Weed of the week: Redroot pigweed

Economic thresholds run from five to 15 plants per 10 metres of row

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Redroot pigweed can be a tough customer. But don’t wallow in despair – control is possible.

Redroot pigweed (Latin name Amaranthus retroflexus) is an annual weed found throughout Ontario in cultivated fields, gardens, pastures, waste places, roadsides and, according to the Ontario Ministry of Agriculture, Food and Rural Affairs, nearly any other disturbed area.

While similar to other types of pigweed, mature redroot pigweed can be distinguished by its comparatively tall physiology and red taproot. It generally grows to a height of 50 to 90 centimetres (20 to 36 inches), although it can get as high as two metres, and features large, broad leaves. Flowers appears as thick branching clusters, with the plant overall showing a dull green colour. Flowering occurs around July and August, and it reproduces by seed only.

Despite its differences, identifying redroot pigweed among other pigweed species can be difficult. It can also hybridize with other pigweed varieties, though the risk posed by hybrid plants is low because they are generally sterile.

Pigweeds in general reduce crop yields through competition for light, water and nutrients.

OMAFRA reports economic thresholds for pigweed emerging simultaneously with corn and soybeans vary from five to 15 plants per 10 metres of row, depending on herbicide cost and crop price.

Pigweed, including redroot pigweed, can accumulate nitrates in the stem and branches in concentrations high enough to poison livestock, particularly if in cattle silage.

The weed is also an alternative host for crop pests such as the green peach aphid, tarnished plant bug, European corn borer, flea beetle, cucumber mosaic virus, and some strains of fungus.

Varied herbicide resistance

Like its geographical range, herbicide-resistant redroot pigweed varieties vary wildly.

According to currently available OMAFRA maps, Group 2 resistance (namely to Pursuit and Classic) exists in 15 counties, from Essex to Parry Sound and Glengarry. Of those 15 counties, resistance to Group 5 herbicides (Aatrex and Sencor) has appeared in four, Group 6 herbicides (Basagran and Pardner) in two, and Group 7 herbicides (Lorox) in four. Hardest hit are Chatham-Kent farmers, who now contend with resistance to all these herbicide groups.

Multiple modes of action and tank-mixes are key in chemical-based management. OMAFRA’s 2019 Guide to Weed Control in Field Crops lists the following active ingredients as possible redroot pigweed controls:

  • Fomesafen (Reflex), provides suppression. An adjuvant (Turbocharge) must be added to Reflex to achieve control.
  • Fomesafen plus glyphosate provides residual control.
  • Saflufenacil (Eragon), when tank-mixed with glyphosate and applied to corn either before or after planting (but before crop emergence) provides residual control.

Other control factors

Though short-lived, the seeds of redroot pigweed are produced in the tens of thousands. Controlling the weed before it seeds greatly reduces pressure on crops. It’s also much easier than controlling mature pigweed plants.

Redroot pigweed seeds generally cannot emerge if covered by more than six inches of soil. Thus, deeper tillage has been shown to be an effective control. Tillage can be particularly effective, even if only performed once, in heavily infested areas. If performed in the fall, too, redroot pigweed can be prompted to emerge earlier, providing more time for pre-planting control.

In addition, University of Missouri research shows cultural practices such as narrower rows can help alleviate pigweed growth (due to a thicker crop canopy), as can cover crops like cereal rye.

OMAFRA resources state both herbicide and crop rotation, and following other integrated weed management practices, is critical in preventing the spread of herbicide resistant redroot pigweed.

About the author


Matt McIntosh

Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.



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