that absolutely no rhizome of the invasive plant is mixed with the roots of the plant you want to keep, otherwise you’ll again be spreading the weedy plant rather than controlling it.

Selective Pruning

If you are not willing to cover a flowerbed or vegetable garden for a full year while the black tarp does it’s job, you can always, if you are diligent enough, remove the weed through selective pruning. Do not hoe or cultivate; again, that will just make things worse. Instead, starting at the beginning of the season, simply cut the weedy plant to the ground. That way you’ll be eliminating its foliage: its only source of energy. It will of course respond by producing new shoots. Cut them back too. And cut any new plants that sprout again. And again. And again. For some plants, the battle will last 2 full years… but there won’t be much regrowth after the first season.

It is important to cut back the new shoots soon as you see them, before they have time to carry out much photosynthesis. But you will see less and less regrowth as time goes on, because by cutting back all the green shoots as soon as they appear, you’re effectively preventing the plant from carrying out normal photosynthesis and it will slowly weaken and eventually die. Selective pruning will work, but as mentioned, you have to be very diligent and never let any sprouts remain in place.

In-Ground Barriers

If you just want to stop progress of invasive plant rather than actually eliminating it, in other words, prevent it from going any further, you can always install a physical barrier in the soil, a barrier the rhizomes can’t cross.

The easiest way to do so is to install the barrier when you plant the potentially invasive plant. Let’s say you love bee balm (Monarda didyma), native to many parts of North America, but you know in advance that it spreads agressively. Before planting it, remove the bottom of a big pot or plastic bucket (the bottom has to be removed to ensure adequate drainage) and sink it into in the soil. Now plant the future invader inside this barrier. For most plants, a barrier 1 foot (30 cm) high will amply suffice. In fact, a 6-inch (15 cm) barrier will be enough for some plants, especially ones (short plants tend to have shallow rhizomes).

Be careful with plants that produce deep rhizomes, though, like Japanese knotweed (Fallopia japonica): it is an illusory to think of controlling this plant, whose rhizomes can reach down 10 feet (3 m) into the sol, with a barrier. It will escape from pretty much any barrier you can imagine.

Under no circumstances should you use plants considered to be invasive exotics in your region or in a neighboring region, as they may harm local biodiversity, invade natural habitats and upset the ecological balance in competition with native species.

For plants with rhizomes that are fairly deep (over 1 foot/30 cm), there is a commercial product called rhizome barrier or bamboo barrier: a semi-rigid plastic film about 2 feet (60 cm) in height that you can insert into the ground around the invasive plant. This product is widely available in areas where bamboo is commonly grown, such as Europe and the US Northwest. You may be able to purchase some at a bamboo nursery if there is one nearby. If not, order the product on the Internet. In the US, try Bamboo Garden.

The Most Laidback Method

There’s a much more effective and laidback way of dealing with native weeds: learn to appreciate them! Horsetail (Equisetum arvense) is not without its charm, with its beautiful mossy foliage.

When you don’t see the plant as a problem, but as a solution, you’ll have solved the problem without lifting a finger!