Onopordum is the Latin name for thistles and is derived from the Greek onus meaning donkey and porde meaning flatulence because it was believed that donkeys farted more after eating these plants.
Acaulon is from the Greek words meaning stemless.
Stemless Thistle - because it does not produce a flower stem.
Other names;
Horse Thistle (SA)
Stemless Onopordum
Summary:
A prickly, low lying, woolly, grey rosette that has white to purple thistle flowers nestled in the centre in summer at ground level.
Description:
Cotyledons:
Two. Oval. Tip slightly notched. Sides convex. Base tapered. Surface hairless. Petiole shorter than the blade.
First Leaves:
Oval, edges spiny, tip pointed. Long woolly hairs on the top and underneath. 20 mm long by 10 mm wide.
Leaves:
Grey, lie flat on the ground, arranged in a large spiny rosette 400-1000 mm diameter.
Petiole - Short.
Blade - Oblong, 200-500 mm long, narrow, lobed almost to the midrib. Wavy edged, spiny, lobes. Dense, soft, white, woolly hairs.
Stems:
None.
Flower head:
One to several, globular, 40-60 mm diameter, hairless, clustered in the centre of rosette of leaves on no stalk or very short stalk. Spiny bracts surround the head.
Flowers:
White to purple.
Bracts - Hairless and end in a long spine.
Florets - Tubular.
Ovary - Receptacle is fleshy with no scales, honeycombed with pits with jagged edges.
'Petals' - White to purple.
Stamens -
Anthers -
Fruit:
Brown or grey, egg shaped to quadrangular, crosswise wrinkles with lengthwise ridges, achene, 4-5 mm long, pitted.
Pappus of many bristles, cream coloured, barbed, 20-25 mm long, about 5 times longer than the seed. Pappus bristles joined in a ring at the base and falls off easily
Seeds:
Enclosed in the fruit.
Roots:
Taproot with many side roots.
Key Characters:
No stem. Flower heads sessile or nearly so.
Biology:
Life cycle:
Annual or biennial. Seed germinate in autumn and grow slowly to form a small rosette over winter. As temperatures increase in spring the plant grows very rapidly to about 800 mm wide, then flowers and sets seed in late spring to early summer. Nearly all plants die in summer, however a few (especially those that germinate late in the season) may survive to flower in the following season.
Physiology:
Reproduction:
By seed.
Flowering times:
Spring in western NSW.
November to December in SA.
Early summer in WA.
Seed Biology and Germination:
Produces dormant seed that may remain viable in the soil for many years.
Vegetative Propagules:
None.
Hybrids:
Allelopathy:
Population Dynamics and Dispersal:
Spread by seed that is mainly dispersed by wind. Most spread is in the direction of the prevailing winds. Water, agricultural produce, animals and vehicles are minor forms of dispersal.
Most abundant when good autumn rains follow a dry summer.
Origin and History:
Spain. Southern France. Mediterranean.
Probably introduced as an ornamental.
In Adelaide, SA in 1845 and spread to the wheat belt by the 1890's.
Spread to Victoria by 1925
First recorded in WA in 1955 and now infests over 100 farms.
Distribution:
NSW, SA, VIC, WA.
1.6 million hectares is infested in Australia.
Habitats:
Climate:
Warm temperate regions usually with less than 450 mm of winter dominant rainfall.
It occurs but is less common in areas with up to 800 mm of rainfall annually.
Soil:
Red brown earths, calcareous red earths, solonized brown soils and heavier calcareous or sandy soils. Mainly on disturbed or bare areas.
Plant Associations:
Mallee, Belah Rosewood and many other communities.
Significance:
Beneficial:
Honey and pollen plant.
Detrimental:
Weed of roadsides, pasture, range land, fallows, cereal crops and irrigated vegetables.
Unpalatable.
Toxicity:
It is rarely eaten by stock and it is suspected of causing impaction and liver problems when stock are forced to consume it.
Legislation:
Noxious weed of NSW, TAS, VIC, and WA.
Management and Control:
Cultivation at the seedling stage is effective. They have a large hairy pappus (parachute) attached to the seed which allows them to be effectively spread by wind (a particular problem when donkeys are present!). Most spread is down wind from the original infestation. Movement in produce, by water, animals or machinery is of lesser importance. It is important to prevent seed set to control this weed. Seed survives in the soil for several years. Mechanical removal and cultivation are effective. Cultivation may bury seed, enforcing dormancy and making eradication difficult. Establishment of competitive pasture reduces the infestation (lucerne appears to be particularly useful). A number of herbicides provide varying degrees of control but most damage legumes in the pasture.
Thresholds:
Eradication strategies:
Prevent seed set. Manually remove isolated plants. Spray larger infestations once or twice a year. Establish competitive pastures. Cultivate soil periodically to force dormant seed to germinate.
Herbicide resistance:
Biological Control:
Weevils feed on the plant in Europe but have not been introduced.
Related plants:
Illyrian Thistle (Onopordum illyricum)
Scotch Thistle (Onopordum acanthium L. ssp. acanthium)
Onopordum tauricum is only found in Victoria
Plants of similar appearance:
Spear Thistle (Cirsium vulgare).
References:
Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P108-109. Photo.
Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P934. Diagram.
Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P725-726. Photo.
Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P102. Photo.
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). #902.2.
Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P219-220. Photos.