Gardening Invasive plants Weeds

Tips On Controlling Plants With Wandering Rhizomes

Common reed

By Larry Hodgson

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.

 creeping rhizomes of common reed
The creeping rhizomes of common reed can travel long distances. Photo: Kenraiz, Wikimedia Commons

The invader may well be an actual weed, like horsetail, quackgrass or common reed, but sometimes ornamental plants turn out to be terribly invasive. That’s notably the case of Japanese knotweed (Fallopia japonica), goutweed (Aegopodium podagraria) or lily-of-the-valley (Convallaria majalis), all offered in garden centers and nurseries. Unfortunately, I have yet to see a garden center or plant nursery mention that a plant they sell is invasive, so… caveat emptor!

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!

Rototiller in garden
Rototilling a plant with rhizomes will just compound the problem. Photo: Nwdata, Wikimedia Commons

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.

What About Herbicides?

I’m not a fan of herbicides, but you can always try a nonselective herbicide (one which kills all vegetation) such as glyphosate, spraying the intruder’s foliage. Unfortunately most plants with invasive rhizomes seem to be at least somewhat resistant to herbicides. The result is that the plant may be somewhat weakened, but it isn’t killed outright, so you have to repeat the treatment over and over. And each time you spray a nonselective herbicide, even if you are very careful, you tend to poison and kill desirable plants nearby.

But there is a way of using nonselective herbicides much more effectively and with no damage to nearby plants. Rather than spraying it on, paint it on. To do so, cut the plant you want to kill down close to the ground, leaving only a stub visible. Now apply the herbicide with a brush, directly to the stub. The herbicide will be rapidly absorbed by through the open wound on the stub and will travel down into the rhizomes. Often you get total control with just one application… and without contaminating the surrounding environment or damaging neighboring plants.

Remember to wear protective gloves as you apply the herbicide, of course.

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…

Black tarp with bricks on top.
A black tarp will keep light out and starve the weed to death. Ill.: Claire Tourigny

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.

For some plants, including Japanese knotweed (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 black 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 no rhizome of the invasive plant is mixed in 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

Scissors cutting plant short.
If you repeatedly cut back plants to the ground, leaving no green growth, you’ll starve them to death. Photo: pxfuel.com

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. Again, don’t hoe or cultivate; 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 the 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.

Pots used as root barriers
Pots used as root barriers. Photo: laidbackgardener.blog

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), but you know ahead of time that it’s invasive. 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 smaller 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 pot or bucket barrier. It will escape from pretty much any barrier you can imagine!

Rhizome barrier.
Rhizome barrier. Photo: Amazon.ca

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 2 feet (60 cm) or more 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 GardenCanada’s Bamboo World  is one source in Canada. You can also find this kind of product on Amazon.com.

The Most Laidback Method

But there is still a more efficient and much more laidback method for dealing with invasive weeds: learn to value them.

Goutweed (Aegopodium podagraria ‘Variegata’), for example, with its white and green leaves, makes an excellent and colorful groundcover that will smother any weeds. Lily of the valley (Convallaria majalis), invasive though it may be, can also be seen as a pretty groundcover plant thanks to its fragrant spring bells. Even horsetail (Equisetum arvense) is not without its charm, thanks its beautiful light and ferny foliage.

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

Article adapted from one posted in this blog on September 12, 2015.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

18 comments on “Tips On Controlling Plants With Wandering Rhizomes

  1. I fought lily of the valley this spring until I almost dreamt about digging it up. Lately, I’ve been tackling the japanese knotweed which makes the lilies look like a piece of cake. I hardly make a dent in it. The goutweed is at least attractive and not quite as hard to dig although as you note I never get it all. 🙂

  2. Erez Manela

    Hello Larry, I really enjoy your posts. Could you please do one on how to combat creeping Charlie (Glechoma hederacea) in the lawn? Thanks, from Belmont MA

  3. Thanks for your interesting article. I’ve had a problem with invasion of my blacktop driveway with Zoysia grass, a grass that propagates from rhizomes and seed. The rhizomes are surprisingly strong enough to travel under the blacktop edge of the lawn and actually erupt the blacktop with mounds and cracks. My solution now is to make a mix of a gallon of white distilled vinegar and a pound of cheap Morton table salt. (Yes, the vinegar can absorb all the salt.) I then stream this mix into the driveway edge – and especially into the zoysia-made cracks – with a quart spray bottle set to steady stream. This year it worked well, and I suspect the remedy will last longer than a herbicide. Last year, I used a smaller spray bottle and don’t think I got enough liquid in the ground.

    I like your idea of a plastic cover to starve the zoysia edge of sunlight. The grass doesn’t like shade. But perhaps the barrier is the best solution because the rhizomes don’t go very deep, only a couple inches. Maybe I can hammer a 6-inch wide galvanized steel strip along the asphalt edge. The zoysia will probably grow over the strip, which will be level with the driveway, but I doubt those rhizomes could establish roots from the top of the asphalt. Thanks for your suggestions.

  4. I have Coastal Bermudagrass & wild dew berry, both are proof that your article is true. I got down on my knee’s after tilling up the soil & removed all the grass & dew berry roots I could find. Then as the roots spouted I removed them, after two years my
    half acre was clean. I was diagnosed with Afib & did not garden for eighteen months & was back to square one.
    I gave up & used solarization to kill all the weeds, it is not great for the soil health, but it is fast & less harmful then poisoning the ground for five years. This is of course MHO. I also have wild garlic (allium vineale), originated in Europe, it is better than poison Oak & that’s the only good thing I can say about it.
    Thanks, for another good article.

  5. Japanese knotweed has some wonderful medicinal benefits, particularly curing lyme disease. So, many of these ‘weeds’ can help us in some way!

  6. I seem to have avoided the knotweed but our house came with a large bed around a small waterfall/pond containing goutweed (not varigated) lily of the valley, yellow flag iris and chameleon plant (Houttuynia cordata) which while pretty was even taking over the other invasives. I finally went to a heavy cardboard cover and mulch hoping to replant in two years. The few plants I wanted to keep were dug and quarantined in nursery pots for the summer so I can move them elsewhere.

  7. On rare occasion, I needed to inspect sites that had been infested with bamboo. Sometimes it was for a lawsuit, and sometimes it was for an insurance company. One insurance company assumed the expense of removing the bamboo from a neighboring property, as well as new landscaping. Heck, they even got appraisals for material that was not salvageable as a result of the infestation, and reimbursed the neighbor.

  8. I do not understand why people do not use clump bamboo or the retaining wall made for bamboo. I know of a homeplace with a few sprigs of bamboo, that was kelp under control with a lawn mower. the owner moved away & now 1.5 acres is covered.

  9. I am in the process of planting comfrey along the edges of all my garden beds. It is a fantastic barrier and provides for the pollinators. Also a soil building plant.

  10. Thank you for sharing this article, it helps a lot.

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