Some amaranth species are among the most aggressively invasive weeds on the planet, while others are harmless crops grown for grain, leafy greens, or garden color. The difference comes down to species. The amaranth family (Amaranthus) contains roughly 70 species, and only a handful cause serious problems. If you’re growing ornamental or grain amaranth in your garden, you’re almost certainly fine. If you’re dealing with wild amaranth in a farm field, you may be facing one of the hardest weeds in modern agriculture to control.
Which Amaranth Species Are Invasive
Three species stand out as the most widespread invasive amaranths: Palmer amaranth (A. palmeri), redroot pigweed (A. retroflexus), and smooth pigweed (A. hybridus). Of these, Palmer amaranth is the most feared in North American agriculture, while redroot pigweed is one of the most common weeds across Europe and temperate regions worldwide. Smooth pigweed is categorized as one of the most economically important weeds globally.
Tumble pigweed (A. albus) is another introduced species that has naturalized across crop fields, particularly in vegetable and field crop areas. It’s been present in parts of Europe since the late 18th century. Waterhemp (A. tuberculatus) rounds out the list of major problem species, especially in the U.S. Midwest.
Meanwhile, grain amaranth (A. cruentus, A. hypochondriacus) and leafy vegetable amaranth (A. tricolor) are cultivated crops with thousands of years of agricultural history. These species can self-seed freely in a garden, which some gardeners find annoying, but they don’t spread into wild ecosystems or resist control the way their weedy cousins do.
Why Invasive Amaranth Is So Hard to Stop
A single female Palmer amaranth plant growing without competition can produce over 613,000 seeds. Even when competing with crops like cotton or peanut, individual plants still produce 400,000 to 500,000 seeds. In a soybean field, that number drops to around 173,000, and corn provides the strongest competition, limiting seed output to roughly 51,000 per plant. Those are still enormous numbers for a single weed.
The seeds are tiny, spread easily by wind and water, and can remain viable in soil for years. This means a single plant that escapes control can repopulate an entire field the following season. Palmer amaranth also grows fast, sometimes adding two to three inches per day in warm conditions, quickly overtopping crops and stealing light.
Redroot pigweed uses a different competitive edge. Its leaf extracts have allelopathic properties, meaning they release chemicals that inhibit the root growth of neighboring plants. Research shows this effect becomes stronger in soils with higher nitrogen levels, which is exactly what you find in fertilized farmland. This chemical suppression of competitors likely plays a role in its success as an invader, particularly in warmer climates.
Herbicide Resistance Makes Things Worse
What truly sets invasive amaranths apart from other weeds is their extraordinary ability to evolve resistance to herbicides. Both Palmer amaranth and waterhemp have developed resistance to six different classes of herbicides. These include glyphosate (the active ingredient in Roundup), ALS inhibitors, triazines, and several other major chemical families that farmers depend on.
The resistance mechanisms are varied and sophisticated. Some populations have altered the genetic target that a herbicide is designed to attack, essentially making the lock not fit the key anymore. Others have ramped up their internal detoxification systems, breaking down herbicides before they can do damage. A single Palmer amaranth population studied in Kansas carried resistance to six herbicide groups simultaneously, and five of those resistances worked through metabolic detoxification, a mechanism that’s particularly difficult to overcome because it can provide cross-resistance to chemicals the plant has never even been exposed to.
This means farmers can’t simply rotate herbicides and expect the problem to go away. Populations that evolved resistance through one pathway often develop tolerance to entirely new chemical classes as a side effect.
Crop Damage From Invasive Amaranth
The yield losses are severe. As few as three Palmer amaranth plants per square meter can cause 60% soybean yield loss. Cotton is even more vulnerable: densities below one plant per square meter have caused 65% yield reductions. In northern Great Plains soybean fields, Palmer amaranth densities above 15 plants per square meter caused 33% yield loss, with significant damage even at lower densities.
These numbers translate directly into economic harm. Palmer amaranth has become the single most costly weed in U.S. cotton and soybean production, and it continues expanding its range northward as climate conditions shift.
How to Tell Invasive Amaranth From Garden Varieties
If you’ve spotted amaranth growing somewhere unexpected and want to know what you’re dealing with, a few physical features help distinguish the major species. Palmer amaranth has long petioles (the stem connecting the leaf to the main stalk) that are typically longer than the leaf blade itself. The leaves are pointed and smooth. Waterhemp has narrower, more rounded leaves with medium-length petioles, and its stems are completely hairless. Redroot pigweed is the easiest to identify: its leaves and stems are noticeably fuzzy or hairy, and its petioles are medium length.
Cultivated grain amaranth tends to have large, showy flower plumes in deep red, purple, or gold. The plants are bushy and full rather than tall and spindly. If what you’re looking at is a tall, lanky plant with greenish flower spikes growing uninvited in a crop field or disturbed ground, you’re likely looking at one of the invasive species.
Is Garden Amaranth a Problem?
Ornamental and grain amaranth varieties (love-lies-bleeding, prince’s feather, and similar cultivars) do self-seed readily. You’ll find volunteer seedlings popping up in garden beds the following spring. But these plants pull easily, don’t carry herbicide resistance, and won’t spread aggressively beyond your garden. They’re no more “invasive” than a self-seeding tomato or dill plant.
That said, if you live in an area where Palmer amaranth or waterhemp is already a problem, be aware that cultivated amaranth can potentially cross-pollinate with wild relatives. This is mainly a concern for farmers rather than home gardeners, but it’s worth knowing if you’re near active cropland dealing with resistant amaranth populations. Keeping ornamental amaranth deadheaded before it sets seed eliminates the concern entirely.