Waterhemp

Amaranthus tuberculatus (Moq.) Sauer

Amaranthus rudis Sauer

Images above: Upper left: Waterhemp foliage (Lynn Sosnoskie, Cornell University). Upper right: Waterhemp plant (Antonio DiTommaso, Cornell University). Bottom: Waterhemp seedlings (Aaron Hager, University of Illinois).

Identification

Other common names:  common waterhemp, tall waterhemp

Family:  pigweed family, Amaranthaceae

Habit:  Tall, upright, summer annual herb.

Taxanomic note:  Many sources separate common waterhemp (Amaranthus rudis) from tall waterhemp (Amaranthus tuberculatus), but recent authorities have considered the two as a single species (Pratt and Clark 2001, USDA Plants).  Though probably not good species, the two forms may deserve recognition as varieties (A. tuberculatus var. rudis and A. tuberculatus var. tuberculatus) (Costea and Tardif 2003).  Common waterhemp, appears to be more common as an agricultural weed (Costea et al. 2005) 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 these characteristics intergrade extensively in the central Midwest where the species is most problematic (Costea et al. 2005, Pratt and Clark 2001), we here treat them as a single species which we refer to simply as waterhemp.

Description:  Seedling stems are light green to red-pink and 0.1-0.2” (0.25-0.5 cm) tall.  Cotyledons are oval to lance-shaped, 0.2-0.4” (0.5-1 cm) long by 0.08-0.16” (0.2-0.4 cm) wide, hairless, 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 ft (0.8-2.4 m) tall on branching, green to reddish, hairless, occasionally ridged stems.  The leaves are alternate, shiny, hairless, 0.6-6” (1.5-15 cm) long by 0.2-1.5” (0.5-4 cm) wide, and oblong to lance shaped with an abruptly tapered, notched tip.  Leaf stalks are up to 0.4-2.8” (1-7 cm) long, and are usually shorter than the leaf blade.  Upper leaves are smaller and more lance-shaped 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” (0.18-0.28 cm) long, and clustered to form upright, 4-8” (10-20 cm) tall, occasionally branching spikes at branch tips and leaf axils.  Male flowers have 5 green sepals, while female flowers have 0-2 green sepals.  Female flowers are replaced by 0.06” (0.15 cm) long oval capsules.  Each capsule contains a single shiny, round to oval, 0.03” (0.08 cm) 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 has 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

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 (Hartzler and Battles 2004, Nordby and Hartzler 2004).  Tillage greatly reduces waterhemp emergence, and moldboard plowing is more effective than chisel plowing for reducing emergence (Govindasamy et al. 2020, Leon and Owen 2006, Refsell and Hartzler 2009).  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 (Buhler et al. 2001). 

Given its late emergence, probably few waterhemp will mature in a winter grain crop, but they may be able to mature in spring oats (Buhler et al. 2001).  If hay is overseeded into the grain, regrowth of waterhemp following grain harvest will likely create massive seed rain if mowing is delayed until fall (Buhler et al. 2001).  An earlier cutting in August at about 4” (10 cm) 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.

The late emergence of waterhemp means that neither a cultivated fallow period nor a stale seedbed before planting will be effective for controlling the species, except possibly for a midsummer planted vegetable crop.  Effective post-plant cultivation of corn and soybean can provide good control of waterhemp (Buhler et al. 2001,  profile of Mugge farm in Chapter 5).  Establishment from near the soil surface and the tiny size of waterhemp seedlings makes 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. 

 As with other pigweed species, straw or other organic mulch materials should suppress emergence of this species, both by preventing germination cues and by blocking emergence of the tiny seedlings.  Rye cover crop residue reduced emergence and improved early-season control of this weed in soybean, but only if soil weed seed populations were not high (Bish et al. 2021).  A multi-tactic combination of a rye cover crop and narrow soybean row spacing reduced waterhemp biomass and seed production by at least 80% whereas each tactic alone was less effective (Yadav et al. 2023).

Topping waterhemp that emerge above soybeans or other short crops with a mower will not be effective as this species recovers rapidly from clipping.  Electrocution with a Weed ZapperTM provided 51 to 97% control, and was most effective as the height difference between weed and crop increased (Schreier et al. 2022).  Pulling plants will prevent competition with the crop, but the rapid flowering habit of the species means that plants should be removed from the field to prevent seed production.  The high retention of seeds on plants at harvest provides an opportunity for seed capture and destruction during harvest operations (Schwartz et al. 2016, Schwartz-Lazaro et al. 2021).  A Seed TerminatorTM destroyed 94% of waterhemp seeds that passed through the terminator; but, after accounting for 31% of seeds that shattered at the combine head and seeds that evaded passing through the terminator, the total waterhemp seeds returned to the soil when harvesting with the Seed Terminator™ was only reduced by 56% compared to harvesting with a conventional combine (Winans et al. 2023).  Concentration of weed seed bearing chaff into a narrow windrow during harvest operations, a process called “chaff lining”, offers options for improved control of waterhemp in succeeding crops (Bennett et al. 2023).

Ecology

Origin and distribution:  Waterhemp is native to the U.S.A.  It is most problematic as a weed in the Midwest but occurs through most of the eastern and southern U.S.A., and the southern margin of Canada from Quebec to Saskatchewan (USDA Plants).  It’s scattered occurrence from the Rocky Mountains westward is probably due to introduction.  It has also been introduced to Europe (Costea et al. 2005).

Seed weight:  0.19 mg (Stevens 1932); 0.2 mg (Harbur and Owen 2004, Shergill et al. 2020); 0.20-0.24 mg (Wu and Owen 2015); 0.22 mg when established in May or June, 0.26 mg when established in July (Heneghan and Johnson 2017); 0.27 mg (Sellers et al. 2003).  Seed weight was reduced by the presence of herbicide resistant traits in the population, such that seeds of an herbicide-susceptible population were modeled to weigh 0.20 mg whereas seeds from a population resistant to five herbicides were modeled to weigh 0.16 mg (Jones et al. 2019).

Dormancy and germination:  Seeds collected from recently matured plants are dormant (Hartzler et al. 1999, Leon and Owen 2003, Bell and Tranel 2010, Wu and Owen 2015). Populations vary in the after-ripening conditions needed to allow germination.  Some populations germinate best after 12 weeks of wet, cold (e.g. 39 °F = 4 °C) 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 (Leon and Owen 2003, Leon et al. 2006).  Seeds begin germination at mean daily temperatures of 41-59 °F (5-15 °C), depending on the population (Leon et al. 2004, Steckel et al. 2004), but percentage germination is low at such cool temperatures.  Peak germination occurs at mean daily temperatures of 68-91 °F (20-33 °C) (Leon et al. 2004, Guo and Al-Khatib 2003, Steckel et al. 2004).  Some germination can occur at day/night temperatures as high as 113/104 °F (45/40 °C) (Guo and Al-Khatib 2003).  Germination at any constant temperature is relatively poor whereas day-to-night temperature fluctuations substantially promote germination (Leon et al. 2004, Steckel et al. 2004).  A large range in day-to-night temperatures of 32-43 °F (18-24 °C) maximizes germination (Leon et al. 2004).  The after-ripening requirements of agricultural populations coupled with peak germination at warm temperatures insures that most seeds will not germinate until the summer following production (Bell and Tranel 2010).  In warm, fluctuating temperature conditions, seeds germinate in as little as one day (Leon et al. 2004).  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 (36 °C), can overcome this latter inhibition and allow germination under plant canopies (Leon and Owen 2003).

Seed longevity:  Waterhemp seeds mixed into the top 2” (5 cm) of soil and stirred annually in spring had an average annual mortality of 40% (computed from Buhler and Hartzler 2001).  In Nebraska, the computed annual seed mortality rate over the first three years was 30-44%, however 1-3% of seeds still germinated after 17 years of burial at 8” (20 cm) (Burnside et al. 1996).  In Illinois, seed mortality after one year was 61%, while mortality was >99% after four years (Steckel et al. 2007).  In several midwestern states, the average mortality of seed buried 6” (15 cm) deep for 12 months was 78% (Korres et al. 2018).

Season of emergence: Waterhemp typically emerges from mid-May to late July in Iowa, with peak emergence in mid- to late-June.  It begins emerging when the average weekly soil temperature reaches 54 °F (12 °C) (Steckel et al. 2007).  Reduced tillage tends to delay peak emergence (Govindasamy et al. 2020, Hartzler et al. 1999, Leon and Owen 2006, Refsell and Hartzler 2009, Wu and Owen 2014).  Waterhemp is classified as a late emerging weed with a long duration of emergence in Nebraska (Werle et al. 2014).  In Ontario, emergence occurs from June through August (Costea et al. 2005).  Waterhemp required 14-17 days to emerge when planted in May in Missouri, and had the slowest emergence of six pigweed species tested (Sellers et al. 2003).  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 (Heneghan and Johnson 2017).

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” (2.5 cm) or less of the soil.  The great suppression of emergence by tillage further supports a shallow emergence pattern (Leon and Owen 2006, Refsell and Hartzler 2009).

Photosynthetic pathway:  C4 (Costea et al. 2005).

Sensitivity to frost:  Flowering and seed set stop at the first frost (Costea et al. 2005).

Drought tolerance:  Waterhemp is not drought tolerant (USDA Plants).  Plant growth and seed production declines linearly as soil water content declines and as the number of days without water increases (Sarangi et al. 2015).  The pattern of seed production in several short pulses over an extended period appears to be an adaptation to ensure reproductive success in the face of unpredictable summer rainfall patterns (Wu and Owen 2014).

Mycorrhiza:  Waterhemp is probably non-mycorrhizal in most circumstances (Costea et al. 2005).

Response to fertility:  In sand, increased ammonium nitrate raised waterhemp relative growth rate 5 fold, similar to the response of corn (Harbur and Owen 2004).  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 soybean (Menalled et al. 2005).  This effect would make the weed both more competitive against crops and also more difficult to control with cultivation at high compost rates.  In soybean, application of a high rate of composted swine manure increased waterhemp dry weight by 25-50% (Menalled et al. 2004).  In an experiment with corn, application of 24-33 t/A (54-73 Mg/ha) of composted swine manure roughly doubled the dry weight of waterhemp (Liebman et al. 2004).  On the other hand, composted swine manure inhibited waterhemp emergence but had no effect on emergence of the crops (Menalled et al. 2005).  Waterhemp tolerates a pH from 4.5-8.0 (Costea et al. 2005).

Soil physical requirements:  The species natural habitat is the wet soils of marshes and the edges of lakes, ponds and rivers (Costea et al. 2005).  Consequently, it tolerates wet, anaerobic soils (USDA Plants), but does best in well drained agricultural soils (Costea et al. 2005).  It tolerates a wide range of soil textures (Costea et al. 2005), but grows best in medium to fine textured soils (USDA Plants).    

Response to shade: Waterhemp is considered shade intolerant (USDA Plants).  Shade from the crop leaf canopy of  >98% resulted in early cessation of emergence, namely in early July rather than August (Steckel et al. 2007).  Considering established plants, shade of 68% reduced final plant weight by about 50%, but plants emerging in May still produced 400,000 seeds/plant and June emerging individuals produced 90,000 seeds/plant.  With 99% shade, mortality was substantial, and remaining plants were small and produced few seeds (Steckel et al. 2003).

Sensitivity to disturbance:  Waterhemp recovers well from clipping.  In greenhouse studies, removing half the shoot had no effect on plants 4-8” (10-20 cm) tall and only reduced weight of 12-16” (30-40 cm) plants by 22%.  In the field, removing half the shoot of 6” (15 cm) 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 each (Mager et al. 2006).  Waterhemp has a taproot (Costea, et al., 2005), 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 pigweed species, begins flowering in response to shortening daylength (Costea, et al., 2005).  Populations established in May and June required 5 to 7 weeks after emergence to begin flowering, whereas populations established in July required 3 to 4 weeks (Heneghan and Johnson 2017, Wu and Owen 2014).  Time to initiation of flowering was consistently a few days (Wu and Owen 2014) to 1.5 weeks (Jones et al. 2019) earlier for male than female plants, ensuring sufficient pollen availability when females began flowering.  Some seeds first become viable 7-9 days after pollination at which time they are brown, but seed weight and percentage viability increases until 12 days after pollination, at which time they are black (Bell and Tranel 2010).  Seed maturation occurred similarly in five cohorts emerging from May to July, namely, at 20-27 days after flower initiation and 6-13 days after pollination (Wu and Owen 2015).

Pollination:  Since male and female flowers occur on separate plants, the species necessarily outcrosses.  It is primarily wind-pollinated (Costea et al. 2005).  Pollen-mediated gene flow accounts, in part, for spread of herbicide resistance in this species (Jhala et al. 2021), and no doubt could facilitate spread of traits conferring tolerance to cultural practices as well.

Reproduction:  Female waterhemp plants can produce as many as 1,000,000 seeds, but 35,000-200,000 seeds is more typical (Costea et al. 2005, Stevens 1932).  Several populations produced 470,000 to 1,290,000 seeds per plant in Indiana when established in May or June, but produced 200,000 to 340,000 seeds when established in July (Heneghan and Johnson 2017).  In soybeans, seed production declined exponentially with an increasing lag between crop planting and waterhemp emergence (Hartzler and Battles 2004).  Thus, waterhemp establishing simultaneously with soybeans in Iowa produced 300,000 seeds/plant, whereas those emerging 50 days later produced 3,000 seeds/plant (Hartzler et al. 2004).  On the other hand, waterhemp seed production per plant is highly density dependent, so individual plant production will vary depending on plant density (Wu and Owen 2014).  Waterhemp produced an average 288,000 seeds/plant in Missouri and had the highest seed production per unit plant weight of six pigweed species tested (Sellers et al. 2003).  Waterhemp plants retained at soybean harvest from 95-100% of seeds in one report (Schwartz et al. 2016) and from 70 to 99% at 8 of 10 site-years in another report (Schwartz-Lazaro et al. 2021).

Dispersal:  Waterhemp seeds float and probably disperse by overland water flow, in irrigation water, and along streams (Costea et al. 2005).  Although passage through ruminants has not been studied, the seeds are similar to those of other pigweed species that disperse readily in feces and manure (Costea et al. 2005).  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 (Hilty 2010).  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 (Costea et al. 2005, van der Laat et al. 2015).  These species preferred waterhemp seeds to those of several other prominent weed species (van der Laat et al. 2015).  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) (Costea et al. 2005).  Microsphaeropsis amaranthi has potential as a bioherbicide for control of waterhemp provided formulations can avoid dry leaf surfaces during the first 12 hr after application (Smith et al. 2006).

Palatability:  Palatability of waterhemp for grazing animals is low (USDA Plants).

References:

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