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Keep an Eye out for New Pigweeds

By Clark Brenzil, PAg., Provincial Specialist Weed Control, Regina

July 2022

Waterhemp and Palmer amaranth are two new pigweed species that have already been found in Manitoba. Currently, waterhemp can be found in northern counties in North Dakota that share a border with Saskatchewan as well as in northeastern counties in Montana. Palmer amaranth can be found in several counties in North Dakota including Ward County surrounding the city of Minot.

Waterhemp and Palmer amaranth
Waterhemp (right) and Palmer amaranth have no hair compared to other pigweeds like redroot pigweed (left). Photo: Manitoba Agriculture

Waterhemp and Palmer amaranth have many similarities but also have key differences. Both are members of the larger pigweed family, that includes more common pigweeds such as redroot, tumble, prostrate, green (or Powell amaranth) and smooth pigweeds. Both waterhemp and Palmer amaranth have smooth hairless stems, while the other pigweeds have hairy stems to varying degrees. Most plants have both male and female reproductive organs on the same plant, but both waterhemp and Palmer amaranth are dioecious plants that have separate male and female plants.

Being dioecious also means that waterhemp and Palmer amaranth are very genetically diverse, which has led to the final trait they both share, prolific resistance to many herbicide groups. Both waterhemp and Palmer amaranth have developed resistance to Groups 2, 5, 14, 15, 27 and glyphosate (Group 9). Palmer amaranth has also demonstrated resistance to Group 3, 4 and 10 (glufosinate) herbicides. This tendency to develop herbicide resistance to a broad range of herbicide chemistries is what makes the plant such a concern.

Waterhemp with shiny elongated leaves.
Waterhemp has shiny, elongated leaves that can be broader towards the base. Lynne Sosnoskie, Cornell University.

The key visual characteristics of waterhemp and Palmer amaranth are strait forward. Most pigweeds will have roughly egg-shaped leaves that are about two to 2.5 times as long as wide, where waterhemp has more elongated leaves that have a length to width ratio of about four to one. Waterhemp leaves are also shiny because of the lack of hair. Palmer amaranth has a leaf stalk (petiole) that is longer than its diamond-shaped leaf blade. Sometimes the leaf blade of Palmer amaranth will have a light or dark chevron pattern on it.

Flowering structures are less definitive and harder to distinguish between waterhemp and Palmer amaranth, but both are tall spindly spikes with multiple branches that extend well above the leaf canopy. Each female plant can produce upwards of 250 thousand of seeds. Waterhemp grows at 1.5 times the rate of redroot pigweed and the growth rate of Palmer amaranth is more than double that of redroot pigweed. As a result, these new pigweeds are more competitive with crops and because they are C4 type plants, this competitive ability is even greater when it is hot.

Palmer amaranth with leaves where the stalk is longer than the blade.
Palmer amaranth has leaves where the stalk is longer than the blade. Lynne Sosnoskie, Cornell University.

Saskatchewan Association of Rural Municipalities Plant Health Officers (PHOs) and the Ministry of Agriculture provincial specialist weed control, will be conducting a roadside survey of fields in southeastern and south-central Saskatchewan for primarily waterhemp and potentially Palmer Amaranth as was done in 2020. So, if you see someone looking out into your soybean or lentil field with binoculars, stop and say hello and we can discuss pigweeds and other weed issues. If you get a call from Saskatchewan Crop Insurance Corporation, indicating that the provincial weed specialist would like to talk to you, it means we have seen something suspicious and would like a closer look. If we confirm something of concern, we can help get rid of it before it spreads.

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