Palmer amaranth

Amaranthus palmeri S. Watson, 1877

Palmer pigweed · English, North America
caruru-palmeri · Portuguese, Brazil
yuyo colorado / bledo · Spanish, Argentina and Mexico
amarante de Palmer · French, Europe

Palmer amaranth is a fast-growing pigweed that has become one of the most damaging and herbicide-resistant weeds in the world. It can grow several centimetres a day, reach over 2 m, and a single female plant can produce hundreds of thousands of seeds. Native to the south-western deserts of North America, it has spread through the cotton and soybean regions of the Americas and is a regulated alert-list weed in other regions.

Palmer amaranth plant. Photo: Yercaud-elango, CC BY-SA 4.0, via Wikimedia Commons.

Type: Weed · Weed (annual broadleaf, separate male and female plants)
Classification: Plantae › Magnoliopsida › Caryophyllales › Amaranthaceae
Main crops: Soybean, Cotton, Maize, Groundnut, Sorghum
Found in: North America, South America, Europe and the Mediterranean, Middle East, Africa and Asia
Chemical groups: HRAC 15, HRAC 14, HRAC 5, HRAC 10, HRAC 27, HRAC 4 (mode-of-action groups, not products)
Last reviewed: 2026-10-19

Palmer amaranth scientific name, order and family

The scientific name of Palmer amaranth is Amaranthus palmeri S. Watson, 1877, in the order Caryophyllales, family Amaranthaceae.

Where it occurs

Native to the south-western United States and northern Mexico. It spread across the southern and midwestern United States from the 1990s and has been found in South America (Argentina, Brazil from 2015), and introduced locally in parts of Europe, the Middle East, southern Africa and Asia, often with contaminated grain, seed or machinery. It is on the EPPO alert list and regulated in some countries.

Region	Status
North America	Native in the south-west; invasive across much of the US
South America	Invasive in Argentina and Brazil
Europe and the Mediterranean	Occasional introductions; established locally in some areas
Middle East, Africa and Asia	Recorded locally; regulated in some countries

Symptoms and identification of Palmer amaranth

Palmer amaranth seed head (herbarium voucher). Photo: Forest and Kim Starr, CC BY 3.0 US, via Wikimedia Commons.

The plant

An upright, usually hairless annual, often 1-2 m tall or more, with diamond- to egg-shaped leaves arranged in a symmetrical, poinsettia-like rosette when young. A reliable sign: the leaf stalk (petiole) of older leaves is as long as or longer than the leaf blade. Some plants have a V-shaped white or purple 'watermark' on the leaf. Plants are either male or female; female seed heads are long (up to about 60 cm), dense and spiny to touch because of stiff bracts.

Signs in the crop, stage by stage

Crop stage	What you see
Crop emergence	Flushes of hairless amaranth seedlings, with emergence continuing over many weeks.
Crop vegetative growth	Weeds outgrowing and shading the crop within a few weeks; very fast stem growth.
Crop maturity and harvest	Tall weeds with long seed heads above the canopy; thick stems interfering with harvest machinery.

Life cycle of Palmer amaranth

Life cycle of Palmer amaranth — diagram drawn by Laisha from the stages below. Durations depend on temperature.

Seeds germinate in flushes from late spring through summer, mostly from the top centimetre or two of soil. Plants grow very fast in heat, and male and female plants must cross-pollinate (by wind), which spreads resistance genes through pollen as well as seed. Female plants shed huge numbers of small seeds that persist in soil for several years.

Stage	Duration	Notes
Seed in soil	Several years; most die within about 3-4 years	Shallow seeds germinate best.
Emergence	Over many weeks of the warm season	Continuous flushes.
Rapid growth	About 4-8 weeks	Can grow several centimetres a day.
Flowering and seed production	From about 6-8 weeks after emergence	Hundreds of thousands of seeds per female plant.

Temperature

A C4 plant favoured by high temperatures (about 25-35 °C) and full sun; tolerant of drought.

Generations

One main generation a year, with staggered emergence.

Conditions that favour it

Reduced tillage that leaves seeds near the surface, wide crop rows that close late, reliance on a single herbicide group, and spread through contaminated seed, feed, manure, cotton gin trash and machinery.

Economic threshold level (ETL)

There is no tolerated threshold in practice: because one female plant can add hundreds of thousands of seeds to the soil, extension services recommend zero tolerance of plants going to seed ('zero seed rain'), and early control of every flush.

Thresholds are local. They depend on crop price, control cost, variety and natural-enemy activity, so use the figure your local extension service publishes where one exists.

Damage and yield loss from Palmer amaranth

Competition studies in North America report yield losses of up to about 65-80% in soybean and cotton and over 90% in maize at high densities; harvest is also slowed or prevented by large weeds.

Monitoring

Scout fields within 2-3 weeks of crop emergence and after each control measure, identifying amaranth species by the petiole-length test.
Note survivors after herbicide applications and send samples for resistance testing.
Inspect field edges, gin trash and machinery entering from other farms.

How to control Palmer amaranth (integrated management)

Control works best as a sequence: prevent, monitor, then act only when a threshold or a risk forecast says so, using the least disruptive option that works.

Cultural control

Prevent seed production: remove escapes by hand before they flower, and clean machinery between fields.
Use narrow rows, competitive crops and cover crops (for example cereal rye) that suppress emergence.
Deep inversion tillage once, to bury the surface seed bank, followed by shallow or no tillage, has reduced infestations in some systems.
Rotate crops to allow different herbicide groups and timings.

Mechanical and physical control

Hand-hoeing or pulling of survivors before seed set; harvest weed seed control (seed destructors or collecting chaff) at harvest.

Biological control

No biological control is available; seed predation by insects and rodents reduces the soil seed bank somewhat.

Host resistance

Not applicable (it is a weed).

Chemical control of Palmer amaranth — by mode-of-action group only

Laisha names chemical options only by their resistance-committee mode-of-action group (IRAC for insects, mites and nematodes, FRAC for fungi, oomycetes and bacteria, HRAC for weeds), never by active ingredient or product. What is registered differs by country and changes over time; the label and your local plant-protection authority decide what may be used, on which crop, at what rate and with what pre-harvest interval.

Programmes rely on overlapping residual pre-emergence herbicides plus timely post-emergence applications on weeds under about 10 cm, using several effective groups. What is registered differs by country and changes over time; follow the label and your local plant-protection authority.

Scheme	Group	Mode of action	Notes
HRAC	15	Inhibition of very-long-chain fatty acid synthesis (VLCFA)	Residual pre-emergence; layered.
HRAC	14	Inhibition of protoporphyrinogen oxidase (PPO)	Pre- and post-emergence; resistance documented.
HRAC	5	Inhibition of photosynthesis at photosystem II (serine 264 binders)	Residual in some crops.
HRAC	10	Inhibition of glutamine synthetase	Post-emergence in tolerant crops; resistance reported.
HRAC	27	Inhibition of hydroxyphenyl pyruvate dioxygenase (HPPD)	Maize and tolerant crops.
HRAC	4	Auxin mimics	Tolerant crops; resistance reported.

Resistance management

Palmer amaranth populations resistant to HRAC Groups 2 and 9 are widespread, and resistance to Groups 3, 4, 5, 10, 14, 15 and 27 has been documented, including populations resistant to several groups at once (International Herbicide-Resistant Weed Database).
Use at least two effective groups at every application timing, layer residuals, and never let survivors set seed.

Look-alikes and how to tell them apart

Waterhemp (Amaranthus tuberculatus) — Leaves longer and narrower with petioles shorter than the blade; seed heads softer, not spiny.
Redroot pigweed (Amaranthus retroflexus) — Hairy stems and leaves, shorter petioles, and short, bristly, compact seed heads.
Spiny amaranth (Amaranthus spinosus) — Sharp paired spines at the leaf nodes.

Host range

As a weed it infests soybean, cotton, maize, groundnut, sorghum, vegetables and field margins, especially in warm summer cropping systems.

What is still uncertain

Its establishment status outside the Americas is changing and incompletely documented.
Yield-loss figures come mostly from North American trials.

Grow guides for its host crops

How to grow the crop well is the first line of defence. Laisha's free grow guides cover each crop from sowing to harvest:

How to grow Soybean How to grow Cotton How to grow Maize How to grow Groundnut How to grow Sorghum

Not sure this is what you are seeing?Ask Laisha: describe the symptoms or send a photo, and get a reasoned first read with what to check next — for your crop, your stage and your region.Ask Laisha

Sources

Amaranthus palmeri (Palmer amaranth) datasheet — CABI Compendium. https://doi.org/10.1079/cabicompendium.4649
Amaranthus palmeri (AMAPA) — EPPO Global Database. https://gd.eppo.int/taxon/AMAPA
International Herbicide-Resistant Weed Database — HRAC / WeedScience.org. https://www.weedscience.org/
Palmer amaranth management — University of Arkansas Division of Agriculture. https://www.uaex.uada.edu/

Written by Laisha from the sources above, in its own words. This page is general reference, not a prescription: confirm identification locally and follow the label and your plant-protection authority.