Aztec's revenge: the superweed Palmer amaranth
After having defeated the Aztecs with a clever 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 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 it 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 U.S. In the late 1980s it appeared in the Carolinas, and by 1995 was labeled the worst 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 height and very fast growth rate, it chokes out soy and corn, lowering crop yields between 70 and 90 percent.
Even a stressed plant will produce 250,000 seeds, and one in good growing conditions will make around a million seeds, and possibly up to 1.8 million. This problem might sound like an opportunity to raise Palmer amaranth as a food crop, but we need to be clear that it would become our only crop. Ever.
If you’re 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 it leaves. The stems are smooth, without any fine hairs, and the petioles, or leaf stems, are longer than the leaves themselves, neither of which 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 animal feed, so it could show up anywhere. If you think you may have spotted this novel weed, please contact the New York State Department of Environmental Conservation at [email protected] or contact your Cornell Cooperative Extension office. Mechanical cultivation, flame-weeding and hand-pulling are the only effective means of control. Cover crops such as winter rye interfere with Amaranth germination. With a plant that can beat the conquistadors, we don’t want to mess around.
