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How to Identify Palmer Amaranth

If you have waterhemp in your fields and are bracing yourself for a Palmer amaranth infestation, you’re not alone. Glyphosate-resistant biotypes of this pugnacious pigweed family member have migrated from the southwest into the Mid-South and have penetrated as far north as Michigan. 

Identification is the first step in fighting it. It’s hard because Palmer amaranth can closely resemble other pigweed family members like waterhemp and redroot and smooth pigweed. Here are some of its hallmark signs, according to Travis Legleiter and Bill Johnson, weed scientists at Purdue University, and Kevin Bradley, University of Missouri weed scientist. 

• Palmer amaranth plants sometimes have a white chevron- or V-shape watermark on their leaves. (See story photo.) Palmer amaranth doesn’t always have a watermark, but if it does, it rules out other pigweed family members.  

• Stems without hairs are another characteristic of Palmer amaranth. The bad news, though, is that waterhemp doesn’t have any hairs, either. However, it’s a way to rule out the redroot and smooth pigweeds that have stem and leaf surface hairs.

• Leaf shapes on Palmer amaranth leaves are wider and ovate- to diamond-shape. Meanwhile, waterhemp leaves are generally long, linear, and lanceolate. Redroot and smooth pigweed leaves are similar to Palmer amaranth leaves, with a round to ovate shape. However, they have leaf hairs, while Palmer and common waterhemp do not.

Mudding up the works is that plants that have been sprayed and survive multiple herbicide applications can exhibit variable leaf shapes that may not correctly represent Palmer amaranth.

• Palmer amaranth petioles — especially on older leaves — will be as long or longer than the leaf blade itself. Meanwhile, waterhemp petioles will be shorter than their long and lance-shape leaves.

• Poinsettias aren’t just for Christmas. They’re akin to Palmer amaranth’s rosette-like appearance with a top-down look. Meanwhile, waterhemp’s short petioles and long linear leaves form a less patterned rosette appearance. 

• Heads of Palmer amaranth females can extend up to 3 feet. Their seed heads also feature stiff, sharp bracts that give them a prickly feeling when touched. Meanwhile, waterhemp’s multiple-branched seed heads are similar in length and lack the stiff prickly bracts. 

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