These non-native weeds are big problems in Manitoba

Keep an eye out for these noxious weeds in the coming growing season

Palmer Amaranth.

The Province of Manitoba has declared the last full week of April as Invasive Species Awareness Week.

The Manitoba Weed Supervisors Association (MWSA) recognizes this week by highlighting just a few of the invasive plants considered to be a significant threat to the landscape of our province. Many of these invasive species are mistaken for wildflowers, but unlike native wildflowers, these species threaten agricultural productivity of both cultivated and non-cultivated land, as well as waterways and natural areas.

The Noxious Weeds Act of Manitoba (NWA) requires control or destruction measures for different invasive plants. A comprehensive listing of noxious weeds is found in the Noxious Weeds Regulation, which contains schedules that ranks plants according to their threat levels and specifies the areas of the province to which these levels apply. The act requires that Tier 1 weeds must be eradicated without conditions. Examples of Tier 1 weeds that are currently negatively affecting Manitoba’s ecosystems are diffuse and spotted knapweed, orange hawkweed, red bartsia and tall waterhemp. Tier 2 weeds are also a significant concern, and include leafy spurge, common tansy, field scabious, and nodding thistle.

The MWSA is comprised of and represents weed supervisors who are individually employed by weed control districts formed by one or more municipalities. Weed supervisors are authorized to ensure problematic weeds are dealt with appropriately on all lands in the districts they cover.

Weed control districts, first started in 1964, have developed programs to deal with invasive plants such as leafy spurge and red bartsia. Prevention measures, early detection and rapid response by weed supervisors are critical for protecting habitats and agricultural land from these types of invasive species.

Tall waterhemp

Waterhemp is native to the U.S. but was not considered a major agronomic problem until the 1980s. Herbicide resistance and changing production systems that included more corn and soybean favoured the “weediness” of this plant. This member of the pigweed (amaranth) family can rapidly establish patches and greatly decrease crop yield. The native habitat of waterhemp is wet, low-lying areas, but it is quite at home in reduced-tillage and no-till environments. Watch for patches to pop up along field edges or near field entrances, along ditches and waterways. The plant typically grows five to six feet tall, with glossy, hairless and more elongated leaves compared to redroot or smooth pigweed. Waterhemp is well adapted to warm growing temperatures and intense sunlight and is capable of producing up to a million seeds per plant that can germinate over the whole summer. Waterhemp has very tiny seed, (similar to redroot pigweed seed), so it is easily transported on equipment, with water and by wildlife, and can be a contaminant in seed lots.

Tall Waterhemp. photo: MARD

Removing small patches of waterhemp prior to seed set, and destroying the plants, is one of the most effective strategies to eradicate this weed. Tall waterhemp populations can be resistant to multiple herbicide groups, making it very hard to control this weed in field crops. Waterhemp samples in Manitoba have been tested and confirmed resistant to two herbicide groups. This has serious implications for local producers.

New populations have been discovered since it was first detected in Manitoba in 2019, with confirmed presence in eight municipalities by 2020.

Palmer amaranth

This is an aggressive, invasive weed native to the desert regions of the southwest U.S. and northern Mexico. It was accidentally introduced to other areas and has devastated crops in the south and Midwest U.S. as it rapidly became herbicide resistant. With the ability to emerge all season, grow two or three inches per day and set seed over the entire season this highly invasive weed can drastically reduce crop yield. Infestations have slowly moved north through contaminated seed, equipment, animal feed and bedding and the digestive tract of wild birds. Palmer amaranth was first detected in North Dakota in 2018, and is spreading throughout the state. Considered to be the No. 1 weed in the U.S., this plant can grow six to eight feet tall and produce one million seeds and heavy infestations can reduce soybean and corn yields by as much as 90 per cent.

A member of the amaranth family, like redroot pigweed and tall waterhemp, Palmer amaranth can be difficult to distinguish from its cousins. Smooth stemmed like tall waterhemp, its leaves are a little wider, more like redroot pigweed, but can be distinguished by the long petiole (stem-like structure that attaches the leaf to the main stem). Petioles of Palmer amaranth are longer than the leaves, while its cousins have shorter petioles. Long, snaky seed heads that can be up to two feet long are a distinctive feature of Palmer amaranth. Identification is crucial, and removal of individual plants and small patches is critical to prevent this weed from establishing in our province. Unconfirmed reports of individual plants in Manitoba have been noted, and we ask everyone to be diligent in identifying and reporting this weed.

Leafy spurge

This invasive perennial, first recorded in Manitoba in 1911, is a serious pest of forage and grazing land. A study presented in 2010 by the Rural Development Institute in Brandon estimated that leafy spurge infested over 1.2 million acres and caused a staggering annual economic loss (direct and indirect costs) of approximately $40 million. If we compare that to a 1999 report from the Leafy Spurge Stakeholders Group stating a $20-million annual impact and approximately 340,000 acres infested, we can see how much this problem had grown in 10 years and has likely grown in the most recent decade.

Leafy Spurge. photo: Michelle Ammeter

Unfortunately, leafy spurge has managed to gain a foothold in many places in southern Manitoba and numerous producers are familiar with the damage it can cause and the struggle to control it. In other areas of the province it is rarer and should be treated as a dangerous invader. Prevent its spread from the movement of contaminated hay or machinery. Early detection and rapid response to new patches will pay dividends as your first cost will always be your least cost with perennial invaders. All landowners have the responsibility to control this weed, either eradicating new infestations or stopping the spread from existing patches. Tier 2 weeds like leafy spurge, by legislation must be destroyed if patches are less than 20 acres while larger infestations must be controlled.

More information on invasive plants either threatening or already present in Manitoba can be found in the Noxious Weeds Act (C.C.S.M. c. N110) and the Noxious Weeds Regulation (Man.Reg.42/17), by contacting your local weed Ddistrict or at the MWSA website,

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