Palmer amaranth

Images above: Left: Palmer amaranth foliage and petioles (Joseph DiTomaso, University of California, Davis). Right: Palmer amaranth plant (Joseph DiTomaso, University of California, Davis).

Images above: Left: Palmer amaranth inflorescence (Mike Stanyard). Right: Palmer amaranth inflorescence, closeup (Joseph DiTomaso, University of California, Davis).

Identification

See Pigweed Identification for more information on how to identify Palmer amaranth.

Other common names:  carelessweed, Palmer pigweed

Family:  pigweed family, Amaranthaceae

Habit:  Tall, erect annual herb.

Description:  Seedling stems are usually red, sometimes green, and sometimes slightly hairy.  Cotyledons are lance shaped, hairy, and red to green in color.  Young leaves are alternate, egg shaped, and roughly 5 times longer (0.4-0.5” = 1-1.3 cm) than wide, with notched tips.  The coarse, red stem of mature plants will reach 1.5-6.5 ft (0.5-2 m) tall with long, thin branches spaced far apart on the stem.  Upper stems and branches are greener, less hairy, and have narrower, pointier leaves than those located lower on the plant.  Leaves are alternate, hairless egg to lance shaped, 2-8” (5-20 cm) long by 0.5-2.5” (1.3-6 cm) wide, with conspicuous white veins on the undersides, and long leaf stalks.  The relatively shallow taproot is reddish near the soil surface.  Plants are either male or female, with individual, inconspicuous, green flowers grouped in spikes; spikes are soft or lightly spiny, thin, 0.3-0.8” (0.8-2 cm) wide, often nodding, and largely unbranched.  The longest spike is located on the main stem; smaller spikes are present at branch tips and in upper leaf-stem joints.  Terminal spikes of male plants are yellow and 0.5-1.5 ft (15-45 cm), with thin, triangular, green bracts around individual flowers.  Female spikes are similar, with thick, stiff, notched bracts with a small spine located in the notch.  Fruits are brown, triangular or pyramid shaped sacs, with 3-5 spines at their tip; sacs rupture at maturity, releasing one glossy, round, dark red-brown to black, 0.04-0.06” (0.1-0.15 cm) long seed.

Similar species:  Palmer amaranth seedlings are similar to redroot pigweed (Amaranthus retroflexus L.), smooth pigweed (Amaranthus hybridus L.), Powell amaranth (Amaranthus powellii S. Watson), and waterhemp [Amaranthus tuberculatus (Moq.) Sauer] seedlings.  Mature Palmer amaranth has thinner stems than redroot pigweed, and its combination of non-wavy, egg shaped, round-tipped leaves with very long leaf stalks and white veined undersides sets the mature plant apart.  Other Amaranthus spp. have crowded, highly branching spikes, different from the long, nodding, thin, lightly branching terminal spikes of Palmer amaranth.  Only waterhemp is like Palmer amaranth in having separate male and female plants.

Management

Since Palmer amaranth has a relatively short lived seed bank, rotation of land into several years of a perennial sod should reduce population levels substantially.  It will rarely be a problem in winter grain crops and early planted spring grains since these crops do most of their growth before this weed emerges in late spring.  Since they are harvested before it matures, control after grain harvest is required to interrupt the weed’s life-cycle.  Over 95% control of plants remaining after wheat harvest was required to prevent seed production by this weed (Kumar et al. 2021).

Since Palmer amaranth seeds must be near the soil surface for successful emergence, inversion tillage reduces densities of the weed relative to reduced tillage systems (Timper et al 2011, Ward et al. 2013).  Nevertheless, because the species often produces such a prodigious number of seeds, even in competitive crops, annual tillage will still not eliminate all seeds that can emerge to infest the crop.  High seed retention on plants at harvest suggests the potential utility of seed collection and destruction during combining (Schwartz et al. 2016, Schwartz-Lazaro et al. 2017, 2021).  Removal of harvested residue and chaff from the field reduced subsequent Palmer amaranth populations by up to 70% (Norsworthy et al. 2016).  Narrow-windrow burning of soybean residue destroys most Palmer amaranth seeds (Norsworthy et al. 2020, Spoth et al. 2022).

Palmer amaranth is among the fastest growing of all weeds.  Its photosynthetic rate is very high, even for a C4 species, and its leaves track the sun, which insures a high photosynthetic rate throughout the day (Ehleringer 1983).  When plants are small they can double in size every 2-3 days (Guertin 2003), and plants can reach 4” (10 cm) tall 2 weeks after the seeds begin germination (Sellers et al. 2003) and over 6.6 ft (2 m) by maturity (Ward et al. 2013).  Root growth is also five-fold greater than that of soybean (Wright et al. 1999b).  This gives the weed an advantage in competition for water and nutrients (Ward et al. 2013).  The extremely rapid emergence and growth of Palmer amaranth means that seedlings are only briefly susceptible to rotary hoeing and tine weeding, and good timing of operations is particularly important when managing this weed.  In corn and sorghum, two pre-emergence blind cultivations may be useful, one about three days after planting and the second just before crop emergence.  Similarly, tine weed or rotary hoe at close intervals (e.g. 4 days) after crop emergence will also reduce populations.  This will require slowing tine weeders down when the crop is very small, but early control is critical: the rapid growth means that any early emerging seedlings that escape will be too big to bury by the time row crops are large enough to hill. 

The competitiveness of Palmer amaranth was attributed primarily to shading imposed by tall plants growing above the crop leaf canopy (Moore et al. 2021), so any management practice that prevents this weed from overtopping the crop is required.  High corn density and irrigation improved corn competitiveness against Palmer amaranth in Kansas (Curie and Klocke 2008).  Increasing the population of drilled soybean suppressed Palmer amaranth emergence in Arkansas (Bell et al. 2015) as well as suppressed growth and seed production of this weed (Korres et al. 2020).  Since soybean will germinate and grow at soil temperatures too low for germination and growth of Palmer amaranth (Wright et al. 1999a), early planting can give the soybeans a competitive head start against this weed.  Early planting was also shown to delay the beginning of the critical period for controlling this weed in pickling cucumber (McGowen et al. 2018).  Alternatively, a period of fallow with repeated shallow tillage in late spring can flush a large proportion of the seeds out of the surface seed bank, resulting in less weed pressure in the crop.  The long season of emergence, rapid growth and exceptional seed production potential of Palmer amaranth makes all of the tactics discussed above only partially effective.  In addition, development of glyphosate resistant populations appears to have facilitated the rapid development of taller, more aggressive plants that are better adapted to compete in various crop canopy structures (Bravo et al. 2017).  Good control may require hand pulling of plants before they go to seed.

Black plastic mulch is effective for suppressing most Palmer amaranth in vegetable crops but plants that emerge through planting holes can overwhelm crops.  In bell pepper production, Palmer amaranth plants should be removed from planting holes within 5 weeks of transplanting the crop (Norsworthy et al. 2008).  Cover crop mulches can contribute to integrated management of this small-seeded species (Ward et al. 2013).  Residue from a rye cover crop suppressed Palmer amaranth population by 57-58% in Arkansas (Norsworthy et al. 2016).  Even a relatively light layer of rye mulch 1,000-2,000 lb/A (1,100-2,200 kg/ha) can reduce Palmer amaranth density by over 70% (Timper et al. 2011), but rye mulch biomass generally needs to exceed 8,900 lb/A (10,000 kg/ha) for greater than 80% Palmer amaranth suppression (Webster et al. 2016).  This level of rye requires early fall planting and high residual soil nitrogen (Webster et al. 2016).  Two years of a rye cover crop following a single inversion tillage to bury surface seeds controlled Palmer amaranth by 92% compared to 69% by inversion tillage alone (DeVore et al. 2013).  Incorporation of winter brassica cover crops, including turnip, garden cress, oilseed rape, and Indian mustard, suppressed Palmer amaranth in transplanted bell peppers by over 40% (Norsworthy et al. 2007).

An eradication program can be successful with a robust noxious weed program, collaboration among all levels of the agricultural community and local jurisdictions, and rapid identification through genetic testing.  Prescribed fire and propane torching which produced at least 1,093 °C provided sufficient heat to kill Palmer amaranth seeds (Yu et al. 2021).

Ecology

Origin and distribution: Palmer amaranth is native to the canyons and desert washes of the Southwest (Sauer 1957).  In the past 50 years it has become a problem weed in much of the Southeast, and southern Midwest (Guertin 2003).  It now occurs sporadically northward to Wisconsin, Ontario and Massachussets (USDA Plants) and can successfully compete with crops and complete its lifecycle throughout Illinois (Davis et al. 2015).  It has also been introduced to Europe, Asia, and Australia (FNA).

Seed weight:  0.44 mg (Sosnoskie et al. 2014), 0.45 mg (Jha et al. 2010), 0.49 mg (Sellers et al. 2003).  Seeds produced under water-stress were 18% heavier than seeds produced under well-watered conditions (Matzrafi et al. 2021).

Dormancy and germination:  Some newly matured seeds can germinate immediately, but most are dormant (Jha et al. 2010, Keeley et al. 1987).  Seeds produced from spring emerging plants have higher germinability (40-70%) than those produced by fall emerging plants (15-20%) (Keeley et al. 1987).  Palmer amaranth has a minimum temperature threshold for germination of 63 °F (17 °C) (Steinmaus et al. 2000), and in field conditions, seedlings begin emerging when temperatures near the soil surface reach 64 °F (18 °C) (Keeley et al. 1987).  It germinates best at 86-99 °F (30-37 °C) (Keeley et al. 1987, Steckel et al. 2004, Guo and Al-Khatib 2003) and can germinate in regimens as hot as 113/104 °F (45/40 °C) day/night temperatures (Guo and Al-hatib 2003).  Fluctuating temperatures provide moderate promotion of germination (Steckel et al. 2004, Jha et al. 2010).  After seeds have been buried or left on the soil surface for six months, light increases germination (Jha et al. 2010).  Reduced light levels and reduced day/night temperature amplitude under a closed soybean canopy reduced Palmer amaranth emergence up to 76% (Bell et al. 2015, Jha and Norsworthy 2009).  At optimal temperatures, germination is very rapid, with most seeds germinating in less than one day (Steckel et al. 2004).  Application of poultry litter has been observed to increase emergence (Golden et al. 2006), possibly indicating that germination is promoted by nitrate as are other Amaranthus species.  Maternal plants exposed to water stress produced seeds that were 30% less dormant and could germinate from drier conditions than seeds produced from well-watered plants (Matzrafi et al. 2021).

Seed longevity:  Palmer amaranth seeds are relatively short lived.  In one study, seed survival was 44-61% after one year, and only 9-22% of seeds survived for three years (Sosnoskie et al. 2013).  In another experiment, seed survival after one year at 6” (15 cm) depth was 20% (Korres et al 2018).  A single year of good weed control reduced seeds in the top 2” (5 cm) of soil by 80-99% (Norsworthy 2008).  However, despite 98% reduction of the seed bank after 6 years, 7 million seeds per acre (18 million per hectare) were still present (Menges 1987a).

Season of emergence: In South Carolina, seedlings emerged primarily from mid-May to mid-July, and required ample rainfall (Jha and Norsworthy 2009). In the southcentral Great Plains, peak emergence was from early May to early June with some emergence occurring in summer months (Lui et al. 2022). In California, optimum emergence occurred from May to September (Keeley et al. 1987).

Emergence depth:  Palmer amaranth emerges best from the top 0.5” (1.3 cm) of soil (Keeley et al. 1987).  Although a very small percentage of seedlings emerged from 3” (7.6 cm) when planted in pots, emergence in the field did not occur from below 1.5” (3.8 cm) (Keeley et al. 1987). 

Photosynthetic pathway:  C4 (Elmore and Paul 1983, Ward et al. 2013).

Sensitivity to frost:   Palmer amaranth does not tolerate freezing temperatures (Guertin 2003).

Drought tolerance:  Palmer amaranth is highly drought tolerant.  The leaves can maintain photosynthesis despite substantial water stress (Ward et al. 2013) and the plant forms a deep taproot (Forseth et al. 1984).  Under light to moderate water stress, Palmer amaranth allocates fewer resources to stem growth, but maintains the same leaf number, leaf area, and root mass as nonstressed plants (Chahal et al. 2018).  Plants can germinate, grow to substantial size and complete their life cycle following a single rain (Ehlringer 1983).  Rapid growth and high seed production is the primary response of this species to hot, droughty environments (Ward et al. 2013).

Mycorrhizae:  Palmer amaranth appears to lack mycorrhizae (Wright et al. 1999b).

Response to fertility:  Growth was more responsive to N than to P and K, and N deficiency reduced all Palmer amaranth growth characteristics, particularly under high light intensity (Korres et al. 2017).  Application of 164 lb P2O5/A (183 kg/ha) at planting and a double application of 58 lb N/A (65 kg/ha) at three and six weeks after planting had little effect on the growth of dense, pure stands of Palmer amaranth (Menges 1987b), but in another study, increased density and growth of the weed was observed when rice received poultry litter (Golden et al. 2006).  Palmer amaranth populations with a history of high nitrogen fertilization and glyphosate resistance have developed higher nitrogen-use efficiency than populations with a low nitrogen history and glyphosate sensitivity, suggesting co-evolution of traits for nitrogen-use efficiency and glyphosate resistance (Bravo et al. 2018).

Soil physical requirements:  Palmer amaranth roots penetrate even highly compacted soil layers, and the species maintains high productivity on compacted soil (Place et al. 2008).

Response to shade:  Plant height increased as rapidly in 87% shade as in full sunlight, with shaded plants producing fewer, thinner leaves and fewer branches (Jha et al. 2008).  Time to flowering was doubled in plants growing in 88% shade (Korres et al. 2017).  Palmer amaranth is difficult to shade with a summer annual crop because it grows so fast and tall that it can even overtop crops like corn (Masinga et al. 2003).  However, Palmer amaranth plants that established 2 weeks or more after soybeans never overtopped them and produced substantially lower biomass and seed numbers than plants establishing simultaneously with the crop (Korres et al. 2019).

Sensitivity to disturbance:  Cutting Palmer amaranth stems at 6” (15 cm) above soil level prevented yield loss of cotton, but plants regrew and produced 116,000 seeds per plant (Sosnoskie et al. 2014).  When cut at 0 or 1” (2.5 cm), seed production was reduced, but still added 690 and 28,000 seeds per plant, respectively, to the seed bank (Sosnoskie et al. 2014).  Hail damage favors Palmer amaranth relative to corn (Curie and Klocke 2008).

Time from emergence to reproduction:  Plants emerging from March through June mostly flowered in 5-8 weeks, whereas plants emerging in July or later when day lengths were declining flowered in 3-4 weeks (Keeley et al. 1987, Sosnoskie et al. 2014,  Spaunhorst et al. 2018).  High plant densities can accelerate initiation of flowering by 10 to 20 days (Korres and Norsworthy 2017).  Plants continue to flower over a period of 40 days after flower initiation in Arkansas (Korres and Norsworthy 2017).  Some viable seeds were produced as early as 2 to 3 weeks after the beginning of flowering (Keeley et al. 1987).

Pollination:  Flowers are normally wind pollinated (Ward et al. 2013).  Since male and female flowers occur on separate plants (Ward et al. 2013), self-fertilization is theoretically impossible, but apparently some seeds can develop without pollination (Ward et al. 2013).  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: Without competition, female plants emerging in spring or summer produce on average 200,000 to over 600,000 seeds per plant (Sellers et al. 2003, Keeley et al. 1987, Spaunhorst et al. 2018, Webster and Grey 2015). Palmer amaranth grown in cotton produced 310,000 to 435,000 seeds in Georgia (Sosnoskie et al. 2014, Webster and Grey 2015).  Plant establishment 3 weeks later reduced seed production in cotton by 76% (Webster and Grey 2015). Seed production of plants established at more than 2 weeks after soybean emergence were reduced by one hundred-fold, yet still produced  up to 3,500 seeds per plant (Korres et al. 2020). Maximum seed production per plant was 376,000 in Nebraska, but decreased by 75% as Palmer amaranth population increased by ten-fold (Miranda et al. 2021). When grown in competition with corn, plants produced over 35,000 seeds per plant (Bensch et al. 2003). Plants grown with soybeans across five midwestern states produced 13,000-60,000 seeds (Schwartz et al. 2016) and similar seed production was observed for plants establishing at the same time as soybeans in Arkansas (Korres et al. 2019). Generally, seed production increases linearly by approximately 300 seeds for every 0.04 ounces (1 gram) increase in dry biomass per plant (Miranda et al. 2021). Plants retain 95-100% of seeds at soybean maturity (Schwartz et al. 2016), and lose only 2-3% of seeds during the month after soybean maturity (Schwartz-Lazaro et al. 2017, 2021).

Dispersal: Seeds disperse in moving water (Guertin 2003, Sosnoskie et al. 2011) on mowers and cotton pickers, in mud clinging to tractor tires and by animals (Sosnoskie et al. 2011). Palmer amaranth seeds also can be spread in contaminated conservation seed or in manure from cattle fed on contaminated sunflower or wheat screenings (Yu et al. 2021). Seeds pass intact through the guts of some birds or are regurgitated after being retained for several days so dispersal by birds seems likely (Proctor 1968, Ward 2013). Cotton gin trash can be contaminated with Palmer amaranth, and gin trash composting procedures are frequently inadequate for killing weed seeds (Norsworthy et al. 2009). Plants allowed to grow along field margins also contribute significantly to movement of seeds into fields (Ward et al. 2013). Pollen can disperse at least 980 ft (300 m) allowing for the rapid spread of genetic traits adapted to agricultural practices (Ward et al. 2013).

Common natural enemies:  The pigweed flea beetle, Disonycha glabrata, can cause substantial damage to most pigweed species (El Aydam and Bürki 1997).  Seed consumption by fire ants, ground beetles, and rodants account for high seed losses from the soil surface, especially during summer months (Sosnoskie et al. 2013). 

Palatability:  Native Americans ate both the cooked seeds and cooked leaves (Guertin 2003, Ward et al. 2013).  Plants are considered good forage for livestock at all stages of growth (Guertin 2003).  Heavy consumption for 5-10 days, however, can cause swelling of tissues around the kidneys (perirenal edema) in pigs, cattle, and sheep.  (Burrows and Tyrl 2001)

Note:  Incorporation of 9-13 t/A (20,000-30,000 kg/ha) dry weight of Palmer amaranth residue substantially reduces growth of grain sorghum and several vegetable species (Menges 1987b, Menges 1988).
 

References:

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