Pigweeds

Powell amaranth, Amaranthus powellii S. Watson

Redroot pigweed, Amaranthus retroflexus L.

Smooth pigweed, Amaranthus hybridus L.

Images above: Upper left: Powell amaranth seedling (Antonio DiTommaso, Cornell University). Upper right: Powell amaranth developing inflorescence (Antonio DiTommaso, Cornell University). Bottom: Powell amaranth older seedling (Antonio DiTommaso, Cornell University)

Images above: Upper left: Redroot pigweed seedling (Joseph DiTomaso, University of California, Davis). Upper right: Hairy stem of redroot pigweed (Antonio DiTommaso, Cornell University). Bottom: Redroot pigweed leaves (Antonio DiTommaso, Cornell University)

Images above: Left: Smooth pigweed seedlings (Antonio DiTommaso, Cornell University). Right: Smooth pigweed inflorescence (Scott Morris, Cornell University).

Identification

Visit this page for more information on pigweed identification.

Other common names:

  • Powell amaranth:  rough pigweed, amaranth pigweed, green amaranth, careless weed
  • Redroot pigweed:  rough pigweed, green amaranth, pigweed, wild beet, amaranth pigweed, red-root, careless weed, redroot amaranth, Chinaman's greens, common amaranth
  • Smooth pigweed:  green amaranth, pigweed, wild beet, spleen amaranth, rough pigweed, amaranth pigweed, red amaranth, careless weed, prince's feather, slender pigweed, smooth amaranthus

Family:  amaranth family, Amaranthaceae

Habit:  Erect, often branched, summer annual herbs.

Description:  Seedlings have reddish-pink stem bases and oval shaped true leaves. 

  • Powell amaranth:  Stems are nearly hairless and red tinged.  Cotyledons are 0.5” (1.3 cm) long and lance shaped, with red or purple undersides.  Young leaves are green with red-purple undersides, slightly notched, not wavy along the edges, with a prominent midvein.  Very few hairs are present on stems, stalks, and leaves.
  • Redroot pigweed:  Stems are hairy and pale green.  Cotyledons are 0.5” (1.3 cm) long, lance shaped (4-5 times longer than wide), and with red or purple undersides.  Young leaves are green, wavy edged and notched with a prominent central vein.  Central vein is prominent on upper surface near blade base.  Small, tough hairs are present on leaf stalks.
  • Smooth pigweed:  Stems are hairy and reddish-purple.  Cotyledons are dull green with red-purple undersides, 0.13-0.5” (0.3-1.3 cm) long and less than 0.13” (0.3 cm) wide.  Young leaves are dark green, slightly wavy-edged, and slightly notched with a prominent midvein.  Hair is dense on the stems.

Mature plants have a shallow, sometimes red taproot.  Leaves are green and alternately arranged.

  • Powell amaranth:  Stems are nearly hairless and can reach 5 ft (1.8 m) tall.  Leaves are 0.6-3” (1.5-8 cm) long by 0.2-1.6” (0.5-4 cm) wide, diamond shaped, pointy-tipped, non-ruffled, and shiny green (sometimes red-tinged) with white veins on the blade undersides.
  • Redroot pigweed:  Upper stems are coated with curly hairs and can reach 6 ft (1.8 m) tall.  Leaves are 5-6” (13-15 cm) long by 2.5” (6 cm) wide, oval to diamond shaped, wavy-edged, and dull green with white central veins on the blade undersides.
  • Smooth pigweed:  Upper stems are coated with short hairs and can reach 6.5 ft (2 m) tall.  Leaves are up to 6” (15 cm) long, oval to egg shaped, wavy-edged, long stalked, and dark green with strong veins and light green to magenta-tinged undersides.

Flowers are small, greenish (turning brown upon maturity), and clustered into long groups located at branch ends and in leaf-stem joints.  Clusters on branch ends are larger than those in leaf axils.  Individual flowers are either male or female.

  • Powell amaranth:  Flowers are clustered into thin, stiffly upright, mostly unbranched spikes. Branch end clusters can reach 10” (25 cm) long by 0.4-0.8” (1-2 cm) wide.
  • Redroot pigweed:  Flowers are tightly clustered into stiff, branching panicles.  Branch end clusters can reach 2-8” (5-20 cm) long by less than 1” (2.5 cm) wide.
  • Smooth pigweed:  Flowers are clustered into nodding, branching spikes.  Branch end clusters can reach 6” (15 cm) long; those in axils can reach 0.5-3” (1-8 cm) long.  Clusters are softer than those of the other species. 

Fruit and Seeds:  Seeds develop singly in small, bladder-like fruits (utricles).  Seeds are glossy and dark brown to black, no larger than 0.05” (0.1 cm), oval to ellipse shaped, flattened, and notched at the narrow end.

  • Powell amaranth:  Utricles do not easily rupture, instead, both seed and sac often fall off the plant as an intact, light tan, 0.1” (0.25 cm) long, unit.
  • Redroot pigweed:  The utricles rupture around the middle, dispersing seeds.
  • Smooth pigweed:  Utricles rupture around the middle, dispersing seeds.

Similar species:  Common lambsquarters (Chenopodium album L.) seedlings have opposite, fuzzy or grainy looking, white to pinkish young leaves.  Palmer amaranth (Amaranthus palmeri S. Watson) leaves are hairless with white veined undersides and long stalks.  It has a, soft, 1-1.5 ft (30-46 cm) long inflorescence atop its main stem.  Waterhemp [Amaranthus tuberculatus (Moq.) Sauer] has tapering, thumb shaped leaves, long, coarsely branched inflorescences, and stems that can reach 8 ft (2.4 m) high.  Tumble pigweed (Amaranthus albus L.) stems are pale green to white, spiny amaranth (Amaranthus spinosus L.) stems are grooved, hairless, and spiny, and prostrate pigweed (Amaranthus blitoides S. Watson) is distinguished by its red, flexible, ground-hugging stems.  Livid amaranth (Amaranthus blitum L.) has a matting habit, green to red stems, and deeply notched, 1.5” (4 cm) long, leaves.

Management

Pigweed germination is generally responsive to tillage and cultivation after the soil warms.  Thus, a one- to two-week lag between initial and final seedbed preparation in late spring or summer helps flush out and destroy seedlings.  The final seedbed preparation should be to a depth of no more than 1.5” (3.8 cm) to avoid raising seeds to near the surface.  Tine weed or rotary hoe when seedlings are first emerging.  Once the crop is large enough to tolerate inter-row cultivation, hill up less than 1” (2.5 cm) before true leaves of pigweed seedlings appear.  Even crops like cabbage and squash that are not normally hilled will tolerate this amount of soil against the base.  When growing a crop like corn or potato, that tolerates heavy hilling, pile the soil up as high as possible to kill any remaining seedlings at later cultivations.

For small grain crops, a dense, uniform and vigorous stand is important for maximizing the crop's competitive advantage.  Harrowing spring grains at between the 3-leaf stage and stem elongation is often well timed to eliminate many newly emerged and white thread stage pigweed seedlings. 

Straw mulch and other mulch materials are highly effective for suppressing these species since their small seeds provide little resources for pushing the seed leaves up out of the mulch.  Because the seed-leaves stay together in a vertical position until they reach the light, however, some seedlings will usually penetrate 3,600 lb/acre (4,000 kg/ha) of loose straw (Mohler and Teasdale 1993, Mohler unpublished data), so either use a heavier mulch layer or compact the mulch after application since dense mulches are more difficult for pigweed to penetrate (Teasdale and Mohler 2000).  In addition, residue from a cereal will persist longer and provide longer weed suppression than that from a legume cover crop.  Recent research has concluded that a rye cover crop (either alone or mixed with other species) with a biomass of 6600 lb/acre (7400 kg/ha) is required to suppress redroot pigweed by 80% (Pittman et al. 2020).

Pigweeds are highly responsive to N and P fertility, so avoiding excess fertilization is critical to management.  If possible, apply fertility amendments after the crop is established.  Note that legume cover crop residue, particularly if soil coverage is incomplete, can stimulate pigweed emergence (Teasdale and Mohler 2000).  Thus, mixing rye with a legume cover crop can be beneficial not only for increasing the persistence of the cover crop residue but also for decreasing the stimulatory impact of N on emergence. 

Pigweeds are only moderately persistent in the seed bank so rotating with winter cereals or hay crops can decrease pigweed seed density (Teasdale et al. 2004).  Also, a few years of good control will dramatically reduce pigweed seed bank density (Mohler et al. 2018a), and incorporation of a legume cover crop can accelerate seed bank decline (Mohler et al. 2018b). 

Pigweeds are prolific seed producers, so clean up fields promptly after harvest if these weeds are present.  If possible, remove plants that have flowered, as they can continue to form seeds even after mowing or light tillage that leaves fragments of flowering stalks on the soil surface (Mohler, personal observation).  Hand rogue at least the larger plants out of the crop if this is economically feasible.  Most seeds remain on plants until soybean harvest, which provides an opportunity to capture or destroy seeds during combining (Schwartz-Lazaro et al. 2021).

Ecology

Origin and distribution:  Redroot pigweed is native to eastern and central North America and Powell amaranth to the mountains of western North America.  Smooth pigweed is native from eastern North America through Mexico to South America (Weaver and McWilliams 1980, Costea et al. 2004).  All three species occur throughout most of the U.S.A., though Powell amaranth is absent from inland parts of the Southeast and smooth pigweed from parts of the Rocky Mountain states (USDA Plants).  All three species have been introduced further northward into Canada (Costea et al. 2004), and redroot pigweed occurs in Alaska (USDA Plants).  Redroot pigweed has been introduced throughout Europe and Asia, to Australia and New Zealand, and most of Africa (Holm et al. 1997).  Smooth pigweed has been introduced into Africa, South and East Asia, Australia and New Zealand (Holm et al. 1977).

Seed weight:  Mean seed weight for various redroot pigweed populations ranges from 0.25 to 0.50 mg (Gaba et al. 2019, McWilliams et al. 1968, Schimpf 1977) with cooler and drier locations having larger seeds.  Powell amaranth, 0.40-0.54 mg (Hauptli and Jain 1978, Weaver 1984, Senesac 1985).  Smooth pigweed, 0.33-0.46 mg (Weaver 1984, Senesac 1985, Shergill et al. 2020).

Dormancy and germination:  Germination is stimulated by high soil temperatures of 86-104 °F (30-40 °C) (Costea et al. 2004, Hu et al. 2018).  Higher temperatures are required to stimulate germination in younger relative to older seeds (Costea et al. 2004).  A period of burial in the soil increases germinability (Omami et al. 1999) and decreases the minimum temperature for germination to 68 °F (20 °C) or lower (Costea et al. 2004).  Germination increases with exposure to light (Hu et al. 2018), and redroot and smooth pigweed are sensitive to the equivalent of 0.01 seconds of sunlight (Costea et al. 2004).  Nitrate also stimulates germination of these species (Costea et al 2004, Brainard et al. 2006), possibly by making seeds more sensitive to light (Gallagher and Cardina 1998).  Seeds of redroot pigweed produced under long day length, cool temperatures or nutrient stressed are more dormant, as are Powell amaranth seeds produced in competitive environments (Costea et al 2004, Brainard et al. 2005).  Low soil moisture conditions induce secondary dormancy in pigweed seeds (Forcella et al. 1997, Gallagher and Cardina 1997, Martinez-Ghersa et al. 1997).  The first seeds produced on a plant are less dormant than seeds produced at the end of the growing season (Costea et al. 2004).  This allows a second generation to emerge and reproduce within a growing season in moderate to warm climates, but prevents extensive germination of seeds too late to successfully produce mature plants.

Seed longevity:  When buried in containers at the bottom or below the plow layer, a few redroot pigweed seeds have survived several decades (Burnside et al.1996, Telewski and Zeevaart 2002), and smooth pigweed sustained only 12% annual mortality in one report (Gardarin et al. 2010).  More typically, seed survival of redroot pigweed and Powell amaranth is poor near the soil surface, but even deep in undisturbed soil the annual survival rate is substantially lower than that of many other annual broadleaf weeds (Toole and Brown 1946, Taylorson 1970, Mohler unpublished data).  A natural population of redroot pigweed seeds in Mississippi disappeared completely in three years, both with and without annual spring tillage (Egley and Williams 1990), but this is probably an extreme case.  In Michigan, the annual mortality of buried redroot pigweed seeds was 41-81% (Davis et al. 2005).  In another study with annual tillage, a redroot pigweed seed bank declined 36% per year (Barralis 1988).  In New York, the annual mortality rate of Powell amaranth seeds buried 6” (15 cm) ranged from 45 to 88% (Mohler et al. 2018b).  In a seven-year experiment with monthly tillage to 10” (25 cm), the number of Powell amaranth emerging declined by 43% per year (Popay et al. 1994).  In Maryland and Pennsylvania, mortality of buried smooth pigweed seeds was 39% after one year and 71% after two years (Ullrich et al. 2011).

Season of emergence:  These species emerge mainly in late spring and early summer, but continue to emerge throughout the growing season, particularly after soil disturbance (Weaver and McWilliams 1980).  Redroot pigweed (Werle et al. 2014) and smooth pigweed (Myers et al. 2004) are classified as late emerging weeds with a relatively long (approximately two month) emergence duration.

Emergence depth:  Optimal depth for emergence is 0.2-0.8” (0.5-2 cm).  One greenhouse study of redroot pigweed found an optimum emergence depth of 0.2” (0.5 cm) in typical medium textured soils (Ghorbani et al. 1999), whereas another found that optimum emergence extended to 0.8” (2 cm) (Benvenuti et al. 2001).  A field study of Powell amaranth found reduced emergence at 1.2” (3 cm) relative to 0.2-0.8” (0.5-2 cm) and no emergence from 2” (5 cm) (Mohler, unpublished data).

Photosynthetic pathway:  C4 (Costea et al. 2004, Elmore and Paul 1983).

Sensitivity to frost:  These species are killed by frost (Stevens 1924).

Drought tolerance:  These species are moderately drought tolerant (Costea et al. 2004, Lovelli et al. 2010).  Low soil moisture can reduce redroot pigweed shoot biomass and seed production substantially compared to well-watered plants, but root biomass is only slightly impacted (Khan et al. 2021a).  Redroot pigweed produces at least double the root length of other weed and crop species during the first month of growth, partially by growing thinner roots (Seibert and Pearce 1993).   A fast-growing root system would allow this small-seeded species to access soil moisture more quickly when in competition with other species.  In addition, the C4 photosynthetic pathway of these species is unusual for broadleaf weeds, and allows the amaranths to better tolerate heat and drought conditions than many other species (Patterson and Flint 1983).

Mycorrhiza:  These species are basically non-mycorrhizal (Harley and Harley 1987, Vatovec et al. 2005), but mycorrhizal associations do occur occasionally.

Response to fertility:  Redroot pigweed is highly responsive to N, P and K (Hoveland et al. 1976, Blackshaw et al. 2003, Blackshaw et al. 2004), and will increase in growth with N application rates up to 480 lb/A (540 kg/ha).  Not surprisingly, redroot pigweed becomes more competitive against crops as N application rate increases (Blackshaw and Brandt 2007, Tyker et al. 1991).  Redroot pigweed emergence can be reduced by compost or manure applications, but growth of seedlings that do emerge is increased by compost (Amisi and Doohan 2010).  Powell amaranth had the highest response to varying compost rates of seven crops and weeds tested (Little et al. 2015), and smooth pigweed is probably also highly responsive to compost and other organic fertilizers.  Smooth pigweed growth, however, was unresponsive to P over a range that nearly doubled the growth of lettuce, but it still interfered with lettuce by concentrating P in its tissues (Santos et al. 2004).  Redroot pigweed does poorly on soils with pH below 5.2 (Buchanan et al. 1975) and Powell amaranth grows more poorly at pH 4.8 compared to 6.0 or 7.3 (Costea et al. 2004).

Soil physical requirements:  All three species thrive on a wide range of soil textures from sand to clay and muck (Weaver and McWilliams 1980).  A comparison of redroot pigweed growth on different soil types, however, found best growth on sandy loam and poorest growth on silty clay (Ghorbani et al. 1999).  Most redroot pigweed populations are intolerant of salt, but adapted populations are moderately tolerant (Costea et al. 2004).

Response to shade:  Growth of these species is substantially reduced by low light (Rajcan et al. 2002, Brainard et al. 2005).  In partial shade, plants are less branched and allocate more energy to stem growth (Costea et al. 2004, Rajcan 2002, Brainard et al. 2005), and this sometimes helps them to grow out from under a competing crop canopy.

Sensitivity to disturbance:  Newly emerged seedlings are tiny, fragile and easily broken or buried (Mohler, personal observation).  Plants resprout if cut above the seed leaf node (Costea et al. 2004).  Plants of moderate size or larger will re-root readily in moist soil.

Time from emergence to reproduction:  All three species flower in response to shortening days.  Redroot pigweed emerging in early summer under long days flowered in about 6.5 weeks, with seeds maturing 7 to 8 weeks later (Shrestha and Swanton 2007).  Similarly, in Wisconsin, smooth and redroot pigweed emerging in May flowered in 7 and 8 weeks, respectively (Doll 2002).  In contrast, plants emerging under shorter days in late summer will flower and produce seeds very rapidly with as few as 3-4 leaves at the time of flowering.  For example, under short day conditions, flowering occurred in 3 weeks with seeds maturing 3 weeks later (Huang et al. 2000).  Research in Australia gave similar results, whereby redroot pigweed emerging in spring flowered about 6 weeks later whereas those emerging in summer flowered about 3 weeks later (Khan et al. 2021b).  Although competition can greatly influence the number of leaves at flowering, the number of days to flowering was unaffected under a wide range of competitive environments under field conditions (Brainard 2005).

Pollination:  These species are primarily self-pollinated but, since the flowers are either male or female, wind or gravity is needed to move the pollen (Costea et al. 2004).

Reproduction: Plants continue to flower and produce mature seeds until frost (Weaver and McWilliams 1980).  To detect mature seeds, rub a portion of the flower cluster between the fingers and look for hard black seeds (Mohler, personal observation).  Plants grown in favorable conditions typically produce 25,000 to 120,000 seeds (Brainard and Bellinder 2004), though plants with over 1,000,000 seeds have been reported for redroot pigweed (Costea et al. 2004).  Plants emerging in a well-established crop, however, may produce only a few dozen seeds (Mohler and Callaway 1995).  Also, seed production from plants emerging in summer will be considerably lower than from spring-emerging plants (Khan et al. 2021b), but these late emerging plants can still contribute significantly to the seedbank if not managed.  A very low proportion (2-15%) of redroot and smooth pigweed seeds in 8 of 11 site-years were shattered at the time of soybean harvest (Schwartz-Lazaro et al. 2021).

Dispersal:  Because seeds of these species often reach high densities in soil, they are easily spread from one site to another in soil clinging to boots, tires and tillage machinery. They are also picked up and dispersed by combines. Wind usually only blows the seeds a few feet, but some seeds remain on the inflorescence into the winter and can blow longer distances on crusted snow.  A substantial percentage of seeds survive passage through the digestive tracts of ruminants and rabbits and are dispersed with their droppings (Costea et al. 2004).  Cow manure is commonly contaminated with pigweed seeds and spreading the manure disperses these seeds (Mt. Pleasant and Schlather 1994, Eberlein et al. 1992)Both seeds and bits of the inflorescence float and disperse in irrigation water (Wilson 1980, Kelley and Bruns 1975).

Common natural enemies:  Larvae of the micromoth Coleophora lineapulvella can greatly reduce seed production (DeSousa et al. 2003).  Mice, carabid beetles and crickets consume many seeds after dispersal (Costea et al. 2004).  European corn borer (Ostrinia nubilalis) sometimes causes substantial damage (Weaver and McWilliams 1980).  In addition, the tarnished plant bug (Lygus lineolaris) is often seen on pigweed seed heads and can reduce seed production of closely related grain crops in the Midwest by 80% or more (Olsoni and Wilson, 1990). 

Palatability:  The young foliage can be used in salads or as a potherb.  The foliage may contain high levels of nitrate or oxalate, but these can be removed by cooking and draining off the water.  The seeds can be cooked as grain or ground into flour (Costea et al. 2004).  Smooth pigweed and Powell amaranth are believed to be the wild progenitors of the grain amaranths A. cruentus and A. hypochondriacus, respectively (Weaver and McWilliams 1980). All three species make nutritious and highly digestible forage (Bosworth et al. 1980, Costea et al. 2004).  Consumption of large quantities of fresh pigweed over several days, however, can cause poisoning of pigs, cattle and sheep (Burrows and Tyrl 2006).

References:

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