redroot pigweed, rough pigweed
Amaranthus retroflexus L.
Common amaranth is an introduced annual found as a casual weed on cultivated land and in waste places. It is considered to be a native of North America but is now distributed worldwide. Common amaranth has come into the UK from many different sources including with birdseed. It is more frequent in the warmer drier regions of Britain like East Anglia. It prefers loose friable soils rich in nitrogen.
Mature plants have been used as animal feed but young plants eaten in large quantities can be poisonous to livestock. The green leaves contain oxalates and high levels of nitrate. Sheep, swine and cattle, particularly young calves have been affected. Nevertheless, in many parts of the world it is used as a green vegetable and the seeds are ground to make flour.
Common amaranth plants and residues may have an allelopathic effect on the germination and growth of other plants. It accumulates high levels of caesium 137 and strontium 90 and may be used for bioremediation of contaminated soil.
Common amaranth flowers from July to September. The flowers are primarily wind pollinated but some insect pollination can occur. Seed matures from August to October. The minimum time for seed development is 30 days from flowering. The average seed number per plant is 117,400 but a large plant may have 229,175 seeds. Closely spaced plants have fewer seeds. The 1,000 seed weight ranges from 0.340 to 0.439 g. Seeds on plants that mature earlier in the season are heavier than seeds on plants that mature later. Seeds can be retained in the inflorescence overwinter.
Common amaranth seed is dormant when shed. The level of dormancy is affected by time of emergence and the growing conditions experienced by the parent plant. Fresh seed from distinct populations of common amaranth may differ in their germination characteristics but the differences are reduced during dry storage. Dormancy is lost after a 2-3 months of dry storage. The tough seedcoat does not rupture easily and, while water uptake is not prevented, seed germination is hindered by it. Anything that weakens the seedcoat will aid germination. In laboratory studies, the minimum germination temperature was 10°C. Maximum germination occurred at 35 to 40°C. A temperature of 40°C will produce some germination of ripe seeds soon after harvest. The optimum germination temperature becomes lower as the seed after-ripens. Nitrogen is thought to promote germination but there is some disagreement on the effect of light.
In the field, common amaranth seeds do not germinate until temperatures begin to rise in late spring or summer. The main period of seedling emergence occurs from May to August but some seedlings can appear in April. Seedlings emerge best from between 5 and 30 mm deep in soil. Below 40 mm, seed germination is much less. Although seedling emergence decreases with increasing burial depth, fatal germination does not occur and deeply buried seeds remain ungerminated. Germination is better in a clay than a sandy soil. Cultivation increases seedling emergence. There is some confusion about whether leaving seeds on the soil surface promotes or inhibits germination.
Common amaranth is a C4 plant in terms of carbon fixation during photosynthesis. It grows best at higher temperatures and light levels. The seedlings are frost sensitive.
Common amaranth seed sown in the field and followed over a 5-year period in winter wheat and spring barley showed an annual decline of around 40%. Emerged seedlings represented 8% of the seedbank. Seed buried in soil at 20, 56 and 107 cm deep gave germination of 11, 36 and 48% respectively after 10 years but none after 16 years. In some burial experiments seeds were able to germinate after 40 years, in others viability was lost after just 5 years. After 30 months dry storage at low temperatures, seeds still retained full viability.
Common amaranth seeds are dispersed by wind, water, on farm machinery, in manure, sewage sludge and in compost. It has been found as a contaminant of lettuce seed. It is a constituent of wild birdseed sold in the UK and is spread by birds and animals. Common amaranth seed has been recovered from irrigation water. Seed stored in water for 33 months gave 9% germination when tested.
Some common amaranth seeds were still viable after 2 weeks of windrow composting at temperatures of 50 to 65°C, but after 4 weeks all had been killed. Seeds are said to survive digestion by cows, sheep and horses. Apparently-viable seeds have been found in samples of cow manure. In silage, ensiling for 8 weeks or a combination of ensilage and 24 hrs of rumen digestion killed most seeds. Rumen digestion alone left 27% of seeds still able to germinate. In pig slurry dried to aid transport, common amaranth seeds survived drying for up to 15 minutes at 50°C but did not survive 3 minutes at 75 or 100°C. In dry soil, heating seeds to 60 or 70°C for up to 7 days had little effect on seed viability. In moist soil, viability was gradually reduced over 7 days to around 5%. At 50°C the viability was reduced to 44% but at 40°C there was little effect.
Control is achieved by repeated surface cultivations and the prevention of seeding. Small seedlings are easily controlled by hoeing, rotary cultivations and flaming.
Common amaranth seed is susceptible to soil solarization but seedlings are relatively tolerant of ultraviolet-B radiation
A thick mulch of cover crop residue spread over the soil surface reduces seedling emergence in the field. In greenhouse tests in the USA, corn gluten meal (CGM) applied as a surface and incorporated treatment to soil sown with common amaranth seed has been shown to reduce common amaranth development by up to 99% at application rates of 324, 649 and 973g per m2. Corn gluten hydrolysate (CGH), a water-soluble material derived from CGM, was found to be more active than CGM when applied to the soil surface.
A number of fungi have been evaluated as biological control agents. The seeds are eaten by many species of birds, rodents and insects. In Canada, pre-dispersal seed predation by micro-moth larvae reduces seed production by up to 40%.
Updated October 2007.