You would have seen these along roadsides, especially near ditches. Tall grasses that sway in the wind. This is likely phragmites, also known as common reed (Phragmites australis L.). It is a very widespread weed, in fact one of the world’s most common plant species, being able to grow in wetlands from the tropics to the far north. As a result of its global distribution, various races and types have evolved. In North America, there is a native type of phragmites that has achieved balance with its environment. However, races from Europe have been introduced over the years and since they have different biological features, they have become invasive. This means they will grow unrestricted and can rapidly establish monocultures that exclude native vegetation and have negative effects on wildlife.
Traditionally, management of phragmites involves cutting of stems or even burning, but these have moderate success. These interventions would set back stands, forcing them to regrow. The problem is that phragmites is a perennial with an extensive rhizome network. These very thick underground stems allow storage of food reserves and also cause patches to regrow easily. For cutting or burning to be successful, they would have to be repeated multiple times during a season in order to exhaust the plant’s reserves.
As far as herbicides are concerned, only three active ingredients have phragmites on their label and can legally be used. Glyphosate, the active ingredient in Roundup WeatherMax and other products, is systemic, non-selective and non residual. The label states application on phragmites must be done at rates between 2.0 to 8.0 L/Ha (0.8 to 3.2 L/ac). Imazapyr is another systemic, non-selective herbicide which has soil residual activity. It is generally used for general weed control on non-crop sites. Imazapyr is sold as Arsenal Powerline (for terrestrial applications) and Habitat Aqua (for aquatic applications). The third active ingredient is sodium chloride (table salt) in the herbicide RagWeed Off. Sodium chloride acts as a contact herbicide and the label states two to four applications per year over multiple years should be done in order to attain desirable phragmites stand reduction. While glyphosate and imazapyr are interesting products due to their systemic action it would be favourable to have other, more selective options, that would permit a more rapid re-establishment of native plants.
Herbicide field trials in 2019 and 2020
We conducted two field trials to evaluate herbicide treatments that could extend the portfolio of available options land managers can use against phragmites. These were compared to the standard treatments based on glyphosate and imazapyr. In 2019, we tested Garlon XRT (triclopyr) which is a commonly used product in vegetation management. While triclopyr provided early good results, it did not provide season long control compared to glyphosate and the two formulations of imazapyr (Table 1). Interestingly, imazapyr did not show good control early in the season with 43 to 60% visible injury at 30 days after treatment but it showed effectiveness in the long run. We also included two other herbicides with known activity against tough grasses. Assure II (quizalofop-p-ethyl) is a group 1 herbicide used in a range of crops and is very effective at controlling annual grasses as well as rhizomatous perennials like quackgrass. LongRun (flazasulfuron) is a vegetation management group 2 herbicide which is labelled for control of many grass species including hare barley, creeping bentgrass and tall fescue. Surprisingly neither of these two products were able to provide acceptable control of phragmites. The reasons for this are unknown but could be due to a combination of the extensive network of thick rhizomes as well as the very dense canopy this weed produces. The 2020 trial confirmed that triclopyr provides suppression of phragmites (Table 2). As seen before, glyphosate was able to provide perfect control during the whole season.
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