Kristen Obeid¹, Peter Sikkema², Christian Willemse², Nicole Langdon², David Westerveld ², Marie-Josée Simard³, Martin Laforest³, David Miville⁴ and Tammy Jones⁵

Waterhemp (Amaranthus tuberculatus var. rudis) is a relatively new weed in Canadian agriculture cropping systems¹. It has been suggested that it was introduced via a combine from the southern United States between the late 1990s and early 2000s in Ontario¹ and similarly to Quebec in 2017.

Waterhemp is a small seeded, summer annual, broadleaf weed with many traits which make it particularly troublesome in agriculture²:

1. Waterhemp can emerge throughout the entire growing season (in Ontario)
2. One female plant can produce up to 4.8 million seeds³ in a non-competitive environment
3. Waterhemp is dioecious (male and female plants), which results in vast genetic diversity contributing to rapid herbicide resistance evolution due to obligate outcrossing, unlike most other Amaranthus species¹
4. Waterhemp is extremely difficult to differentiate from other Amaranthus species, especially when small.  However, there are a few key differences, as illustrated in Figures 1 and 2:
   • Waterhemp cotyledons are often more egg-shaped than the long, linear cotyledons of other Amaranthus species.
   • The first true leaves of waterhemp are generally longer and more lance-shaped.
   • Waterhemp seedlings are hairless with leaves that look waxy or glossy.
   • Waterhemp stems are hairless.

Figure 1.  Amaranthus species seedlings

Photo credits: P. Smith and C. Shropshire

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Figure 2. Amaranthus species stems.

Photo credit: C. Shropshire

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Figure 3.  Waterhemp plant

Photo credit: C. Willemse

An Amaranthus genetic test has been developed to differentiate between 6 species (green pigweed, redroot pigweed, palmer amaranth, smooth pigweed, tumble pigweed and waterhemp), and can be used if identification is uncertain.  A pigweed species identification guide has also been developed.  Contact kristen.obeid@ontario.ca for either resource.

The first case of glyphosate-resistant waterhemp was discovered in Lambton county in 2014.  This population was the first confirmed glyphosate-resistant waterhemp in Canada.  Since then, waterhemp has been found in 11 Ontario counties, 7 of which have fields with populations that are 4-way resistant to herbicide groups 2, 5, 9 and 14.  In 2019, the genetic test which differentiates Amaranthus species was instrumental in confirming new waterhemp populations in Ontario (25), Manitoba (7) and Quebec (8). Multiple-resistant waterhemp has now been confirmed in vegetable fields in Elgin and Norfolk (Groups 2, 9 & 14) counties. 

Figure 4. Waterhemp Resistance and Distribution in Ontario

Label Colour	Herbicide Resistance Groups
ORANGE	2, 9
BLUE	2, 5, 9
GREEN	2, 9, 14
RED	2, 5, 9, 14

Figure 5. Waterhemp Distribution in Canada

Waterhemp has the potential to become one of the most problematic weeds in Canada, as it is in the United States corn belt.  It is easily spread to new areas as a contaminant of seed or hay, as a hitchhiker on vehicles or equipment and through natural dispersal by water and wildlife⁴.  Once established, waterhemp will thrive in many agricultural production areas due to its prolific seed production, extended emergence pattern, high genetic diversity and its current documented herbicide resistance to Groups 2, 5, 9 and 14 (Ontario and Quebec) and Groups 2 and 9 (Manitoba).

References

¹ Costea, M., Weaver, S.E., and Tardif, F.J. 2005. the biology of invasive alien plants in Canada. 3. Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif. Can. J. Plant Sci. 85: 507-522.

² Schryver, M.G., Soltani, N., Hooker, D.C., Tranel, P.J., and Sikkema, P.H. 2017. Glyphosate-resistant Waterhemp (Amaranthus tuberculatus var. rudis) in Ontario, Canada. Can. J. Plant Sci. 92: 1057-1067.

³ Hartzler, R.G., Battles, B.A., and Nordby, D. 2004. Effect of common Waterhemp (Amaranthus rudis) emergence date on growth and fecundity in soybean. Weed Sci. 52L 242-245.

⁴USDA APHIS Weed Risk Assessment for Amarantus palmeri  (Amaranthaceae) – Palmer’s amaranth. 2020. https://www.aphis.usda.gov/plant_health/plant_pest_info/weeds/downloads/wra/amaranthus-palmeri.pdf

Authors

¹Ontario Ministry of Agriculture, Food and Rural Affairs, Harrow, ON, Canada

²Department of Plant Agriculture, University of Guelph, Ridgetown Campus

³Agriculture and Agri-Food Canada, Saint-Jean-sur-Richelieu Research and Development Centre, Saint-Jean-sur-Richelieu, QC, Canada

⁴Laboratoire d’Expertise et de Diagnostic en Phytoprotection, Ministère de l’Agriculture, des Pêcheries et de l’Alimentation du Québec, QC, Canada

⁵Manitoba Ministry of Agriculture, Carmen, MB, Canada

Acknowledgements

The authors gratefully acknowledge the contribution to this work from agronomists, extension personnel, technicians and students.  As well as, previous graduate students L. Benoit, B. Hedges and M. Schryver.