Outcompeting Invasive Plants, Part 1

By Laura Gardner, Master Gardener in Training

When it comes to dealing with invasive plants in our gardens, some can be quite challenging to control, let alone eradicate. In a system of Integrated Pest Management (IPM), gardeners may need to choose a number of different control methods and the methods considered need to make the least environmental impact. These methods involve cultural, mechanical, biological, and chemical options. In most invasive plant situations, choices will be cultural and mechanical. Chemicals may not be an option for home gardeners due to licensing, legislation, and product label requirements. However, some chemicals may be necessary in situations where there is a health and safety concern—e.g. Heracleum mantegazzianum (Giant Hogweed). Some of the usual methods that are used may include pulling, digging, cutting, removing flowerheads, sifting the soil for root fragments, smothering, or solarizing with tarps. Many invasive plants are difficult to remove in their entirety due to their extensive rhizomatous roots or their ability to produce many seeds that can last for years in the soil. Two unwanted plants that have popped-up in my garden in recent years are Campanula rapunculoides (Creeping Bellflower) and Allilaria petiolata (Garlic Mustard). My property is fenced but it is not closed off and so is open to seeds coming from other areas (including adjacent neighbouring properties). Because of this, my approach is about control of existing plants but also in preventing new ones from becoming established.

In addition to some of the methods mentioned above, two others you can add to your arsenal is to use mulch and to plant more densely.

Add Mulch

While the above-mentioned weed seeds do travel by wind, the majority of them will fall near the parent plant on the other side of the fence. All along the open fence lines I have added a thick layer of arborist wood chips. Adding a 4” layer will inhibit the germination of weed seeds as light is prevented from reaching them. Any seedlings that do germinate can be easily pulled as the roots cannot take a firm hold within the mulch. The mulch will break down over time and will need to be replenished.

Add Dense Plantings

e.g. Solidago Flexicaulis (Zig-Zag Goldenrod)

There is a wooded area near my house that is densely covered by a native plant called Solidago flexicaulis (Zig-Zag Goldenrod). Recently I noticed that there were only two or three Garlic Mustard plants amongst it. The Goldenrod was beating it! The Ontario Invasive Plant Council advises planting certain native plants at a density of 9 or 11 plants/m2 in order to compete with Garlic Mustard.[i] In addition to Zig-Zag Goldenrod these other plants are recommended as Garlic Mustard competitors:

  • Anemonastrum canadense (Canada Anemone)
  • Hydrophyllum virginianum (Virginia Waterleaf)
  • Hydrophyllum canadense (Canada Waterleaf)
  • Geum canadense (White Aven)
  • Matteucia struthiopteris (Ostrich Fern)
  • Viola sororia (Woolly Blue Violet)
  • Carex blanda (Woodland Sedge)
  • Mianthemum stellatum (Starry False-Solomon’s-Seal)
  • Athyrium filix-femina (Lady Fern)
  • Ribes americanum (American Currant)
  • Diervilla lonicera (Bush Honeysuckle)
Solidago flexicaulis (Zig Zag Goldenrod) and Parthenocissus vitacea (Woodbine), and Allilaria petiolata (Garlic Mustard) together in a wooded area in Peterborough

[i] Garlic Mustard (Allilaria petiolata): Best Management Practices in Ontario. Ontario Invasive Plant Council. p. 22. Online: https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/OIPC_BMP_GarlicMustard.pdf

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