Redroot Amaranth

Amaranthus retroflexus L.

Family: - Amaranthaceae.

Names:

Redroot Amaranth refers to the roots that are often red. Amaranthus is from the Greek meaning "not to wither" and refers to the persistent flower spike.

Other Names:

Pigweed (USA)
Reflexed Amaranth
Red root
Redshank is sometimes used locally in Tasmania for this species.

Summary:

An erect, summer growing, stout, hairy annual herb to 1000 mm high. Many greenish flowers are packed into broad, conical spikes in the leaf axils or at the ends of branches. It often has a rosy red taproot.

Description:

Cotyledons:

Two. Spear shaped. Tip rounded. Hairless. Short merging stalk.

First leaves:

Triangular and hairless with a notched tip.

Leaves:

Alternate.
Stipules -
Petiole - yes.
Blade - Broadly oval or egg shaped, 30-120 mm long. Somewhat undulating near the edges. Often reddish. Obvious veins.

Stems:

Round, 1000 mm tall, often with a red tinge. Densely hairy. Hairs crisped and short.

Flower head:

Erect, oval, dense, bristly, cylindrical (up to 15mm thick) leafless spikes in the axils of upper leaves and a dense, bristly, leafy panicle with short thick branches at the ends of stems.

Flowers:

Greenish.
Bracts - 4-6mm long, longer than perianth, with cusp tips.
Perianth - 5 segments, shorter than bracts and the same length or longer than the fruit. Spade shaped with pointed tip that becomes rounded then a broad shallow notch as fruit develops.
Ovary -
Stamens - 5
Anthers -

Fruit:

Small bladdery bag, 2-3mm long. Nearly as long as the perianth. Opening by a transverse line around the circumference.

Seeds:

Shiny. Flattened. Almost circular in outline with a white ring near the middle. About 1mm wide.

Roots:

Often rosy red. Taproot.

Key Characters:

Biology:

Life cycle:

Spring germinating annual. Flowers December to March. Grows in summer and autumn.

Physiology:

Reproduction:

By seed.

Flowering times:

December to March.

Seed Biology and Germination:

Fluctuating temperatures are required to break dormancy.

Vegetative Propagules:

Hybrids:

Allelopathy:

Population Dynamics and Dispersal:

Origin and History:

North America.
Has spread to most temperate regions.

Distribution:

ACT, NSW, SA, TAS, VIC, WA.

Courtesy Australia's Virtual Herbarium.

Habitats:

Often in wet areas.

Climate:

Soil:

Plant Associations:

Often found on fallows.

Significance:

Beneficial:

Detrimental:

Weed of summer crops, gardens, vegetables, roadsides, seepage areas, cultivated land and disturbed areas.

Toxicity:

Possibly toxic due to nitrate and possibly oxalate. No cases have been reported in Australia.

Symptoms:

Cattle and sheep are affected. Sudden death after consuming large quantities of the plant.
Infested hay has been implicated in some cases.
Kidney trouble in cattle and pigs(perirenal oedema) has been reported from overseas. Symptoms usually appear 5-7 days after exposure.
Weakness, trembling, incoordination, knuckling of pastern joints, paralysis of hind limbs, coma and death.

Treatment:

Avoid grazing Redroot Amaranth areas with hungry stock or feeding Amaranth infested hay during the warmer months.

Legislation:

None.

Management and Control:

Thresholds:

Eradication strategies:

Herbicide resistance:

Biological Control:

Related plants:

Boggabri weed (Amaranthus mitchellii)
Dwarf Amaranth (Amaranthus macrocarpus)
Foxtail (Amaranthus paniculatus)
Green Amaranth (Amaranthus viridis)
Love lies bleeding (Amaranthus caudatus)
Native Amaranth (Amaranthus interruptus)
Needleburr (Amaranthus spinosus)
Powell's Amaranth (Amaranthus powellii)
Redshank (Amaranthus cruentus)
Rough fruited Amaranth (Amaranthus muricatus)
Slim Amaranth (Amaranthus hybridus)
Spreading Amaranth (Amaranthus deflexus)
South American Amaranth (Amaranthus quitensis)
Tumble-weed (Amaranthus albus)
Amaranthus graecizans.
Batchelor's Buttons (Gomphrena spp.)
Cockscomb (Celosia spp.)

Plants of similar appearance:

Redroot Amaranth is similar to Slim Amaranth (Amaranthus hybridus) in foliage and habit but it has densely hairy stems.

References:

Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P71. Photo.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). p330. Diagram.

Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). p158-159. Diagram.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P284. Photo.

Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney). P69-70.

Hyde-Wyatt, B.H. and Morris, D.I. (1975). Tasmanian weed handbook. (Tasmanian Department of Agriculture, Hobart, Tasmania). P73.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #68.14.

McBarron, E.J. (1983). Poisonous plants. (Inkata Press, Melbourne). P75. Diagram.

Wilding, J.L. et al. (1987). Crop weeds. (Inkata Press, Melbourne). P38. Photos. Diagram.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.