Khaki weed

Alternanthera pungens Kunth

Synonyms - Achyranthes repens, Alternanthera achyrantha, Alternanthera echinata, Alternanthera repens.

Family: Amaranthaceae.

Names:

Alternanthera is from the Latin words meaning 'alternating anthers' referring to the arrangement of fertile stamens next to infertile ones in the flower. Pungens is from the Latin word 'to prick' and refers to the prickly seed head.
Khaki weed probably refers to the colour of the heads or dying leaves.

Other Names:

Creeping Chaffweed (USA).
Khakiklits (South Africa).

Summary:

A dense mat forming plant with annual tops, a fleshy, perennial rootstock, reddish, hairy stems and prickly burrs.

Description:

Cotyledons:

Two.

Leaves:

Opposite. Pairs of unequal size.
Stipules -
Petiole - Short.
Blade - Oval to egg-shaped. 10-50 x 6-25 mm. May have a rounded or pointed tip. Hairless or sparsely hairy. Obvious veins on the underside.

Stems:

Hairy. Low lying. Root at nodes. Reddish. Up to 600 mm long. Finely toothed, soft, silky hairs. Several stems from each crown. Forms thick mats on the ground.

Flower head:

Short dense, brownish spike in the leaf axils. Egg shaped. 15 mm long x 10 mm diameter. Prickly.

Flowers:

Bisexual. No stalks. Inconspicuous.
Bracts - Flowers surrounded by sharp pointed yellow bracts.
Ovary - Style - short.
Perianth - 5 segments, cream to white, prominent midrib, sharply pointed. Unequal. 2 outer ones, oval, 5-6 mm long, prickly pointed. Inner one rectangular 4 mm long and not prickly. 2 side ones narrowly oval, 3 mm long, folded lengthwise, hairy on the back and weakly spined.
Stamens - Five. 2 or 3 may have no anthers.
Anthers -

Fruit:

A yellow prickly burr about 10 mm wide made up of several small bladdery bags, like a slightly flattened ball about 1.5 mm wide.

Seeds:

Circular, flattened, brown. Shiny. 1-2 mm diameter.

Roots:

Deep woody or fleshy taproot. Often black.

Key Characters:

Two perianth segments are longer than the other three.

Biology:

Life cycle:

Annual stems, perennial rootstock. Seeds germinate in spring and summer after rain and produce creeping stems and a stout rootstock over summer. Stems take root at the nodes forming a dense mat. Flowering occurs from March to April and the top growth dies in winter. New shoots emerge from the rootstock in spring.

Physiology:

Reproduction:

By seed and layering of stems.

Flowering times:

March to April.

Seed Biology and Germination:

Vegetative Propagules:

Stem and root fragments.

Hybrids:

Allelopathy:

Population Dynamics and Dispersal:

Spread via root and stem fragments as well as seed in prickly "burrs" that attach to animals and tyres. It is commonly distributed along stock routes and at stock yards.

Origin and History:

Tropical South America.

Distribution:

ACT, NSW, NT, QLD, SA, VIC, WA.

Courtesy Australia's Virtual Herbarium.

Habitats:

Climate:

Tropical and sub-tropical regions.

Soil:

Light soils or loams mainly.

Plant Associations:

Significance:

Beneficial:

Fodder plant, moderately palatable when young.

Detrimental:

Forms competitive dense mats with prickly burrs that may injure the feet and mouths of stock and dogs. Readily dispersed by adhering to stock and tyres. Weed of lucerne, orchards, turf, recreational areas, saleyards, pastures, cultivated areas and roadsides.

Toxicity:

May cause hay fever, dermatitis and asthma in some people. Toxic to stock but not usually eaten. Sheep will graze young plants without any apparent ill effect. Causes a skin ailment in cattle.

Symptoms:

Treatment:

Legislation:

Noxious weed of NSW, NT, SA, Vic.

Management and Control:

Spread by seed and root or stem fragments. Roots must be cut well below the surface to stop re shooting. Repeated cultivations are effective. A single cultivation usually makes the infestation worse by spreading root and stem fragments. A number of herbicides provide effective control, including dicamba, amitrole and picloram applied before flowering or bromacil for residual control.

Thresholds:

Eradication strategies:

Herbicide resistance:

Biological Control:

Related plants:

Alligator weed.(Alternanthera philoxeroides)
Common Joyweed (Alternanthera nodiflora)
Hairy Joyweed (Alternanthera nana)
Lesser Joyweed (Alternanthera denticulata)
Narrow leaved Joyweed (Alternanthera angustifolia)

Plants of similar appearance:

In other Alternanthera species the perianth segments are equal whereas in Khaki weed 2 are longer than the other three.

References:

Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P68. Photo.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P282-283. Photo.

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #63.6.

Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). p92.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P158-159 Photos.

Acknowledgments:

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