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Waterhemp (Amaranthus tuberculatus): Biology, Identification, and Management

Preventing the spread of waterhemp in Montana is a high priority. This guide provides information on waterhemp identification, why it’s problematic, and what actions to take if you suspect you have found it.

Last Updated: 06/22
by Tim Seipel, MSU Extension Cropland Weed Specialist and Assistant Research Professor in Land Resources & Environmental Sciences; and Lovreet Shergill, Assistant Professor of Weed Science in MSU Southern Agricultural Research Center

Waterhemp

Waterhemp (Amaranthus tuberculatus) is a common and problematic broadleaf weed in the pigweed family (Amaranthaceae) (Figure 1). It is widespread in the south and Midwest of the United States. Waterhemp was first reported in Montana in 2020 and has been documented twice along the Yellowstone River in eastern Montana. The Weed Science Society of America ranked waterhemp as the seventh most troublesome weed in grass crops, pasture, and turf.

A photo of a waterhemp plant in a field of irrigated durum.

Figure 1. Waterhemp (Amaranthus tuberculatus) plants found in irrigated durum.

This pigweed is highly competitive and can grow rapidly, up to 1 inch per day under favorable conditions. Waterhemp is a prolific seed producer and female plants typically produce up to 250,000 seeds per plant. However, in non-competitive environments and under optimal conditions some plants can produce 1 million or more seeds. Up to 80 percent of seeds produced are viable and remain viable in the soil seedbank. Between 1 and 12 percent of seeds remain viable for up to four years.

Waterhemp is also closely related to another very problematic pigweed, Palmer amaranth, that is spreading toward Montana but has yet to be documented (Palmer Amaranth Montguide). The most common pigweeds in Montana are redroot pigweed, prostrate pigweed, tumble pigweed, Powell’s amaranth, and slender pigweed. Pigweeds are common agricultural weeds around the world. Pigweeds, including waterhemp, are warm-season annual plants that grow quickly and aggressively, compete with crops, reproduce through prolific seed production, and have some of the highest cases of herbicide resistance. Prevention of waterhemp establishment is a high priority in Montana; for more information, visit (https://agr.mt.gov/Noxious-Weed-Task-Force-Program).

Waterhemp Dispersal

Seeds of waterhemp are small (0.8 - 1.0 mm diameter) and can easily be dispersed by contaminated machinery, water, birds, and other animals, via contaminated manure and compost, birdseed, and to some degree, wind. Waterfowl, particularly ducks and geese, have been documented to disperse pigweed seeds, including waterhemp and Palmer amaranth, with an estimated 1700-mile range. Waterhemp is also a common contamination in bird feed and other seed mixtures. When buying seed, ensure the seed is certified and check the label to determine if seed samples contain dioecious pigweeds, which indicate waterhemp or Palmer amaranth seeds. If you suspect pigweeds in the seed, it can be tested. Seed samples can be submitted to the Montana State seed lab (https://plantsciences.montana.edu/seedlab/) for identification.

Identification of Waterhemp

Waterhemp looks superficially like many other pigweed species in Montana. Some characteristics that can be used to distinguish waterhemp are described below.

SEEDS, SEEDLINGS, AND VEGETATIVE CHARACTERISTICS

Seeds of most pigweeds are difficult to distinguish from each other. Waterhemp seeds are small (0.8 - 1.0 mm diameter), black to dark red, and shiny under light (Figure 2).

A photo of small, black waterhemp seeds and a ruler showing the seeds to be approximately 0.8 to 1 millimeter in diameter.

Figure 2. The seed of waterhemp is small and black and approximately 0.8 to 1mm in diameter.

Seedlings of waterhemp have egg-shaped true leaves with a notch at the tip of the leaf. The first true leaves of other common pigweeds in Montana are wider, elliptical, or more round. The only exception is the leaves of slender pigweed. Waterhemp leaves are without hairs and oval to lanceolate in shape and have a waxy appearance. The petiole, or leaf stalk, of waterhemp is often longer than that of redroot pigweed, but shorter than the length of the leaf blade itself, especially compared to other pigweeds in Montana. Waterhemp has a smooth stem with no hairs (appear glossy) and contrasts with redroot pigweed, which has fuzzy stems. Waterhemp stems are brightly colored and range in color from murky red or pink to green.

FLOWERING

Waterhemp and Palmer amaranth are the only two dioecious pigweeds likely to occur in Montana. Dioecious means there are separate male and female plants, so these plants will only produce pollen or seed. This is one of the best characteristics to distinguish these species from redroot pigweed and slender pigweed, which have both male and female flowers on the same plant. The male inflorescences of both male and female plants are long and narrow compared with redroot pigweed. The inflorescences of waterhemp males and females (Figures 3 & 4) are narrower and more elongated when compared to redroot pigweed.

A photo of a female waterhemp plant in flower.

Figure 3. Female plant of waterhemp (A. tuberculatus) in flower.

A photo of a waterhemp plant next to a redroot pigweed plant.

Figure 4. Waterhemp plant on the left next to the shorter inflorescences of redroot pigweed (A. retroflexus).

Prevention is the Key

Waterhemp is one of the most problematic weeds and has evolved resistance to multiple groups of herbicides. A population from Missouri has been documented to be resistant to six groups of herbicides including group 2 (chlorimuron), group 4 (2, 4-D), group 5 (atrazine), group 9 (glyphosate), group 14 (fomesafen), and group 27 (mesotrione). Currently, two populations of waterhemp have been found in eastern Montana. Preventing waterhemp establishment in Montana is vital. Once established, waterhemp increases input costs and decreases yields. Below are a few key points that will be helpful to prevent waterhemp establishment in Montana.

Scout fields and identify pigweeds.
Check seed labels and inspect seed. A common vector of the introduction of Palmer amaranth has been in Millet seed.
Be sanitary, keep equipment clean, and when buying used equipment, check and thoroughly clean it.
If a field is infested, use equipment in it last and then clean it. Try not to move seeds from a contaminated field to another field.
Waterhemp and Palmer amaranth have also been found in birdseed; check areas under bird feeders and remove the weeds if found.

Management

Optimize crop growing conditions using row spacing, planting density, fertilization, and pest management to maximize crop competitiveness and enhance overall weed control.

CROP ROTATION

It is an important aspect of weed management. The rotation of various crops creates diverse environments over time through the difference in crop competitiveness and the use of different herbicide active ingredients, which helps suppress weed populations.

TILLAGE

Waterhemp seeds are small and need to emerge from near the soil surface. Tillage or cultivation, if done timely, may be an effective method of waterhemp control. Primary tillage can control early emerging seedlings and strategic or occasional moldboard plowing can be used to bury seeds beyond the germination zone.

CHEMICAL

Few in-crop herbicides for waterhemp control are available for pulses, or oilseeds. It is very difficult to control waterhemp or Palmer amaranth in these crops and they should be avoided until the infestation is eliminated. Cereal grains or fallow offer the most herbicide options and include pre- and post-applications of herbicides to manage a long emergence period of waterhemp. Residual herbicides can protect crop yield by delaying waterhemp establishment early in the growing season. Some examples of residual herbicides for waterhemp control may include Group 3 [pendimethalin (Prowl H2O), trifluralin (Treflan)]; Group 5 [metribuzin (numerous), atrazine]; Group 7 [linuron (Lorox)]; Group 14 [fomesafen (Reflex), sulfentrazone (Authority products), flumioxazin (Valor)]; Group 15 [acetochlor (Harness/Warrant), pyroxasulfone (Zidua), and S-metolachlor (Dual II Magnum)].

Post-herbicides should be applied with the addition of an overlapping soil residual herbicide. Post-herbicides should be applied before the waterhemp reaches four inches in height. Keep in mind that waterhemp can grow more than one inch per day, so scouting to identify the right stage for herbicide application will be necessary. Resistance to glyphosate (Roundup) has been reported from North and South Dakota as well as nationwide, so it should be assumed that any population found could be resistant to glyphosate. Group 2 herbicide [imazamox (Raptor), chlorimuron (Classic)] resistance in waterhemp is widespread and resistance to Group 14 herbicides [fomesafen (Reflex or Flexstar), lactofen (Cobra)] is also common. Therefore, the characterization of herbicide resistance is necessary to develop an effective herbicide program. Group 27 herbicides [mesotrione (Callisto), tembotrione (Laudis), and topramazone (Impact or Armezon)] are highly effective when used in combination with atrazine. Other post- herbicide groups in appropriate crops that could be effective in controlling waterhemp include Group 10 [glufosinate (Cheetah, Interline, Liberty)] with Liberty Link crops, and Group 4 [dicamba (Clarity, or 2,4-D]. Use premixes or tank-mixes with multiple effective sites-of-action at each application and rotate sites-of- action when possible. Applicators should follow the herbicide label, use full-labeled rates, spray volume, and apply herbicides in a timely manner.

Waterhemp plants that have survived a post-emergence application and begin to regrow by two weeks after the application will need to be removed before they produce seed. Follow-up scouting of the field should be conducted for years after an infestation is discovered and plants removed from the field. Waterhemp is a broadly adapted weed found not only in crop fields but along field edges, near sloughs, roads, and other disturbed sites. It is important to scout these areas and remove or prevent the annual weed from going to seed. Seeds on the surface can be expected to survive for 3 to 5 years. Preventing waterhemp or Palmer amaranth from setting seed is crucial as large plants can produce up to one million seeds. In areas where infestations are small, hand pulling can be effective in removing plants.

Take action if a suspicious plant is found.

Early detection and rapid response are critical.

Contact the local Extension agent or an agronomist to help identify the plant.
If waterhemp or Palmer amaranth is suspected, a specialist will visit the site and make visual confirmation.
Record the location with GPS.
Genetic testing should occur to confirm pigweed identity.
Work with agricultural professionals to determine the source of waterhemp or Palmer amaranth.
Develop an action plan. Plants must be pulled and removed from the site before they go to seed, and the area should be searched and monitored for additional plants for at least three years.

Waterhemp has been found twice in eastern Montana. Monitoring and correct identification is important and is often made through genetic testing. If you have questions, please contact the local Extension agent or Montana State University Extension specialists (https://agr.mt.gov/Noxious-Weed-Task-Force-Program).