Centaurea cyanus L.
Summary:An annual or biennial herb to 80 cm tall with long lasting blue, white or pink flowers on tough wiry stems. Most plants have blue flowers. The leaves are narrow, covered with cotton like hair and may have a few teeth on the margins.
Two. Cotyledons green on top and greyish green underneath and smooth. The mid vein is an indistinct gentle depression on the lower half on the upper surface. On the underside, the mid vein is an indistinct ridge on the lower half. The petioles are smooth and flat on the upper surface and joined at the base by a V shaped ridge across the stem axis.
Hypocotyl is stout, light green and smooth.
First leaves:Opposite. The petiole is about 10 mm long, smooth and flat on the upper surface and joined at the base by a V shaped ridge across the stem axis. Otherwise they are similar to older leaves.
Stipules - None.
Petiole - Indistinct.
Blade - Narrowly elliptical to lance shaped. Initially has grey, cobwebby hairs becoming short, spreading stout hairs especially on the upper surface. Edges often slightly toothed. Mid vein a distinct depression on the upper surface and a ridge on the lower surface.
Leaves folded along the mid vein in the bud and covered in cobwebby hairs.
Stems:Elongates just before flowering usually.
Flower stem - Erect, angled in cross section, freely branched, 300-800 mm tall, greyish green, covered with cobwebby hairs.
Flower head:Single flowers on long stalks.
Flowers:Blue, white or pink. Outer flowers 15-30 mm long, deeply toothed at the tip. Inner flowers smaller
Fruit:Flower head that retains the seed is 30-38 mm long, purplish grey near the top and cream near the base, oval in outline, elliptic in cross section. Glossy surface with fine spreading hairs. One side is convex and the other is straight with a distinct notch near the base. Rounded base and flat top with a dense cluster of stiff brown white bristles
Leaves covered with cottony hair.
Usually has blue flowers.
Flowering times:Seed Biology and Germination:
Population Dynamics and Dispersal:
Spread by seed.
Origin and History:Europe.
Distribution:NSW in the Cooma area as a garden escape, VIC.
Courtesy Australia's Virtual Herbarium.
Ornamental. Sold in wildflower mixes.
Detrimental:Weed of crops, roadsides and disturbed areas.
Very widespread in the USA.
Management and Control:Difficult to control once established.
Prevent seed set for several years.
Glyphosate provides control but damages surrounding vegetation which assists reinfestation. Consider blanket or wick application if Cornflower is amongst lower growing species.
Hormone herbicides or Lontrel® should provide more selective control.
Replant grass and other species to reduce re infestation.
Herbicide resistance:Biological Control:
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)
Creeping Knapweed (Acroptilon repens)
Plants of similar appearance:References:
Bodkin, F. (1986). Encyclopaedia Botanica. (Angus and Robertson, Australia).
Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney).
Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).
Lazarides, M. and Cowley, K. and Hohnen, P. (1997). CSIRO handbook of Australian Weeds. (CSIRO, Melbourne). 221.4.
Randall, J.M. and Marinelli, J. (1996) Invasive Plants. (Brooklyn Botanic Gardens Inc. Brooklyn). P73. Photo.
Stucky, J.M. (1981). Identifying Seedling and Mature Weeds Common in the Southeastern United States. (The North Carolina Agricultural Research Service and The North Carolina Agricultural Extension Service, North Carolina State University, Raleigh). P106-7. Photos.
Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.