Amaranthus is a large genus of about 70 species, and some of them are among the most aggressive agricultural weeds on the planet, while others are cultivated as nutritious grain and leafy vegetable crops. So the answer depends entirely on which species you’re talking about and where it’s growing. The same genus that ancient civilizations relied on as a dietary staple now includes plants that can destroy up to 91% of a corn or soybean harvest.
Why Some Amaranthus Species Are Weeds
Several amaranthus species are classified as serious weeds, particularly in farming systems. The most notorious is Palmer amaranth, which the USDA describes as “one of the most widespread, troublesome, and economically damaging agronomic weeds in the southeastern U.S.” Other weedy species include redroot pigweed and waterhemp, both common problems in row crops across North America.
What makes these plants so problematic is a combination of biological traits that let them outcompete crops. They use a type of photosynthesis called C4, the same system found in corn and sugarcane, which lets them fix carbon dioxide more efficiently than most broadleaf plants. This translates to rapid growth in hot, dry conditions where other plants struggle. C4 plants waste less water per unit of growth, giving weedy amaranth a built-in advantage during droughts and heat waves.
A single female Palmer amaranth plant can produce between 100,000 and 500,000 seeds in one season, and those seeds stay viable in the soil for three to five years. That kind of reproductive output means even a small number of plants left unchecked can create a massive seed bank that persists for years.
How to Identify Weedy Amaranth
Palmer amaranth, the most problematic species, has diamond-shaped leaves with smooth to wavy edges. A white or purple V-shaped marking (called a chevron) sometimes appears on the leaf surface. The most reliable identification feature is the leaf petiole: on older leaves, the stalk connecting the leaf to the stem is as long as or longer than the leaf blade itself. This distinguishes it from waterhemp, which has longer, narrower leaves that lean more oval-shaped.
The flower spikes are hard to miss. Palmer amaranth produces terminal seed heads that can reach two to three feet long, sometimes more. These dense, cylindrical spikes tower above the rest of the plant and are a clear visual giveaway during the growing season.
The Herbicide Resistance Problem
Weedy amaranth species, Palmer amaranth in particular, have developed resistance to nine different classes of herbicides. This includes glyphosate, the active ingredient in Roundup and the most widely used herbicide in modern agriculture. In resistant populations, about half the plants survive a standard glyphosate application that would kill 100% of non-resistant plants.
The mechanism behind glyphosate resistance in Palmer amaranth is striking. Rather than developing a mutation that blocks the herbicide, these plants carry extra copies of the gene that glyphosate targets. Researchers have found anywhere from 13 to 123 copies of this gene in resistant individuals, essentially overwhelming the herbicide through sheer biological redundancy. On top of that, eight different mutations have been identified that confer resistance to another major herbicide class, and some populations carry resistance to multiple herbicide types simultaneously.
This resistance is spreading geographically. Palmer amaranth is native to the American Southwest and Mexico, but it has expanded aggressively into the eastern United States and naturalized in Australia, Spain, Portugal, India, Sweden, Romania, and possibly Canada.
Crop Damage From Weedy Amaranth
The economic toll is severe. Yield losses of 70 to 91% have been documented in corn, soybeans, peanuts, dry beans, and sugar beets when Palmer amaranth infestations go unmanaged. The plants grow fast enough to shade out crops, and their root systems compete aggressively for water and nutrients.
Farmers managing resistant populations have to rely on integrated strategies rather than herbicides alone. Cover crops can reduce pigweed density by roughly 50% in many cases. Row-crop cultivation (mechanical weeding between rows) significantly cuts weed biomass in most growing seasons. Planting crops in narrower rows also helps: switching from standard 76-centimeter row spacing down to 19 centimeters reduced late-season weed growth by about 46%, because the crop canopy closes faster and blocks light from reaching weed seedlings.
Amaranthus as a Crop and Food Source
The same genus that produces some of the world’s worst weeds also includes species that have been cultivated as food for thousands of years. Amaranth grain was first domesticated 6,000 to 8,000 years ago in South and Central America and became a staple for pre-Columbian civilizations, including the Aztecs. It held both dietary and religious significance: the Aztecs built statues of their deities using amaranth grain mixed with honey, which were worshipped, then broken apart and eaten.
When the Spanish arrived under Cortez, they outlawed amaranth as part of their campaign to suppress indigenous religious practices. Fields were burned and cultivators were punished. The grain survived only in a few remote areas, which is why it remained relatively obscure for centuries before its recent revival as a health food.
Cultivated amaranth leaves contain roughly 39 grams of protein per kilogram of fresh weight, along with meaningful amounts of iron. The grain is notable for being high in protein compared to other grains, with a particularly strong amino acid profile. Both leaves and seeds are eaten in many parts of Africa, Asia, and Latin America today.
Weed or Crop Depends on Context
The line between weed and crop in the amaranthus genus is entirely contextual. A plant growing in a soybean field in Georgia that shrugs off herbicide applications and produces half a million seeds is unambiguously a weed. The same genus, cultivated in a garden or small farm for its grain and greens, is a resilient, nutritious food source that thrives in heat and drought precisely because of those same C4 traits that make its weedy cousins so hard to kill.
Even within a single species, the classification shifts. Palmer amaranth is native to the desert Southwest, where it’s part of the natural ecosystem. In the Southeast, where it was introduced, it’s an invasive agricultural pest. The biology doesn’t change. What changes is whether that biology is working for you or against you.