What Is Palmer Amaranth and Why Is It a Problem?

Palmer amaranth (Amaranthus palmeri) is a broadleaf weed native to the desert Southwest of the United States that has become one of the most economically damaging agricultural weeds in the country. Its combination of explosive seed production, rapid growth, and an extraordinary ability to evolve resistance to herbicides makes it a threat that no single chemical or tillage pass can reliably control. What started as a regional desert plant has spread across the Midwest and Southeast, forcing farmers to spend dramatically more on weed management while still losing significant portions of their harvests.

A Desert Native That Went Mainstream

Palmer amaranth is an annual plant originally found in the Sonoran Desert region, spanning southern California, Arizona, and parts of northwestern Mexico. It belongs to a large genus of pigweeds, but it stands apart from its relatives in several ways. Unlike most weeds that farmers deal with, Palmer amaranth is dioecious, meaning individual plants are either male or female rather than carrying both reproductive parts on the same plant. Recent genomic work has pinpointed a roughly 2.84-megabase region on chromosome 3 that is specific to male plants and likely drives sex determination.¹ That dioecious biology forces outcrossing between plants, which shuffles genetic diversity with every generation and gives the species a deep well of variation to draw from when facing new selection pressures like herbicides.

The plant can grow two to three inches per day under favorable conditions, reaching heights well above a person’s head by the end of summer. Research on growth under elevated carbon dioxide found that Palmer amaranth grown at 750 parts per million CO₂ was about 15% taller, had 10% more leaf area, 18% more stem dry matter, and nearly 30% more overall plant volume compared to plants grown at current ambient CO₂ levels around 410 ppm.² That finding is sobering because it suggests the plant is well positioned to thrive as atmospheric CO₂ continues to rise.

The Seed Problem

A single female Palmer amaranth plant can produce a staggering number of seeds. Growing alone without crop competition, one plant can generate up to 600,000 seeds. Even in a crop canopy where it is fighting for light and nutrients, a single female plant routinely produces more than 100,000 seeds.³ Research in dry edible bean found that seed output per plant ranged from about 91,000 to 376,000 depending on how crowded the Palmer amaranth plants were, with the highest per-plant production at the lowest weed density. Even at higher densities, individual plants still churned out tens of thousands of seeds.

Those seeds are tiny, roughly the size of a grain of sand, and they can persist in the soil for years. Germination is heavily influenced by temperature: lab work showed that alternating day/night temperatures of 35°C/25°C produced germination rates around 83%, while cooler alternating cycles of 25°C/15°C still yielded about 56%. At temperature extremes, whether very hot (45°C/35°C) or cold (15°C/5°C), almost no seeds germinated.⁴ Soil moisture matters too: seedling emergence peaked at 85% when soil was at full or three-quarter field capacity, but dropped to 37% when moisture fell to half capacity. Salt and osmotic stress also suppress germination, but under the warm, well-watered conditions typical of Midwestern cropland in spring, a huge fraction of that seed bank is ready to emerge. The sheer volume of seeds means even a small surviving percentage translates into a massive infestation the following year.

What It Does to Crop Yields

Palmer amaranth competes aggressively for light, water, and nutrients. Its height advantage lets it overtop most row crops, shading out soybeans, cotton, and corn. In soybean fields in the Northern Great Plains, researchers documented yield losses of 33% once Palmer amaranth density exceeded about 15 plants per square meter.⁵ Losses at lower weed densities were often worse than expected from simple models, meaning even a few Palmer amaranth plants scattered through a field can drag yields down significantly.

Cotton, historically the crop most plagued by Palmer amaranth in the Southeast, suffers similar or worse losses. The thick, woody stems of mature plants can even damage harvesting equipment, forcing farmers to slow down or hand-pull weeds before harvest. That physical toughness compounds the competitive damage: it is not just a weed that steals yield, but one that can bring machinery to a halt.

Glyphosate Resistance Through Gene Amplification

The feature that transformed Palmer amaranth from a serious nuisance into an agricultural emergency is its capacity to resist glyphosate, the active ingredient in Roundup and the backbone of weed control in herbicide-tolerant crops for decades. The resistance mechanism is unusual and, frankly, remarkable. Rather than mutating the target enzyme so that glyphosate no longer binds to it, Palmer amaranth simply makes vastly more of the enzyme than glyphosate can knock out.

Glyphosate kills plants by blocking an enzyme called EPSPS that is essential for making certain amino acids. In resistant Palmer amaranth, the gene encoding EPSPS has been amplified across the genome. Early work on a Georgia population found that resistant plants carried anywhere from 5 to more than 160 extra copies of the EPSPS gene compared to susceptible plants, and the gene copies were scattered across every chromosome.⁶ More copies of the gene meant more messenger RNA, which meant more EPSPS protein, which meant the plant could keep producing amino acids even while glyphosate was busy inhibiting a portion of the enzyme pool. Follow-up research estimated that between 30 and 50 genomic copies of EPSPS were needed for a plant to survive standard field rates of glyphosate.⁷

This amplification-based resistance has spread widely. A population studied in Connecticut, far from the weed’s desert origins, showed 33 to 111 relative copies of the EPSPS gene, with no point mutations in the enzyme’s active site that would otherwise explain survival.⁸ Genomic analysis has also identified extrachromosomal circular DNA (eccDNA) carrying EPSPS copies in Palmer amaranth, a mechanism previously associated mainly with drug resistance in cancer cells and rarely documented in plants.¹ The gene amplification is heritable, so once it appears in a population, it passes efficiently to the next generation through the obligate outcrossing that dioecious reproduction demands.

Resistance Far Beyond Glyphosate

If glyphosate resistance were the only issue, farmers could switch to alternative herbicide classes. But Palmer amaranth has proven disturbingly capable of stacking resistance to multiple herbicide families at once. A Kansas population was documented with resistance to five different herbicide modes of action simultaneously: roughly 12-fold resistance to glyphosate, 14-fold to atrazine, 13-fold to mesotrione, 5-fold to chlorsulfuron, and 3-fold to 2,4-D, along with reduced sensitivity to fomesafen.⁹

Researchers in Kansas later confirmed a population with six-way resistance, surviving field-recommended rates of herbicides spanning ALS inhibitors, photosystem II inhibitors, EPSPS inhibitors (glyphosate), PPO inhibitors, HPPD inhibitors, and the synthetic auxin 2,4-D. This was the first report of six-way resistance in any single Palmer amaranth population.¹⁰ The predominant mechanism behind most of these resistances was metabolic, meaning the plant’s internal detoxification enzymes break down the herbicides before they can do damage. Metabolic resistance is especially worrisome because it can provide cross-resistance to herbicides the plant has never been exposed to, since the same enzyme families can deactivate many different chemical structures.

How It Spreads to New Territory

Palmer amaranth’s rapid geographic expansion across the United States, from a plant confined to the arid Southwest to an invader of Midwestern grain fields and even Northeastern farmland, has multiple drivers. Contaminated seed is a major vector. Feed grain, cover crop seed, and conservation seed mixes can all carry Palmer amaranth seeds as unintended hitchhikers. Because the seeds look nearly identical to those of related pigweed species, visual identification during routine seed testing is unreliable. Minnesota’s Department of Agriculture classified Palmer amaranth as a prohibited noxious weed seed in 2016, requiring genetic testing of any Amaranthus seeds found as contaminants during seed inspections.¹¹

Wildlife also plays a role. A study examining waterfowl digestive tracts confirmed that birds consume seeds from Palmer amaranth and other agricultural weeds, and that those seeds can remain viable after passing through the birds’ guts.¹² Migratory waterfowl could therefore carry viable seeds hundreds of miles in a single flight. Combine that with contaminated equipment moving between fields and the natural spread of seed by wind and water, and the pathways for long-distance dispersal are numerous.

An additional genetic complication is hybridization with related species. Research using next-generation sequencing found strong evidence of gene flow between Palmer amaranth and waterhemp (Amaranthus tuberculatus), another major agricultural weed. Herbicide-resistance alleles from Palmer amaranth were found widespread in surveyed waterhemp populations, indicating that resistance genes can jump between species.¹³ This means that even controlling Palmer amaranth in a particular field does not necessarily contain its resistance traits if related species in the neighborhood can pick them up through pollen exchange.

The Financial Toll

Glyphosate-resistant Palmer amaranth has forced a fundamental and expensive shift in weed management. A survey of Georgia cotton growers, where resistance appeared early and hit hard, found that herbicide input costs more than doubled after resistance spread through the region. But even that increased spending was not enough: 52% of the cotton crop had to be hand-weeded at an average cost of $57 per hectare, representing a cost increase of at least 475% compared to the pre-resistance era.¹⁴ Growers also returned to more intensive tillage, partially reversing the soil-health gains that conservation tillage and no-till systems had delivered over the previous two decades.

Those Georgia numbers are from the early years of resistance, and the situation has not gotten cheaper. As resistance has stacked to include more herbicide classes, the menu of effective chemical options has shrunk, pushing costs higher and reliability lower. For smaller farm operations, a bad Palmer amaranth year can be the difference between a profitable season and a loss.

Adapting to Farming Itself

Palmer amaranth does not just resist herbicides. Research has shown that populations with long histories of exposure to high-nitrogen fertilization evolved greater nutrient-use efficiency, more than 43% higher than populations from low-nitrogen backgrounds. Glyphosate-resistant populations in particular showed over 47% higher nutrient-use efficiency and altered their canopy architecture in response to nitrogen fertilization compared to susceptible populations.¹⁵ In other words, the weed is evolving to extract more benefit from the very fertilizer farmers apply for their crops. The high genetic diversity driven by obligate outcrossing gives Palmer amaranth the raw material to adapt to nearly any consistent selection pressure a farming system imposes.

Non-Chemical Control and Biological Approaches

Given the limitations of herbicides, researchers have explored a range of alternative or complementary approaches. Harvest weed seed control (HWSC) is one strategy gaining traction. The concept is straightforward: instead of letting weed seeds that pass through the combine fall back onto the field, you collect or destroy them. Methods include chaff lining (concentrating chaff into narrow rows for later management) and impact mills that pulverize seeds during harvest. These tools have shown promise in Australian farming systems and are being evaluated for North American conditions, though commercial-scale data for Palmer amaranth specifically is still accumulating.¹⁶

Biological control research is at an earlier stage but has shown some interesting leads. A fungus called Phomopsis amaranthicola, isolated from lesions on an Amaranthus plant, was pathogenic to 22 Amaranthus species in testing. In field trials, the highest application rate achieved 100% mortality in most tested species within inoculated plots, reaching that endpoint 25 days earlier than in untreated plots.¹⁷ On the bacterial side, researchers screened rhizobacteria (soil bacteria living around plant roots) for bioherbicide potential. Four isolates, identified as Pseudomonas, Enterobacter, and Bacillus species, showed the ability to inhibit Palmer amaranth seed germination or plant development in lab and greenhouse settings without harming corn or common bean.¹⁸ Neither approach is ready for commercial deployment, but they represent tools that could eventually complement herbicide and mechanical strategies.

Why It Keeps Getting Harder to Identify

One underappreciated challenge with Palmer amaranth is that it looks a lot like several other pigweed species common in agricultural fields. Waterhemp, redroot pigweed, and smooth pigweed can all be confused with Palmer amaranth in the seedling stage, when control measures are most effective. By the time the plants are tall enough for easy visual identification, they may already be too large for many herbicides to kill. The dioecious flowering habit of Palmer amaranth (long, spiky seed heads on female plants, looser pollen-shedding structures on males) helps distinguish mature plants, but waiting for flowering means you have already lost the control window.

This identification problem extends to the seed level. As noted earlier, Minnesota’s requirement for genetic testing of Amaranthus seeds found during seed inspections reflects how difficult visual sorting is. Researchers have worked to optimize real-time PCR protocols to make genetic testing faster and cheaper, since the number of seed lots requiring screening continues to grow as the weed spreads into new states.¹¹ For farmers who suspect Palmer amaranth in their fields but are not certain, university extension services typically offer identification assistance, and getting a confirmed ID early matters enormously because the management strategies for Palmer amaranth are more aggressive than what most other pigweeds demand.

Climate Change as an Accelerant

Palmer amaranth evolved in a hot, dry desert environment, so it already has heat tolerance that many temperate crop weeds lack. The finding that elevated CO₂ boosts Palmer amaranth’s height, leaf area, and biomass adds another dimension to the concern.² Most of the growth differences in that study were driven by CO₂ levels rather than temperature, and importantly, both glyphosate-resistant and susceptible plants responded similarly. A bigger, faster-growing Palmer amaranth is harder to control at any given herbicide rate and competes more aggressively with crops for light. Warming temperatures are also extending growing seasons in northern states, giving the plant more days to complete its life cycle and produce seed in regions where it was once limited by frost. The combination of range expansion northward and enhanced growth under rising CO₂ makes the weed’s trajectory a moving target for management planning.