Weeds to Watch: Invasive Pigweeds – Waterhemp and Palmer Amaranth

Kristen Obeid, OMAFRA Weed Management Specialist – Horticulture

Dave Bilyea, Weed Management – Horticulture, University of Guelph, Ridgetown Campus

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.

Table 1. Key Tips to Identify Waterhemp and Palmer amaranth Compared to Other Pigweed Species

Pigweed (Amaranth) Species	GrowthHabit	Identification Tips
Green Pigweed (Powell amaranth) (Amaranthus powellii)	Erect, branched0.6-2.1 M	First true leaves narrower and more tapered toward tip than redroot or smooth; plant may be slightly hairy but less than redroot.
Palmer Amaranth (Amaranthus palmeri)	Erect 1-3 M	Male and female plants ( dioecious) Male seed heads soft (similar to smooth pigweed) Female seed heads spiny Plants are virtually hairless with very long petioles Occasionally a dark red/purple or white water or “V” mark or patch on leaf blade Leaves can sometimes have a single short hair at the tip of the leaf blade As plants become older, they can often assume a poinsettia-like appearance Plants can grow up 7 cm in a day
Prostrate Pigweed (Amaranthus blitoides)	Prostrate mat to 1 M across	Thin shiny leaf blades can form dense circular mats Stems fleshy and pliable Leaves are paddle shaped Often confused with purslane and spurge species
Redroot Pigweed (Amaranthus retroflexus)	Erect,branched 0.6-2.1 M	Upper stems and leaves usually covered with fine hair Leaf margins tend to be wavy Older leaves form a diamond shape
Smooth Pigweed (Amaranthus hybridis)	Erect,branched 0.6-2.1 M	Similar to redroot pigweed may hybridize with closely related species Leaves are hairless, upper stems densely hairy Leaves less wavy than redroot Seed head more branched than redroot or green
Spiny Amaranth (Amaranthus spinosus)	Erect to bushy0.3-1.1 M	Pair of stiff, sharp 0.25 cm spines at base of each leaf; stems smooth, hairless, often reddish Maybe mistaken for female Palmer amaranth
Tumble pigweed (Amaranthus albus)	Erect, bushy0.3- 1 M	Leaves are usually spatulate, smaller than other pigweed species, bushy and branched Plants break off at the base and roll in the wind dispersing seeds
Waterhemp (Amaranthus rudis)	Erect 1-3 M	Male and female plants (dioecious) Plants develop paddle shaped leaves Plants are virtually hairless Plant resembles redroot and green pigweed as a seedling

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.

Figure 3. Pigweed (Amaranth) species leaves and petioles

This picture shows the longer petiole of Palmer amaranth.  The petiole is longer than the leaf when folded over.

Figure 4. Pigweed (Amaranth) species stems

Palmer amaranth       Waterhemp          Green pigweed           Redroot pigweed

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 https://extension.psu.edu/invasive-pigweeds-palmer-amaranth-and-waterhemp

Palmer Amaranth Biology, Identification, and Management, Purdue Extension
https://www.extension.purdue.edu/extmedia/ws/ws-51-w.pdf

Identification of the weedy pigweeds and waterhemps of Iowa, Iowa State University Extension
https://store.extension.iastate.edu/product/Identification-of-the-Weedy-Pigweeds-and-Waterhemps-of-Iowa