Acroptilon repens (L.) DC.
Synonyms - Acroptilon picris, Acroptilon obtusifolium, Acroptilon subdentatum, Acroptilon serratum, Acroptilon angustifolium, Centaurea picris, Centaurea repens, Serratula picris.
Acroptilon is from the Greek akros meaning tip and ptilon meaning feather and refers to the feathery tipped inner floral bracts.
Repens is from the Latin repe meaning to creep referring to the creeping habit of the root system.
Creeping Knapweed - Creeping refers to the creeping nature of the root system. Knapweed is derived from Knopweed where knop means a knob or bud and refers to the obvious flower buds.
Other Names.Blue Weed - refers to the bluish older leaves on the plant,
Hardhead Thistle - Hardhead refers to the hard flower buds,
Summary:A spineless thistle to 1 m tall that occurs in patches, with purple flowers, annual stems that are black to dark brown near the base. It has an extensive perennial underground network of roots.
Two. Club shaped. Tip indented. Sides convex. Base tapered. Surface hairless. Petiole very short.
First leaves: Club shaped. Woolly hairs. Pointed tip. Teeth on leaf edge.
Rosette leaves: Lobed, blue green with a whitish midrib.
Petiole - Short.
Blade - Silvery green when young turning grey green with age. Juvenile leaves entire but become toothed or irregularly to deeply lobed with age. Lance shaped, 50-150 mm x 10-50 mm. Short woolly hairs or slightly hairy. No spines. Tip pointed
Stem leaves: Narrowly oblong to linear lanceolate with upper leaves progressively smaller and less lobed.
Petiole - Very short or none.
Blade - Silvery green when young turning grey green with age. 10-50 mm long. Lower leaves lobed or toothed. Upper leaves may be toothed or entire and are shorter than lower leaves.
Stems: Slender and stiff. Erect. Up to 1 m but usually 2-10 cm tall. Usually corymbosely branched. Short woolly hairs especially on the younger stems. Grooved and ridged. Usually dark brown to black at the base and green above.
Flower head:Involucre, up to 25 mm diameter when open. Urn shaped, 9-17 mm long x 5-8 mm wide. Single on the ends of stems on short leafy stalks.
Bracts - Four rows, unequal, spineless, broad, white or pale yellow, thin with papery edges and woolly near top.
Flowers: Purple, lilac, pink or rarely white turning straw coloured at maturity. Florets are all the same, numerous, tubular and bisexual. Tube 6.5-7 mm long with 5 deep lobes 2-3.5 mm long.
Ovary - Stigma 3.5 mm long.
"Petals" - Many, narrow, spreading.
Anthers - 4-5.5 mm long. No tails. Pollen spherical 48-51 um diameter, finely granular, 3 pored, thin walled about 2 um thick.
Seeds:Grooved longitudinally, somewhat curved. 2-5 mm x 2-3 mm. Wedge to egg shaped and compressed. Ivory white and sometimes mottled. 3-5 stripes on each face. Smooth. Loosely attached pappus of white, stiff barbed bristles. Some hairs are half as long as the seed and the rest twice as long.
Sub basal scar immediately lateral to the base of the seed.
Seeds tend to remain in the tightly closed flower head.
Roots:The extensive root system comprises of the original vertical root from the seedling and one to many permanent horizontal roots with many stem buds giving rise to vertical shoots.
Vertical roots grow to 7 m deep from the horizontal roots. They may grow down to 4-5 m within the first two years of growth and have been recorded to 20 m deep in old plants.
Horizontal roots (rhizomes) are in the top 300 mm of soil extending for several metres. Older roots are dark and scaly.
Key Characters:Attachment scar on the achene is sub basal rather than lateral as on Centaurea species.
The rust Puccinia acroptili is morphologically distinct from the Puccinia species that attack Centaurea species (Gerowitt and Heitefuss, 1990)
Annual stem with perennial rootstock. Seed germinates autumn/winter/spring and forms a rosette of leaves and an extensive root system to 2.5 m deep in the first season. The top growth usually dies off over summer after flowering. New growth emerges from the horizontal roots and stem bases in the following spring and produce a rosette of leaves followed by flowering stems by early summer. It is usually dormant over winter but plants disturbed in late summer and autumn will produce new shoots through winter.
Some plants have survived for more than 80 years.
Physiology:The leaves are bitter, but are grazed by sheep. The seeds are also bitter and taint flour from contaminated grain.
Disaccharide (sugar) reserves in roots peak at the end of the growing season (summer) at 8.5% and decrease to 6.8% at the beginning of spring and bottom at 3.6% by the end of spring around flowering. Inulin appears to follow a similar pattern. Sugar and nitrogen reserves are lower in shaded plants.
Overseas, shoots emerge from the root system soon after the soil temperatures rise above freezing.
No mycorrhizal associations have been found.
Reproduction:By seed, root fragments and spreading underground horizontal roots.
Root fragments greater than 2.5 cm long will establish and produce shoots when planted at shallow depths. 5 cm fragments established when buried 15 cm deep but not at 30 cm. Root fragments will withstand 3 days of desiccation at 15/5 degrees C but higher temperatures or longer times caused death.
Chromosome number 2n =26 which distinguishes Acroptilon from other Centaurea (Hallgren, 1990)
Plants are probably self incompatible and pollinated by insects.
Seedlings flowering in their first year did not produce viable seed.
Flowering times:Mainly in early summer with some in late winter to spring.
Seed Biology and Germination:Seed can remain viable in the soil for 2-3 years with odd reports of up to 5-8 years. Seed may remain viable after soaking for several months.
Produces viable seed but seedlings are uncommon in the field.
Most report that little seed is produced and many ovules abort. One report of 1200 seeds/plant.
Seed will germinate over a range of temperatures from 0.5-35 degrees C with and optimum of 20-30 degrees C.
Seed is initially dormant and this can be broken by alternating temperatures.
Alternating light and dark increased germination from 48% in dark to 69% but there appears to be no obligatory light requirement for germination.
Scarification of the seed coat increases germination.
Germination rarely occurs in the field.
Vegetative Propagules:Rhizomes. Root systems have lasted 80 years in Canada.
Shoots can still emerge when all roots down to 1.2 m are removed. Emergence was delayed by 16 weeks with this treatment but was not delayed by root removal down to 15 cm deep (Mikulka, 1987).
Hybrids:Two forms are recognised in Russia that differ in their leaf shape and susceptibility to picloram.
It is not known to hybridise with other species.
Allelopathy:It produces polyacetylene compounds that reduce the growth of oats, beans and tomatoes. These compounds also stimulate the growth of some other species.
They appear to be produced in the leaves and flower heads and are sesquiterpene lactone compounds.
Population Dynamics and Dispersal:Spreads by creeping perennial roots and root fragments and occasionally by seed. Seedlings are rarely seen in the field in Australia and probably only establish in favourable seasons. The main method of spread is by roots fragments being transported by cultivation or earthmoving equipment.
The plant extends radially and can cover 12 m2 in 2 years. In Victoria patches increased in area by 33-53% (or 0.9-1.5 m) per year. The increase in size occurs both when the plant has shoots over spring/summer and when there is no top growth present over autumn/winter. Expansion was often greater under cropping with cultivation but occasionally greatest growth was recorded in the annual pasture plots (Mikulka, 1987).
Most of the seed falls within a metre of the plants and is not well adapted for transport. However, seed is moved by flowing water, in mud and as a contaminant of produce such as hay and lucerne seed. Hay is thought to be the major source of spread in Canada as the pappus is ineffective and most of the seed remains within the bracts of the seed head. Grazing animals spread the seed because it remains viable after passage through the gut and some adheres to coats. Barbed bristles on the seed help it attach to clothing, fur and bags. Seed remains viable after soaking for several months.
Dense patches have up to 100-300 shoots/m2 overseas. In Victoria up to 69 shoots/m2 have been recorded with normal densities around 20-40 /m2 (Mikulka, 1987).
It generally increases with irrigation.
Seed production was greater in rangeland than roadside situations.
Most of the infestations are small with more than 50% being less than 0.4 ha in Canada.
Origin and History:Originates from Eastern Europe near the Caspian Sea, South western and central Asia, Southern Russia and Mongolia.
It has spread to most temperate countries.
It arrived in Australia around 1900 and first recorded in Wangaratta, Victoria in 1907. It was first recorded in SA in 1930 near Renmark and over the next 16 years it spread 200 km down the Murray River. The infestation in SA was estimated to be 70 ha in 1946 (Catizone and Viggiani, 1990).
It was probably introduced to the USA in 1929-30 in Lucerne seed from Turkestan and had spread to all of the Western and central states within 10 years. The first reports in Argentina were in 1944 (Catizone and Viggiani, 1990).
Distribution:NSW, QLD, SA, VIC.
Once found at near Ravensthorpe in WA and eradicated.
Originally it was a weed of irrigated vines along the Murray but soon spread to adjacent dryland cereal areas with a 275-375 mm annual rainfall.
Serious weed of New Zealand, Argentina, Canada, California, India, South Africa and USA.
It occurs in both irrigated and arid regions of Canada and is more common in drier areas.
The northern limit is latitude 54 degrees N.
In Victoria in 1983 there was 3260 ha spread over 36,700 ha of dryland properties and 940 ha on irrigated and non agricultural land (Mikulka, 1987)
Courtesy Australia's Virtual Herbarium.
Semi arid to subhumid temperate regions.
Often more common on moist sites but this depends on the competition from companion perennial grasses.
Soil:Tolerates a wide variety of soils and often more vigorous on clay to clay loam soils.
Plant Associations:Tends to exclude other plants and grow as a pure stand.
Often associated with Hoary Cress (Lepidium draba) which comes from the same region.
Fodder. Used in ancient medicine.
Detrimental:Weed of irrigated crops, orchards, vines, maize, sugar beet, sunflower, safflower and dry land cereals in the 300-600 mm rainfall zone, roadsides, irrigation ditches and wastelands.
Major weed of the Victorian Mallee and the Murray River irrigation area.
Weed of pasture and wastelands.
Very competitive and rarely displaced by agricultural species.
Forms dense patches that excludes most other species.
Contaminant of lucerne seed.
It can reduces Wheat yields by up to 28-75% with shoot densities of 11-64/m2 and Corn yields by 64-88% with shoot densities of 19- 65/m2. In Victoria, 30 shoots/m2 is caused cereal yield losses of 54-75% and it was the most competitive weed studied (Mikulka, 1987).
The seeds produce a bitter flavour in contaminated flour with a 0.01% contamination.
It is a serious noxious weed of dryland crops in its home range in southern USSR.
Competes with companion species for soil moisture and nutrients. Its main effect in cereals is to deplete soil moisture before winter crops are planted as the weed shoots only emerge when the crop is some three months old (Mikulka, 1987)
In Victorian pastures a density of 24.6 shoots/m2 reduced Annual Ryegrass/Medic pasture production by 42% (Mikulka, 1987).
Toxicity:Overseas it is toxic to stock and causes a nervous disorder in horses (chewing disease or Nigropallidal encephalomalacia) when freshly cut hay is eaten. In Australia toxicity has not been significant. The palatability appears to variable with reports ranging from quite palatable in Australia to unpalatable and avoided by stock overseas.
Legislation:Noxious weed of NSW, SA, VIC and WA.
Noxious weed of Canada (since 1936) and the USA.
Management and Control:Cultivation is generally ineffective and often leads to increased spread. It must be repeated every 3 weeks during the growing season and even then is of questionable value.
Root fragments up to 40 cm long did not survive if buried more than 30 cm deep. Root fragments quickly desiccate and die if brought to the surface in summer but there is usually plenty left buried to continue the infestation.
It rarely invades healthy irrigated summer pastures but establishes readily on disturbed or bare areas.
In perennial pastures it is reduced by encouraging rapid growth of the pasture combined with regular mowing or grazing.
Flooding in summer to 150 mm deep over rosette plants (preferably on ploughed fields) for 4 weeks on light soils or 8 weeks on heavy soils is used in irrigation areas in California as a control technique.
Clopyralid applied in summer over 2-3 years has provided high levels of control. Dicamba at 2-20 kg a.i./ha, Glyphosate and Picloram at 1-2.5 kg a.i./ha are also useful.
2,4-D at 10-40 kg a.i./ha is required for reasonable levels of control but rarely provides eradication.
Simazine at 10 kg a.i./ha in a band 28-30 deep has provided good levels of control in triazine tolerant crops such as Corn.
Mowing is ineffective and increased the production of aerial shoots and reduced desiccation of underground storage organs.
Grass crops appear to compete better with Creeping Knapweed than broadleaf crops.
Thresholds:It is one of the most competitive weeds and little agricultural production occurs within patches.
Eradication strategies:Treat infested area with 10 L/ha Tordon® 75-D each spring until no rosettes appear.
Herbicide resistance:Biological Control:
A stem galling nematode is sprayed onto the weed in Russia. There is some potential for rusts and other agents. Many of the insects that attack Creeping Knapweed are polyphagous.
Related plants:None in the same species are in Australia, however, the Centaurea species is very closely related and contains the following species in Australia.
Black Knapweed (Centaurea nigra)
Dusty Miller (Centaurea cineraria)
Maltese Cockspur (Centaurea melitensis)
Panicled Knapweed (Centaurea paniculata)
St Barnaby's Thistle (Centaurea solstitialis)
Star Thistle (Centaurea calcitrapa)
Plants of similar appearance:California Thistle and Sorrel have a similar creeping root system.
References:Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P81.
Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P719. Photo.
Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #16.1.
Lloyd, S. (2000). Creeping Knapweed. Agriculture WA Farmnote. No. 42/00.
Moore, R.J., Frankton, C. (1974). 'The thistles of Canada.' (Information Canada: Ottawa, Ontario)
Orchard, H. E. Two Weeds which Menace Primary Production. Journal of Agriculture of South Australia , 75-78. 1-9-1946.
Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P236-239. Photos.
Pritchard GH (1984) Some Aspects of the Biology of Creeping Knapweed (Acroptilon repens) in Victoria. pp. 407-409. (Weed Science Society of Australia: Perth)
Watson, A. K. The Biology of Canadian Weeds. 43. Acroptilon (Centaurea) repens (L.) DC. Canadian Journal of Plant Science 60, 134-145. 1-7-1980.
Wilding, J.L. et al. (1987). Crop weeds. (Inkata Press, Melbourne). P39. Photos. Diagram.
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