Gardening Invasive plants Weeds

Should You Try to Save Plants from a Weed-Infested Bed?

German irises invaded by variegated goutweed.

By Larry Hodgson

Question: One of my perennial beds is full of goutweed and I’ve decided to use the black tarp method you described in How to Control Invasive Plants to redo the bed. This worked perfectly in getting rid of lily of the valley in another part of my garden. However, that time I simply covered the entire section in tarp and didn’t bother trying to recuperate the other plants. This time there are a few perennials I want to save. Can I dig them up and replant them, or will they be infested with goutweed?

Tina Rowe

Answer: Any plant (perennial, shrub, bulb, etc.) dug up from a bed invaded by a plant that spreads by underground rhizomes like goutweed (Aegopodium podagraria), lily-of-the-valley (Convallaria majalis), plume poppy (Macleaya cordata), Japanese knotweed (Fallopia japonica), sorrel (Rumex acetosa) or quackgrass (Elymus repens) likely will have rhizomes of the weedy species mixed in with its roots. So, by trying to save the plant from invasion by moving it, you could just end up spreading the pest species, as the slightest bit of rhizome left by accident will result in a new plant. 

It’s often simplest to just cover the entire area where the invading plant dominates with black tarp and thus simply “clear the slate”, wiping out both the weed species and any other plants still growing there. Ill.: Claire Tourigny

Often the easiest thing is just to abandon the old plants, cover the entire surface with black tarp, thus killing both the weed species and the other plants, and simply start afresh. 

However, there are other possibilities.

New Plants from Cuttings

For one, have you considered taking cuttings of your perennials? 

Taking a cutting from a perennial.
A surprisingly large number of perennials can be grown from stem cuttings. Photo: abc.net.au

Many perennials can be grown from cuttings taken from the aboveground part of the plant and these will, of course, contain no weedy rhizomes. Most perennials with a branching habit (phlox, sedums, asters, chrysanthemums, lamiums, monardas and thousands more) can be readily grown from stem cuttings  as well as almost all shrubs. 

Or, if they are species (most cultivars [cultivated varieties] won’t come true from seed), you could collect seed and sow those.

Dig, Clean and Quarantine

However, you can also try to dig up plants you can’t grow from cuttings and rid them of weed rhizomes. Just be very, very careful. 

Here’s what to do:

Goutweed rhizomes
You might be able to tell goutweed rhizomes from the roots of your perennials by their horizonal growth habit. Photo: Drahkrub, Wikimedia Commons

Dig up the plant and rinse off all its soil. Remove any goutweed rhizomes you see growing through its root ball. Now pot it up. Do not plant it in another part of the garden until it has undergone a 3-month quarantine in its pot. If no goutweed has appeared after 3 months, you’ve successfully eliminated the pest species and you can replant the potted plant.

Hosta in a pot with weeds starting to grow around it.
Pot up your plant and wait to see if the weedy invader sprouts or not. Photo: awaytogarden.com

Often dividing the plant you want to save into smaller parts will make things easier. Divisions of the plant will be easier to check for rhizomes, as there will be fewer roots among which the rhizomes can hide. Saving a division is therefore often a better choice than trying to save the entire mother plant.

So yes, you can save plants from the invaders that have taken over their part of the garden. You just have to be very careful in doing so.

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.

3 comments on “Should You Try to Save Plants from a Weed-Infested Bed?

  1. Yup; I have done well with digging and cleaning, but did so during winter while the affected plants were dormant. I can imagine that it would be difficult with plants that are difficult to distinguish from the weeds.

  2. Laura MacKenzie

    Any tips on what to do about peonies? I now they hate to be moved. Would mulching around them work?

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