Pest Patrol: Control of Phragmites with Herbicides and Biological Agents

You will have noticed these tall grass swaying in the wind along the roads, especially near the ditches. They are probably phragmites, also called common reed (Phragmites australis L.). It is a very widespread weed and one of the most widespread plant species in the world. In North America, there is a native type of Phragmites which has achieved equilibrium with its environment. However, breeds from Europe have been introduced and have become invasive.

Traditionally, the management of Phragmites has involved trimming the stems or even burning, but this has had moderate success. These interventions push back the stands, but they push back. The problem is, Phragmites is a perennial with a large rhizome network. In order for the cutting or engraving to be successful, they should be repeated several times.

When it comes to herbicides, only three active ingredients have Phragmites on their labels and can be used legally. Glyphosate is systemic, non-selective and non-residual. The label indicates that application to Phragmites should be made at rates between 2.0 and 8.0 L / ha (0.8 to 3.2 L / ac). Imazapyr is another non-selective systemic herbicide with residual activity in soil. It is used for general weed control on uncultivated sites and is sold under the names Arsenal Powerline and Habitat Aqua. The third active ingredient is sodium chloride (table salt) in RagWeed Off Herbicide. Sodium chloride acts as a contact herbicide and the label states that two to four applications per year over several years should be made in order to achieve the desirable reduction in Phragmites stands.

We conducted two field trials to assess herbicide treatments that could extend the portfolio beyond standard glyphosate and imazapyr treatments. In 2019, we tested Garlon XRT (triclopyr) which is a product commonly used in vegetation management. While triclopyr performed well initially, it did not provide control throughout the season against glyphosate and the two imazapyr formulations (Table 1 above). Interestingly, imazapyr did not show good control at the start of the season with 43-60 percent lesions visible 30 days after treatment, but it did show long-term efficacy. We have also included two other herbicides with known activity against leathery grasses. Assure II (quizalofop-p-ethyl) is a Group 1 herbicide used in a range of crops and is very effective in controlling annual grasses as well as rhizomatous perennials such as quackgrass. LongRun (flazasulfuron) is a Group 2 vegetation management herbicide that is labeled for the control of many grass species including hare barley, red bentgrass and tall fescue. Surprisingly, none of these products were able to provide acceptable control of Phragmites. The reasons are unknown but could be due to a combination of the large network of thick rhizomes as well as the very dense canopy that this weed produces. The 2020 trial confirmed that triclopyr suppresses phragmites (Table 2 below). As seen previously, glyphosate has been able to ensure perfect control throughout the season.

Evaluation of biological control agents

In collaboration with colleagues from Agriculture and Agri-Food Canada, the Ontario Ministry of Natural Resources, and the University of Toronto, we planned to assess the compatibility of warbler-specific biocontrol insects with herbicides. However, the onset of the pandemic in 2020 forced us to readjust. Limited versions of Archanara sp. insects were carried out on a few sites and their survival in winter is monitored. Hope we can expand the scope of the work in the near future.

This work was funded in part by the Ontario Ministry of Agriculture, Food and Rural Affairs as part of an agreement with Canada under the Canadian Agricultural Partnership.

Have a question you want answered? #PestPatrol hashtag on Twitter for @cowbrough or email Mike at [email protected].

РFran̤ois Tardif and Peter Smith work in the Department of Plant Science, University of Guelph.

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