Wednesday, September 25, 2019

Palmer Amaranth Is Not Your Grandparents’ Pigweed

Amaranthus palmerii courtesy Wikimedia user PompilidAfter having defeated the Aztecs with a fusion of horses, steel, smallpox, and a stunning lack of moral conscience, the Spanish conquistadors wasted no time outlawing amaranth, a grain which constituted most of the Aztec diet at the time. Known to gardeners and farmers these days as pigweed, amaranth has obviously continued to flourish in spite of that military decree.

There are 70 recognized amaranth species, several of which are grown commercially from Mexico south to northern Peru. It is a very nutritious grain, high in protein, magnesium, iron, phosphorous, and selenium, and is eaten roasted or cooked in water. When young, its leaves can be used as a cooked green much like spinach. Amaranth is also grown ornamentally, with a number of varieties available with red, purple, or yellow flower spikes.

As every gardener knows, pigweed makes loads of tiny seeds, and the old adage “One year of seeds gives you seven years of weeds” could have been inspired by this noxious plant. Years ago I allowed some lamb’s-quarters and pigweed to grow in the garden to use as greens, with the intention of pulling them before they set seed. I think you know how that story ended.

Due to the heavy reliance on herbicides for weed control, particularly in cotton, we now have a variety of amaranth considered by many agronomists as the worst weed in the United States. Palmer amaranth (Amaranthus palmeri S. Watson), a.k.a. Palmer pigweed or careless-weed, has become impervious to nearly all herbicides, certainly all types which are economically viable for farmers. Reportedly, some cotton farmers have gone back to hand-weeding, at great cost.

Palmer amaranth is native to parts of Mexico and the American Southwest, but slowly made its way across the US. In the late 1980s it appeared in the Carolinas, and by 1995 was labeled the most dangerous weed in that region. Though once believed a heat-loving plant, it seems quite at home in North Dakota and Minnesota right now. This annual weed is often in the 6’ to 8’ height range, but can be 10 or more feet tall. Because of its fast growth rate and altitude at maturity, it chokes out soy and corn, lowering crop yields between 80 and 90 percent.

Even a stressed Palmer amaranth plant will produce 250,000 seeds, and one growing in good conditions will make around a million seeds, possibly up to 1.8 million. This might sound like an opportunity to raise it as a food crop, but we need to be clear that it would be our only crop.

Ever.

If you become lost in a field of 10-foot-tall amaranth, undoubtedly it will occur to you that those are not your grandparents’ pigweed. In the seedling stage, though, all amaranth species look very similar. Palmer amaranth sometimes has a V-shaped pigment pattern on its leaves. The stems are smooth, without any fine hairs, and the petioles or leaf stems are longer than the leaves themselves. Neither situation is the case with redroot pigweed seedlings. Mature leaves are mucronate (which I mention for the Scrabble players out there), meaning the midrib sticks out at the tip of the leaf like a mini-thorn. Also, seedhead spikes are much longer than in any other pigweed, typically between 2 and 3 feet long.

Palmer amaranth seeds are spread through seed mixtures, vehicles and tillage equipment, and animal feed, so it could show up anywhere. Mechanical cultivation, flame-weeding and hand-pulling are the only effective means of control. Cover crops such as winter rye interfere with amaranth germination. If you think you may have spotted this novel weed, please contact the New York State Department of Environmental Conservation at isinfo@dec.ny.gov or contact your Cornell Cooperative Extension office. With a plant able to beat the conquistadors, we don’t want to mess around.

Photo of Amaranthus palmerii courtesy Wikimedia user Pompilid.

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A Canton, NY-based arborist, educator and writer, Paul Hetzler had intended to be a bear when he grew up, but failed the audition. He settled for an educator position instead, and serves as Horticulture and Natural Resources Educator at Cornell Cooperative Extension of St. Lawrence County.

His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine. He is the author of Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World.

You can reach Paul at the Cornell Cooperative Extension office in Canton at (315) 379-9192.




One Response

  1. Martie says:

    Paul – I love your columns and would like to see you write an article about Japanese knotweed, which is running rampant from Maine to Lewis County, NY, crowding out more delicate native vegetation at an alarming rate. It’s the worst invasive I’ve ever had to deal with – nearly impossible to eradicate, unless it’s spotted in its early stages.

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