Manage Weeds On Your Farm


SARE Outreach
Charles L. Mohler, John R. Teasdale, Antonio DiTommaso | 2021 | 416 pages

Other common names: common waterhemp, tall waterhemp

Amaranthus tuberculatus (Moq.) Sauer

Amaranthus rudis Sauer

Identification of Waterhemp

Family: Pigweed family, Amaranthaceae

Habit: Tall, upright, summer annual herb

Taxonomic note: Many sources separate common waterhemp (Amaranthus rudis) from tall waterhemp (Amaranthus tuberculatus), but recent authorities have considered the two as a single species. Though probably not good species, the two forms may deserve recognition as varieties (A. tuberculatus var. rudis and A. tuberculatus var. tuberculatus). Common waterhemp appears to be more common as an agricultural weed, but tall waterhemp also occurs in farm fields. Since the two forms can only be definitively distinguished using minute characteristics of the female flowers, and since these characteristics intergrade extensively in the central Midwest where the species is most problematic, we here treat them as a single species that we refer to simply as waterhemp.

Description: Seedling stems are light green to red-pink and 0.1–0.2 inch tall. Cotyledons are oval to lanceolate, 0.2–0.4 inch long by 0.08–0.16 inch wide, hairless and red-green above and dark red below. The true leaves are oblong, dark green, prominently veined and reddish pink below, shiny above and shallowly notched at the tip. Mature plants reach 2.5–8 feet tall on branching, green to reddish, hairless, occasionally ridged stems. The leaves are alternate, shiny, hairless, 0.6–6 inches long by 0.2–1.5 inch wide and oblong to lanceolate with an abruptly tapered, notched tip. Leaf stalks are up to 0.4–2.8 inches long and are usually shorter than the leaf blade. Upper leaves are smaller and more lanceolate than lower leaves. The root system is a taproot with secondary fibrous roots. Male and female flowers are produced by separate plants. Flowers of both sexes are green, without petals, 0.07–0.11 inch long and clustered to form upright, 4–8 inch-tall, occasionally branching spikes at branch tips and leaf axils. Male flowers have five green sepals, while female flowers have zero to two green sepals. Female flowers are replaced by 0.06 inch-long oval capsules. Each capsule contains a single shiny, round to oval, 0.03 inch-wide, red-brown to black seed.

Similar species: Redroot pigweed (Amaranthus retroflexus L.), smooth pigweed (A. hybridus L.) and Powell amaranth (A. powellii S. Watson) have hairy stems and leaves, while waterhemp leaves and stems are hairless. These amaranth species also have male and female flowers on a single plant, while waterhemp has separate male and female plants. Like waterhemp, Palmer amaranth (A. palmeri S. Watson) has smooth stems and separate male and female plants. The leaf stalks of Palmer amaranth, however, are generally longer than the leaf blade, while waterhemp leaf stalks are usually shorter than the leaf blade. Palmer amaranth leaves may have chevron or V-shaped watermarks, while waterhemp leaves never have such markings.

Management of Waterhemp

Waterhemp is a late emerging species, so planting crops as early as possible while still ensuring good establishment will improve the competitiveness of the crop. Plants emerging one month or more after corn or soybeans are substantially suppressed. Tillage greatly reduces waterhemp emergence, and moldboard plowing is more effective than chisel plowing for reducing emergence. Although the seeds are moderately persistent when buried deeply, their small size means that seeds must return close to the soil surface to emerge successfully following any future tillage event, and a large proportion of those buried by tillage will never return. Although inversion tillage can be one component of a waterhemp management strategy, waterhemp seed densities in the soil can be decreased using ridge tillage in row crops if the cultivation program is effective. 

Given its late emergence, probably few waterhemp will mature in a winter grain crop, but they may be able to mature in spring oats. If hay is overseeded into the grain, regrowth following grain harvest will create massive seed rain if mowing is delayed long enough to get a hay cutting that fall. An earlier cutting in August at about 4 inches will knock back the waterhemp at a time when the hay species can rebound rapidly from root reserves and shade the waterhemp. Mow with a forage chopper and wagon so that the cut inflorescences are removed; otherwise, they are likely to set seed. The success of this tactic depends on a good stand of hay or cover crop. Since relatively little of the hay leaf area will be removed, a hay cutting in the fall may still be possible. Despite all precautions, anything short of repeated tillage after small grain harvest is likely to result in some waterhemp seed production.

Effective cultivation of corn and soybeans can provide good control of waterhemp. Establishment from near the soil surface and the tiny size of waterhemp seedlings make them highly susceptible to rotary hoeing and tine weeding. Continue in-row weeding as long as possible to get later emerging cohorts out of the row. Begin throwing soil into the row with inter-row cultivation soon after the last in-row operation and mound up around the base of the plants at the final cultivation. Topping waterhemp that emerges above soybeans or other short crops with a mower will not be effective, as this species recovers from clipping rapidly. Pulling plants will prevent competition with the crop, but the rapid flowering habit of the species means that it is unlikely to be effective for preventing seed production unless the plants are removed from the field. The high retention of seeds on plants at harvest provides an opportunity for seed capture and destruction during harvest operations.

The late emergence of waterhemp means that neither a tilled fallow period nor a stale seedbed before planting will be effective for controlling the species, except possibly for a midsummer planted vegetable crop. Straw or other organic mulch materials can completely suppress this species both by preventing germination cues and by blocking emergence of the tiny seedlings.

Ecology of Waterhemp

Origin and distribution: Waterhemp is native to the United States. It is most problematic as a weed in the Midwest but occurs through most of the eastern and southern United States, and the southern margin of Canada from Quebec to Saskatchewan. Its scattered occurrence from the Rocky Mountains westward is probably due to introduction. It has also been introduced to Europe.

Seed weight: Population mean seed weights range from 0.19–0.27 mg. 

Dormancy and germination: Seeds collected from recently matured plants are dormant. Populations vary in the after-ripening conditions needed to allow germination. Some populations germinate best after 12 weeks of wet, cold (39°F) conditions, whereas others germinate best after a period of warm wet conditions, and a population from a natural habitat germinated well following after-ripening in a wide range of conditions. Seeds begin germination at mean daily temperatures of 41–59°F, depending on the population, but percentage germination is low at cool temperatures. Some germination can occur at day/night temperatures as high as 113/104°F. Peak germination occurs at mean daily temperatures of 68–91°F. Germination at any constant temperature is relatively poor whereas daily temperature fluctuations substantially promote germination, with a daily fluctuation range of 32–43°F maximizing germination. The after-ripening requirements of agricultural populations coupled with peak germination at warm temperatures ensures that most seeds will not germinate until the summer following production. In warm, fluctuating temperature conditions, seeds germinate in as little as one day. Light stimulates germination, especially red light, whereas light that has been depleted in red wavelengths (e.g., by passage through a plant leaf canopy) can inhibit germination. However, high temperatures, for example 97°F, can overcome this latter inhibition and allow germination under plant canopies. 

Seed longevity: Waterhemp seeds mixed into the top 2 inches of soil and stirred annually in spring had an average annual mortality of 40%. In Nebraska, the computed annual seed mortality rate over the first three years was 30–44%, but 1–3% of seeds still germinated after 17 years of burial at 8 inches. In several Midwestern states, the average mortality of seed buried 6 inches deep for 12 months was 78%.

Season of emergence: Waterhemp typically emerges from mid-May to late July in Iowa, with peak emergence in mid- to late June. Reduced tillage tends to delay peak emergence. Waterhemp is classified as a late emerging weed with a long duration of emergence in Nebraska. In Ontario, emergence occurs from June through August. Waterhemp required 14–17 days to emerge when planted in May in Missouri and had the slowest emergence of six pigweed species tested. It is generally known for late emergence and a discontinuous germination pattern that extends well into the growing season, thereby allowing waterhemp to emerge and produce seeds after all weed management operations have been completed.

Emergence depth: This has not been studied directly, but the small seed size and similarity of the seeds to related pigweed species indicate that most seedlings likely emerge from the top 1 inch or less of the soil (see the species chapters “Palmer amaranth” and “Pigweeds: Powell amaranth, redroot and smooth”). The great suppression of emergence by tillage further supports a shallow emergence pattern.

Photosynthetic pathway: C4

Sensitivity to frost: Flowering and seed set stop at the first frost.

Drought tolerance: Waterhemp is not drought tolerant. Plant growth and seed production declines approximately linearly as soil water content declines and as the number of days without water increases. The pattern of seed production in several short pulses over an extended period appears to be an adaptation to improve reproductive success in the face of unpredictable summer rainfall patterns.

Mycorrhiza: Waterhemp is probably non-mycorrhizal in most circumstances.

Response to fertility: In sand, increased ammonium nitrate raised waterhemp’s relative growth rate about five fold, similar to the response of corn. Increasing rates of composted swine manure in a soil mix substantially increased the relative growth rate of waterhemp seedlings while having little effect on the relative growth rates of corn, wheat or soybeans. This effect would make the weed both more competitive against crops and also more difficult to control with cultivation at high compost rates. In soybeans, application of a high rate of composted swine manure increased waterhemp dry weight by 25–50%. In an experiment with corn, application of 24–33 tons per acre of composted swine manure roughly doubled the dry weight of waterhemp. Composted swine manure also inhibited waterhemp emergence but had no effect on emergence of the crops. Waterhemp tolerates a pH from 4.5–8. 

Soil physical requirements: The species’ natural habitat is the wet soils of marshes and the edges of lakes, ponds and rivers. Consequently, it tolerates wet, anaerobic soils but does best in well drained agricultural soils. It tolerates a wide range of soil textures but grows best in medium to fine textured soils. 

Response to shade: Waterhemp is considered shade intolerant. Shade of 68% reduced final plant weight by about 50%, but plants emerging in May still produced 400,000 seeds per plant and June-emerging individuals produced 90,000 seeds per plant. With 99% shade, mortality was substantial, and remaining plants were small and produced few seeds. By comparison, corn in Illinois at the V8 growth stage casts 80–98% shade.

Sensitivity to disturbance: Waterhemp recovers well from clipping. In greenhouse studies, removing half the shoot had no effect on plants 4–8 inches tall and only reduced the weight of 12–16 inch plants by 22%. In the field, removing half the shoot of 6 inch plants reduced final height by 11% but had no effect on seed production. Removing all but the seed leaf node reduced seed production by 78%, but the plants still produced an average of 32,000 seeds. Waterhemp has a taproot, which makes large plants difficult to uproot with a cultivator but relatively susceptible to slicing with shallow cultivating knives.

Time from emergence to reproduction: Waterhemp, as other Amaranthus species, begins flowering in response to shortening day length. Populations established in May and June required five to seven weeks after emergence to begin flowering, whereas populations established in July required three to four weeks. Time to initiation of flowering was consistently a few days earlier for male than female plants, ensuring sufficient pollen availability when females began flowering. Some seeds first become viable seven to nine days after pollination, at which time they are brown, but seed weight and percentage viability continues to increase until 12 days after pollination, at which time they are black. Seed maturation occurred similarly in five cohorts emerging from May to July, namely at 20–27 days after flower initiation and six to 13 days after pollination.

Pollination: Since male and female flowers occur on separate plants, the species necessarily outcrosses. It is primarily wind pollinated.

Reproduction: Female waterhemp plants can produce as many as 1 million seeds, but 35,000–200,000 seeds is more typical. Several populations produced 470,000–1.29 million seeds per plant in Indiana when established in May or June but produced 200,000–340,000 seeds when established in July. In soybeans, seed production declined exponentially with an increasing lag between crop planting and waterhemp emergence. Thus, waterhemp establishing simultaneously with soybeans in Iowa produced 300,000 seeds per plant, whereas those emerging 50 days later produced 3,000 seeds per plant. On the other hand, waterhemp seed production per plant also is highly density dependent, so individual plant production will vary among emergence cohorts depending on plant density. Waterhemp produced an average 288,000 seeds per plant in Missouri and had the highest seed production per unit plant weight of six pigweed species tested. Waterhemp plants retained 95–100% of seeds at soybean harvest. 

Dispersal: Waterhemp seeds float and probably disperse by overland water flow, in irrigation water and along streams. Although passage through ruminants has not been studied, the seeds are similar to those of other pigweed species that disperse readily in feces and the spreading of manure. Given the persistent seed bank and prolific seed production of the species, dispersal in soil clinging to shoes, tires, animals and machinery seems likely.

Common natural enemies: Seeds are eaten by mourning doves, ducks and songbirds. After dispersal, the seeds are eaten by field crickets, Gryllus pennsylvanicus, and several species of carabid ground beetles, including Amara aeneopolita, Anisodactylus rusticus, Stenolophus comma and Harpalus pennsylvanicus. These species preferred waterhemp seeds to those of several other prominent weed species. Waterhemp is attacked by several pathogens, including Albugo bliti (white rust), Phymatotrichum omnivorum (Phymatotrichum root rot), Cercospora acnidae (a leaf spot disease) and Phyllosticta amaranthi (a leaf spot disease). Microsphaeropsis amaranthi has potential as a bioherbicide for control of waterhemp provided formulations can prevent dry leaf surfaces during the first 12 or more hours after application.

Palatability: Palatability of waterhemp for grazing animals is low.

Summary Table of Waterhemp Characteristics

Growth habitSeed weight (mg)Seed dormancy at sheddingFactors breaking dormancyOptimum temperature for germination (F)Seed mortality in untilled soil (%/year)Seed mortality in tilled soil (%/year)Typical emergence seasonOptimum emergence depth (inches)
tall, erect0.19–0.27Yescms, li, at68–9130–7840late spring to summer0–1
Photosynthesis typeFrost toleranceDrought toleranceMycorrhizaResponse to nutrientsEmergence to flowering (weeks)Flowering to viable seed (weeks)Pollination Typical & high seed production (seeds per plant)
C4lowlownohigh3–73–4cross200,000 & 1,000,000

Table Key

General: The designation “–” signifies that data is not available or the category is not applicable.

Growth habit: A two-word description; the first word indicates relative height (tall, medium, short, prostrate) and second word indicates degree of branching (erect, branching, vining).

Seed weight: Range of reported values in units of “mg per seed.”

Seed dormancy at shedding: “Yes” if most seeds are dormant when shed, “Variable” if dormancy is highly variable, “No” if most seeds are not dormant.

Factors breaking dormancy: The principle factors that are reported to break dormancy and facilitate germination. The order of listing does not imply order of importance. Abbreviations are:

scd = seed coat deterioration

cms = a period subjected to cold, moist soil conditions

wst = warm soil temperatures

li = light

at = alternating day-night temperatures

ni = nitrates

Optimum temperature range for germination: Temperature (Fahrenheit) range that provides for optimum germination of non-dormant seeds. Germination at lower percentages can occur outside of this range. The dash refers to temperature range, and the slash refers to alternating day/night temperature amplitudes.

Seed mortality in untilled soil: Range of mortality estimates (percentage of seed mortality in one year) for buried seeds in untilled soil. Values were chosen where possible for seeds placed at depths below the emergence depth for the species and left undisturbed until assessment. Mortality primarily represents seed deterioration in soil.

Seed mortality in tilled soil: Range of mortality estimates (percentage of seed mortality in one year) for seeds in tilled soil. Values were chosen for seeds placed within the tillage depth and subjected to at least annual tillage events. Seed losses are the result of dormancy-breaking cues induced by tillage, germination and deterioration of un-germinated seeds.

Typical emergence season: Time of year when most emergence occurs in the typical regions of occurrence for each weed. Some emergence may occur outside of this range.

Optimum emergence depth: Soil depths (in inches below the soil surface) from which most seedlings emerge. Lower rates of emergence usually will occur at depths just above or just below this range.

Photosynthesis type: Codes “C3” or “C4” refer to the metabolic pathway for fixing carbon dioxide during photosynthesis. Generally, C3 plants function better in cooler seasons or environments and C4 plants function better in warmer seasons or environments.

Frost tolerance: Relative tolerance of plants to freezing temperatures (high, moderate, low).

Drought tolerance: Relative tolerance of plants to drought (high, moderate, low).

Mycorrhiza: Presence of mycorrhizal fungi. “Yes” if present; “no” if documented not to be present, “unclear” if there are reports of both presence and absence; “variable” if the weed can function either with or without, depending on the soil environment.

Response to nutrients: Relative plant growth response to the nutrient content of soil, primarily N, P, K (high, moderate, low).

Emergence to flowering: Length of time (weeks) after emergence for plants to begin flowering given typical emergence in the region of occurrence. For species emerging in fall, “emergence to flowering” means time from resumption of growth in spring to first flowering.

Flowering to viable seed: Length of time (weeks) after flowering for seeds to become viable.

Pollination: “Self” refers to species that exclusively self-pollinate, “cross” refers to species that exclusively cross-pollinate, “self, can cross” refer to species that primarily self-pollinate, but also cross-pollinate at a low rate, and “both” refers to species that both self-pollinate and cross-pollinate at relatively similar rates.

Typical and high seed production potential: The first value is seed production (seeds per plant) under typical conditions with crop and weed competition. The second value, high seed production, refers to conditions of low density without crop competition. Numbers are rounded off to a magnitude that is representative of often highly variable reported values.

Further Reading

Costea, M., S.E. Weaver and F.J. Tardif. 2005. The biology of invasive alien plants in Canada. 3. Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea & Tardif. Canadian Journal of Plant Science 85: 507–522. 

Korres, N.E., J.K. Norseworthy, B.J. Young, D.B. Reynolds, W.G. Johnson, S.P. Conley, R.J. Smeda, T.C. Mueller, D.J. Spaunhorst, K.L. Gage, M. Loux, G.R. Kruger and M.V. Bagavathiannan. 2018. Seedbank persistence of Palmer amaranth (Amaranthus palmeri) and waterhemp (Amaranthus tuberculatus) across diverse geographical regions in the United States. Weed Science 66: 446–456.

Leon, R.G. and M.D.K. Owen. 2006. Tillage systems and seed dormancy effects on common waterhemp (Amaranthus tuberculatus) seedling emergence. Weed Science 54: 1037–1044.

Liebman, M., F.D. Menalled, D.D. Buhler, T.L. Richard, D.N. Sundberg, C.A. Cambardella and K.A. Kohler. 2004. Impacts of composted swine manure on weed and corn nutrient uptake, growth, and seed production. Weed Science 52: 365–375.