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October 14, 2026 - The Amaranths Are Advancing

Recognizing Palmer amaranth and waterhemp before they reach Alberta fields.

Many producers and landowners are familiar with a troublesome group of weeds commonly called pigweed. However, pigweed is a general term for plants in the amaranth genus, and invasive species such as Palmer amaranth can be overlooked by anyone unfamiliar with its distinguishing features.

Palmer amaranth and waterhemp are emerging threats moving north from the United States into prairie field crops. Like many invasive plants, they can spread when seeds are carried through irrigation channels, transported on contaminated equipment, or accidentally included in feed mixes. Natural dispersal also contributes to their spread, as seeds can travel by wind or be carried by migrating birds and other wildlife.

Although most Canadian reports of these two species have come from northwestern regions, Alberta and neighbouring areas are not immune. If Palmer amaranth or waterhemp is introduced and becomes established, the consequences could be serious for Alberta producers. In the United States, these weeds have caused significant yield losses, including a reported loss of up to 91 per cent in Kansas (Agriculture Canada), and they are extremely difficult to control. Palmer amaranth can produce 500,000 seeds per plant and twice that number under ideal conditions. It can also accumulate high nitrate levels and may be toxic to cattle when consumed in large quantities.

Both waterhemp and Palmer amaranth are prolific growers, capable of growing five to eight cm per day, and both are resistant to multiple herbicide groups. This combination of rapid growth and herbicide resistance makes them exceptionally difficult to control. Current control options are mainly cultural and include prescribed burning, weed torches, and hand-pulling. Research underway in Saskatoon is examining whether commonly grown Prairie field crops, such as wheat, can suppress these invasive weeds. The study uses grain amaranth as a proxy weed to test whether competitive crops and higher seeding rates can help prevent Palmer amaranth and waterhemp from becoming established.

Early detection begins with learning how to identify Palmer amaranth and waterhemp.

Young Palmer amaranth plants can be recognized by their poinsettia-like leaf arrangement. Female plants can also be identified by green, spiny, thorn-like bracts at the base of the petiole, while male plants have long, drooping flowering structures. On both male and female plants, the flowers form cylindrical spikes that extend from the central stem and can reach three feet in length. Because Palmer amaranth is dioecious, with separate male and female plants, it can develop greater genetic diversity and traits that help populations thrive, including herbicide resistance.

Like Palmer amaranth, waterhemp is dioecious, with separate male and female plants. Mature leaves are tapered and have pointed tips, while the smooth stems range in colour from red and purple to green. Leaves are arranged alternately along the stem. Only female waterhemp plants produce seeds, which develop in long, spiky, clustered seed heads.

The best way to catch these invaders early is to stay alert, understand how they spread, and learn to identify them. A small infestation is much easier to control than one allowed to become established.