Any plant that grows where you don’t want it is a weed. And farmers across the Corn Belt are on high alert for Palmer amaranth and waterhemp — two pigweeds that, left to grow, can double or even triple future weed control costs and outcompete crops.
That’s why Annie Klodd, Extension weed control specialist at Penn State University, put out a warning last week about these notorious, invasive pigweeds. She urges scouting for these emerging weeds now, and controlling them before they reach 4 inches tall.
That’s a real challenge. Once they emerge, Palmer amaranth and waterhemp can top 4 inches in a few days of warm weather. That’s why control programs should include multiple modes of action — and catch these plants at their most vulnerable stage.
Both pigweeds are highly competitive, with widespread resistance to glyphosate and Group 2 (ALS-inhibitor) herbicides, which is why they’re spreading so fast. They’re confirmed on over 40 farms across Pennsylvania, with highest concentrations in southeastern counties.
Not just a field problem
Cropland isn’t the only place these weeds can gain entry to your farm. “Palmer and waterhemp growing in non-production areas are certainly a concern,” points out Klodd. “Any seeds that fall from those plants can easily spread to surrounding fields or get carried away by vehicles.”
Be on the alert for them in roadside ditches and fence lines, plus drainage ditches, lanes, driveways and around buildings. “We just saw an instance of this last year where a farmer had brought in feed containing Palmer and waterhemp seeds. Some of the feed was poured out near his barns, and he discovered Palmer and waterhemp popping up.”
He sprayed them before the weeds produced any new seed. But some remaining seeds germinated this spring, too. Several plants also sprouted up in a drainage ditch, and he hand-pulled them.
“We recommend landowners regularly mow ditches, fence lines, etc., to help prevent spread,” she concludes.
How to spot them
Very young Palmer amaranth and waterhemp may be identified by carefully examining their stems and leaves. See photos.
University of IllinoisPALMER AMARANTH: This seedling has a notched tip; no hairs; broad, ovate-shaped leaves and no waxy sheen.
University of IllinoisWATERHEMP: This seedling has egg-shaped cotyledons, notched tips, no hairs and narrow lanceolate leaves with waxy sheen.
Penn State also has compiled many resources onto a website, including identification, management and timely updates on these pigweeds. It includes a five-part video series on identification and management, identification photos, plus a one-page flier highlighting key points about the species. The Integrated Weed Management Resource Center also is valuable resource on invasive pigweeds.