Strategies to control invasive plants

przez Li Ma

Invasive species are non-native organisms which have been introduced to an area without the predators, pathogens and other natural enemies that would help keep them in check in their native habitats. Some of these invasive species are highly destructive, competitive and difficult to control. They can threaten property and recreational values, infrastructure, our agricultural base, public health, and safety, as well as the biodiversity and ecological function of the areas infested. Conservation biologists have globally ranked invasive alien species as the second most serious threat to biodiversity after habitat destruction. Invasive plants are typically controlled using manual, mechanical, cultural, biological and/or chemical control techniques. This article investigates effective control methods for knotweeds [Japanese knotweed (Fallopia japonica), Bohemian knotweed (Fallopia x bohemica), Giant knotweed (Fallopia sachalenensis) and Himalayan knotweed (Polygonum polystachyum)], English holly (Ilex aquifolium), garlic mustard (Alliaria petiolata), gorse (Ulex europaeus) and wild chervil (Anthriscus sylvestris), which are the priority invasive species in the Metro Vancouver region.

Due to extensive rhizome growth, mechanical control (e.g., cutting) is not an effective control method on its own for knotweeds. Foliar application of 2.5 to 3% glyphosate or 1% imazapyr is effective. Imazapyr has been found to be more effective than glyphosate. Integrated management practices are recommended to avoid the development of herbicide resistance.

Small English holly plants can be pulled or dug up by hand when soil is moist. Stem injection with imazapyr capsules using the EZ-Ject lance tool (EZ-Ject 2011) is the most effective and time-efficient application method for large plants. Frilling or stem injection with triclopyr is also very effective.

For gorse, seedlings and small shrubs can be pulled by hand before they fully establish. Cut-stump treatment with triclopyr is very effective for large plants with low risk of the applicator and non-target plants exposure. Herbicide should be applied to the stump within a few seconds of cutting the stems when cut stump treatment is applied. Repeated treatment will likely be necessary to achieve complete control.

For wild chervil, tillage alone may be effective. An integrated control strategy using herbicide, tillage and grass seeding is recommended for excellent control and re-vegetation.

In the case of garlic mustard, individual plants can be pulled, removing as much root as possible. Flowering plants can be cut at ground level if pulling is not possible and seed production should be prevented for up to 5 years to exhaust the seed bank. Spot application of 1% glyphosate-based herbicide by foliar spray in spring is recommended for larger infested areas.

It is important to acknowledge that eradication may not happen with one treatment method or after the first treatment. Therefore, management requires a long-term commitment and a realistic perception of success. Education and prevention strategies are also keys to managing invasive plants, but these activities go beyond the scope of this report.

Autor: Li Ma
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Li Ma, PhD in plant ecophysiology is a research and teaching assistant at the Faculty of Land and Food Systems, University of British Columbia in Vancouver. Her scientific interests concentrate on the ecophysiology of effects of red/far-red light ratio on selected weed and crop species. She was also a sustainability scholar at Metro Vancouver, where she was determining the priority invasive plant species for local governments and investigating the control methods available for each of those priority species with consideration of local conditions.

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