Weeds to Watch: Invasive Pigweeds, Waterhemp and Palmer Amaranth
The first line of defense against waterhemp and palmer amaranth is proper identification. It is EXTREMELY difficult to identify waterhemp and palmer amaranth from other pigweed species especially as seedlings.
Currently, waterhemp has been found in five Ontario counties: Bruce, Chatham-Kent, Essex , Lambton and Middlesex. Since 2015, Dr. Peter Sikkema's research team (University of Guelph, Ridgetown Campus) has surveyed 76 sites across Ontario and have confirmed waterhemp with cross-resistance to Groups 2 (ALS inhibitors, e.g. Pinnacle, Prism, Pursuit), 5 (Photosystem II inhibitors, e.g. Gesagard, Sencor, Simazine, Sinbar), 9 (EPSP synthase inhibitor, e.g. glyphosate) and 14 (PPO-inhibitors, e.g. Authority, Aim, Chateau, Goal). Waterhemp is the first weed in Ontario with confirmed resistance to PP0-inhibitors (Group 14). The Group 14 resistant samples were all found in Essex County and within 25 kilometers of each other. Even more concerning is the fact that all of these samples have cross-resistance to Groups 2, 5 and 9 herbicides.
Palmer amaranth is not presently listed as being found in Canada, however, historically it has been found in scattered locations in Ontario: Forest - 1966, St. Thomas - 1978 and Niagara Falls - 2007. Palmer amaranth has been found in all of the U.S. states that surround Ontario and the Great Lakes (Figure 1). Palmer amaranth is native to the Sonoran desert of the Southwest United States and Northern Mexico. It is ideally adapted to thrive in dry and high heat conditions. The most recent survey (2016) conducted by the Weed Science Society of America (WSSA) lists palmer amaranth as the most troublesome weed in U.S. agriculture (field crops, fruits and vegetables).
Figure 1. Current documented sighting of Palmer Amaranth in the U.S. (2017) U.S. It is now also found in North Dakota.
Figure 2. Pigweed (Amaranth) species seedlings
- Palmer amaranth seedling - notched tip, no hairs, broad ovate shaped leaves, no waxy sheen.
- Waterhemp seedling - egg shaped cotyledons, notched tip, no hairs, narrow lanceolate leaves, waxy sheen.
- Redroot pigweed seedling - notched tip, small fine hairs, ovate shaped leaves. Green and smooth pigweed very similar.
Pigweed (Amaranth) Species | Growth Habit | Identification Tips |
---|---|---|
Green Pigweed (Powell amaranth) (Amaranthus powellii) |
Erect, branched 0.6-2.1 M |
|
Palmer Amaranth (Amaranthus palmeri) |
Erect 1-3 M |
|
Prostrate Pigweed (Amaranthus blitoides) |
Prostrate mat to 1 M across |
|
Redroot Pigweed (Amaranthus retroflexus) |
Erect,branched 0.6-2.1 M |
|
Smooth Pigweed (Amaranthus hybridis) |
Erect, branched 0.6-2.1 M |
|
Spiny Amaranth (Amaranthus spinosus) |
Erect to bushy 0.3-1.1 M |
|
Tumble pigweed (Amaranthus albus) |
Erect, bushy 0.3- 1 M |
|
Waterhemp (Amaranthus rudis) |
Erect 1-3 M |
|
Figure 3. Pigweed (Amaranth) species leaves and petioles: A. Palmer amaranth female plant, B. Common Waterhemp, C. Green pigweed, D. Redroot pigweed. This picture shows the longer petiole of Palmer amaranth. The petiole is longer than the leaf when folded over. (Photo credit: C. Shropshire)
Figure 4. Pigweed (Amaranth) species stems: A. Palmer amaranth female plant, B. Waterhemp, C. Green pigweed, D. Redroot pigweed (Photo credit: C. Shropshire)
If you suspect you have waterhemp or palmer amaranth on your farm please contact Kristen Obeid: kristen.obeid@ontario.ca or 519-738-1232. You can also contact Kristen to obtain a power point presentation identifying the various pigweeds for educational purposes.
Other resources
- Invasive Pigweeds: Palmer Amaranth and Waterhemp, Penn State Extension
- Palmer Amaranth Biology, Identification, and Management, Purdue Extension
- Identification of the weedy pigweeds and waterhemps of Iowa, Iowa State University Extension
For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: | Kristen Obeid - Weed Management Specialist - Horticulture/OMAFRA; Dave Bilyea - Weed Management - Horticulture/University of Guelph |
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Creation Date: | 01 August 2018 |
Last Reviewed: | 01 August 2018 |