Palmer amaranth found in Ontario

It was only a matter of time before Palmer amaranth was found on a farm in Ontario. Although technically it was found back in 2007 along a rail line outside of Niagara Falls, surveillance of that area in the years following have never detected any plants since the initial finding. In late summer, an individual plant was found on a farm in Wellington County. The most valuable thing that can be done at this point is to know the process to identify any plants you suspect may be Palmer amaranth and destroy any plants before they produce seed.

Both neighbouring New York (2020) and Michigan States (2010) have found Palmer amaranth. In 2022, Manitoba found it in the rural municipality of Dufferin.

Table of Contents

    Why is finding Palmer amaranth bad?

    The main issue with Palmer amaranth is that resistant populations to 9 different modes of action have been documented (WSSA groups 2,3,4,5,6,9,10,14 and 27). Many populations are multiple resistant, and in 2021 a population that was resistance to 6 different herbicide modes of action (multiple resistant to: WSSA groups 2,5,6,9,14 and 27) was discovered in the United States (Source: This significantly limits options to manage this competitive broadleaf weed.

    Locations where populations have been found

    • Wellington county – at the edge of a corn field.

    What made the landowner suspicious?

    The landowner referred to “a weird looking pigweed” that just looked different than anything they had seen before. One’s gut instinct is usually a pretty good screening tool for when you should seek help on identifying either Palmer amaranth or waterhemp.

    Early detection makes it easier to remove plants

    In both cases the amount of plants was small and manageable. Any Palmer amaranth plants could be pulled or dug out to prevent any seed from being produced and dispersed.

    What are the key things to look for?

    There are 4 key items that lead us to the identification of Palmer amaranth and are highlighted below. This is consistent with what others have experienced. Dr. Lynn Sosnoskie (Cornell University) provides an excellent summary of the key diagnostic features associated with common pigweed species. Leaf tissue from these plants were also sent for DNA analysis to verify the visual identification.

    1) Leaf petioles that are longer than the leaf blade

    A leaf blade and petiole from a Palmer amaranth plant. Notice that the petiole is 11 cm long, while the leaf blade is 9 cm long. If you can find leaves that have petioles longer than the leaf blades, then this is a good indication that the plant is Palmer amaranth.
    If you fold the petiole at the base of the leaf blade, you can quickly determine if the petiole is longer than the leaf blade.

    2) Hairless stems

    The stems of Palmer amaranth are hairless. There can be a rough texture (the red lines on the above stem feel rough to the touch). Whiskers often exist at the leaf axils.

    3) Thick and tall stems

    Even when within dense populations, the stems of Palmer amaranth are noticeably thicker than other more common pigweed (e.g. Redroot or green pigweed).
    The stem of this Palmer amaranth plant was around 35mm in diameter, which was around twice the thickness of any redroot and green pigweed that was close to this plant.

    4) Diamond shaped leaves

    Leaf blades of Palmer amaranth tend to have a diamond-shaped outline.
    Palmer amaranth also appeared to have spiky whiskers at the leaf axials. It is uncertain as to how common this is but it is something that we have not observed with either waterhemp or green and redroot pigweed.

    Images from the field

    Management options

    In general, plants from the amaranth family are often not found in fall planted crops such as winter cereals (wheat, barley, triticale) and winter canola, as they create an environment that is not ideal for seed germination. Perennial forage crops (alfalfa) would provide the best opportunity to prevent germination of new seedlings and depletion of the seed bank through predation and other environmental stressors.

    Cover crops planted after crop harvest will general reduce the amount of pigweed species that germinate and are able to produce viable seed.

    For corn and soybean, the use of an effective pre-emergence herbicides is extremely important. It removes a significant amount of the first flush of Palmer amaranth seedlings and ensures that there is not a huge range in weed stage at the time of post-emergent applications.

    List of current herbicides labelled for the control of palmer amaranth

    ID resources

    Canadian Plant Health Council

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