Palmer amaranth (Amaranthus palmeri)

Common Names: Carelessweed, Palmer's pigweed, dioecious amaranth

Description:

Palmer amaranth (Amaranthus palmeri) is a fast-growing summer annual pigweed native to the deserts of the southwestern United States and northern Mexico [1][3]. It escaped that home range, wrecked cotton fields across the Southeast in the 1990s, and has been pushing north ever since ? it is now one of the most economically important weeds of corn, cotton, and soybean, and it was confirmed in Michigan starting in 2010 [3][4]. MISIN tracks it as an early-detection priority for good reason: it grows up to 2 to 3 inches a day, one female plant can set hundreds of thousands of seeds, and many populations ? Michigan's included ? resist multiple herbicides [1][3][4].

Habit:

Palmer amaranth is a big, fast pigweed ? commonly 6 to 8 feet tall, with a main stem that can reach 5 inches across once it bolts [1]. It grows up to 2 to 3 inches a day in good conditions, roughly twice the rate of redroot pigweed and more than 40% faster than waterhemp [1][2].

Two checks separate it from the other pigweeds, and both take seconds. First, fold a leaf back against its own stalk: on mature leaves the stalk (petiole) is as long as or longer than the leaf blade ? no other common pigweed does this reliably [1][2]. Second, run your fingers down the stem: Palmer amaranth is hairless, while redroot and smooth pigweed are hairy [2]. The leaves are oval to diamond-shaped and hairless, and some carry a white V-shaped chevron mark [1].

From mid-summer on, the seed heads are the tell. Palmer amaranth is dioecious ? male and female flowers grow on separate plants ? and both sexes carry long, ropy spikes at the stem tips [1][2]. On female plants those heads are armed: stiff bracts up to 1/4 inch long stick out past the flower parts and turn sharp, so the head is genuinely prickly to grip [1][2]. Male heads are smaller, softer, and droop after pollination [1]. Sharp bracts also sit where the leaves meet the stem, which separates it from other pigweeds [4]. Keep scouting late ? Palmer amaranth keeps emerging through the season, later than waterhemp [2].

At-a-glance cues:

1. Leaf stalk as long as or longer than the leaf blade ? fold the leaf back to check [1][2]
2. Hairless stems and leaves ? no fuzz anywhere [1][2]
3. Long, ropy seed heads; on female plants stiff bracts up to 1/4 inch make them sharp to grip [1][2]
4. Extremely fast growth ? up to 2 to 3 inches a day, reaching 6 to 8 feet tall [1]

Leaves:

1. Arrangement: alternate along the stem [1]
2. Shape: oval to diamond-shaped, and wider than waterhemp's [1][2]
3. Petiole: on mature leaves, as long as or longer than the leaf blade ? the key identifying trait [1][2]
4. Surface: hairless [1][2]
5. Markings: some leaves carry a white V-shaped chevron mark; a few have a barely visible spine at the tip [1]

Stems:

1. Habit: a fast-growing summer annual, commonly 6 to 8 feet (1.8 to 2.4 m) tall [1]
2. Growth rate: up to 2 to 3 inches (5 to 8 cm) a day in ideal conditions ? about twice the rate of redroot pigweed and more than 42% faster than waterhemp [1][2]
3. Thickness: the main stem can reach up to 5 inches (13 cm) across after bolting, though most are thinner [1]
4. Surface: hairless (glabrous) ? unlike redroot pigweed and smooth pigweed [2]
5. Axils: sharp bracts sit where the leaves meet the stem [4]

Flowers:

1. Sex: male and female flowers grow on separate plants (dioecious) [2]
2. Arrangement: long, ropy spikes at the tips of the stems, on both male and female plants [1]
3. Female heads: carry large bracts up to 1/4 inch (6 mm) long that extend past the flower parts and turn sharp as they mature, making the head prickly to touch [1][2]
4. Male heads: smaller and softer, drooping after pollination [1]
5. Thickness: seed heads reach up to 1 inch (2.5 cm) across ? thicker than waterhemp's [2]

Fruit and seeds:

1. Seed: small and dark, and essentially identical to other pigweed seeds ? telling the seeds apart takes genetic testing [1]
2. Output: a single female plant produces roughly 100,000 to 500,000 seeds [1]; a Michigan estimate puts it near 400,000 per plant [4]
3. Emergence: seedlings come up throughout the growing season, with more late-season emergence than waterhemp [2]
4. Conditions: seed does best in no-till or minimum-tillage ground [1]
5. Field check: rub a seed head and look for the small black seed [2]

Habitat:

Palmer amaranth grows in sunny, disturbed places and row-crop fields [1]. It is a plant of hot, open ground, and it does best where the soil is not tilled much [1].

Its nativity is worth being precise about: Palmer amaranth is native to the deserts of the southwestern United States and northern Mexico [1][3]. It is a North American native ? just not here. It was first recorded outside that home range in Virginia in 1915, but did not become a serious problem until the 1990s, when it tore through cotton in the Southeast [3]. It has been moving north since and was confirmed in Michigan beginning in 2010 [4].

Reproduction:

Palmer amaranth does not arrive on its own ? it gets carried, and the carriers are what to watch.

The most likely route is contaminated seed [3]. Contaminated Conservation Reserve Program seeding mixes caused recent Midwest infestations, which means a conservation or pollinator planting can seed Palmer amaranth straight into clean ground [3]. In Michigan the likeliest path was different: contaminated cottonseed fed to dairy cattle, with the weed appearing in fields that had received manure within a year or two [4]. Once it is established in a field, farm equipment moves it [4].

Then the arithmetic takes over. The plant is dioecious ? male and female on separate plants ? and a single female produces roughly 100,000 to 500,000 seeds [1][4]. Seedlings emerge all season long, later than waterhemp [2], and grow up to 2 to 3 inches a day [1], so a seedling you walk past becomes a seed-producing plant within weeks.

Everything about managing it follows from that: stop seed set. Because control gets ineffective once plants are advanced, early detection and manual removal before seed set is the critical intervention window [4].

Impact and Damage:

Palmer amaranth is not a health hazard to people or pets. The one physical caution is the female seed heads, whose bracts turn stiff and sharp ? wear gloves when handling them [1][2].

The damage is agricultural, and it is severe. Palmer amaranth is one of the most economically important weeds of corn, cotton, and soybean [3]. It is highly competitive: it grows faster and builds more bulk than the pigweeds around it, shading crops out [1][2].

What sets it apart is resistance. Palmer amaranth adapts quickly and builds up herbicide-resistant genes [1]. Glyphosate-resistant populations appeared in 2004, and it is now described as extremely resilient and resistant to many herbicides, posing a significant threat to the agriculture industry [3]. Michigan's own populations have been confirmed resistant to both glyphosate and ALS-inhibiting herbicides [4]. That is precisely why it is treated as a species to catch early rather than one to manage after it settles in.

In natural areas it behaves as a weed of sunny, disturbed ground rather than a documented invader of intact plant communities [1]. No dollar figure for its total impact in North America has been established.

Similar species:

Palmer amaranth is a pigweed, and the pigweeds are genuinely hard to tell apart ? this is the identification problem, not a footnote to it. These comparisons come from published sources, since MISIN does not yet track look-alikes for it. Two checks do most of the work: Palmer amaranth's leaf stalk is as long as or longer than its leaf blade, and the plant is hairless [1][2].

Waterhemp (Amaranthus tuberculatus) is the closest match and the hardest call, because it is also dioecious and also hairless. Its leaves are narrower ? Palmer amaranth's are wider and more ovate to diamond-shaped ? and its seed heads are thinner, where Palmer amaranth's reach up to an inch across [2]. Palmer amaranth also grows more than 42% faster [2].

Redroot pigweed (Amaranthus retroflexus) gives itself away at a touch: its stems are hairy, while Palmer amaranth's are hairless [2]. Palmer amaranth also grows about twice as fast [2].

Smooth pigweed (Amaranthus hybridus) likewise has hairy stems, unlike Palmer amaranth's smooth, hairless ones [2].

Monitoring and rapid response:

Signs to watch for:

1. Fast-growing, hairless pigweeds in sunny disturbed ground and crop fields. Check any pigweed by folding a leaf back against its stalk ? if the stalk is as long as or longer than the blade, look closer [1][2]. From mid-summer on, look for long ropy seed heads; on female plants the bracts are stiff and sharp to grip [1][2]. Keep scouting late: Palmer amaranth emerges through the season, later than waterhemp [2].

What you can do:

1. Pull it, and pull it early. Hand weeding new infestations to prevent seed production is the recommended action, and because plants grow 2 to 3 inches a day, a small plant becomes a seed producer fast [1][2][4]. Wear gloves ? mature female heads are sharp.
2. Never let one plant go to seed. A single female makes roughly 100,000 to 500,000 seeds, so one missed plant undoes a season of work [1][4]. Carry pulled plants off the site rather than dropping them where a maturing head could still shed seed.
3. Check what you plant. Contaminated conservation and pollinator seeding mixes have seeded Palmer amaranth into clean Midwest ground ? know your seed source before you sow a habitat planting [3].
4. Know your manure and feed. In Michigan the weed most likely arrived in contaminated cottonseed fed to dairy cattle and then spread through manure applied to fields [4].
5. Clean equipment between fields ? farm equipment moves it once it is established [4].
6. For growers and crews: harvest infested fields last, so the combine does not carry seed into clean fields, and scout repeatedly rather than once [2].

When to call a professional:

1. Any field-scale infestation. Palmer amaranth resists multiple herbicide groups and control is ineffective once plants are advanced, so growers should contact their extension weed specialist at the first find rather than after plants are large [3][4].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. University of Minnesota Extension. Palmer amaranth. extension.umn.edu
2. Hartzler B. Palmer amaranth: ID, biology and management. Iowa State University Extension and Outreach, Integrated Crop Management. crops.extension.iastate.edu
3. National Invasive Species Information Center, U.S. Department of Agriculture. Palmer Amaranth (Amaranthus palmeri). invasivespeciesinfo.gov
4. Sprague C (2012). Palmer amaranth found in more Michigan fields: Now is a good time to scout. Michigan State University Extension. canr.msu.edu

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