What Does It Look Like?
What is it?
Star Thistle (Centaurea calcitrapa) is an upright, much branched, bushy annual or biennial herb, usually growing 400 to 800 mm high but sometimes up to 1 m. The taproot is fleshy and 2 to 3 cm in diameter. Two types of leaves are produced, rosette leavers and stem leaves, all spineless. The rosette (basal) leaves are produced first and are short-lived, having a leaf-stalk (petiole), leaves to 250 mm long by 40–50 mm wide, darkish green, pinnatisect (deeply divided almost to midrib), with toothed lobes, pubescent (downy – covered with short, soft, erect hairs) or woolly when young and glandular. Stem and stem-leaves are then produced. The stems are much-branched, sparsely to densely covered with fine hairs. Most stem leaves are also deeply divided, however, the uppermost leaves of the stem may be undivided. Stem leaves are sessile (without a leaf stalk), scabrous (rough to touch), the lower leaves are lobed to lyrate (deeply lobed, with a large terminal lobe and smaller lateral one) to lanceolate (lance-shaped), 20–80 mm long, 10–30 mm wide, the upper leaves are smaller, entire (having a smooth margin, not lobed, divided or toothed) linear (very narrow in relation to its length, with the sides mostly parallel) to narrow-lanceolate, 2–4 mm wide. (Parsons & Cuthbertson 2001; Navie 2004; VicFlora 2016).
Flower-heads produce tightly packed groups of pale purple, rarely pink or white flowers on stalkless heads that are 10 to 20 mm long by 6 to 10 mm wide and occur at the ends of the branches or between the upper leaves and stems. Each head is surrounded by numerous greenish to brownish coloured bracts (modified leaves), each ending in a rigid, sharp, white or yellowish spine 15 to 30 mm long and with a further 2 to 6 shorter spines at the base. Flowers mainly Nov.–Apr.
Seeds are whitish or greyish in colour with brownish or darker streaks or blotches, 3 to 4 mm long, 2 mm wide, egg-shaped, smooth and without hairs or scales (Parsons & Cuthbertson 2001; Navie 2004).
For further information and assistance with identification of Star Thistle, contact the herbarium in your state or territory.
Purple, White, Pink
Growth form (weed type/habit)
Where it currently grows? Preferred habitat
Star Thistle grows in open, unshaded situations in temperate, and occasionally subtropical regions, on a wide range of soils as a weed of agriculture especially poorer pastures and cropping land, open native vegetation, roadsides, stockyards, crops, and waste land in areas of 700 to 900 mm annual rainfall (Parsons & Cuthbertson 2001; Navie 2004).
Are there similar species?
Star Thistle (Centaurea calcitrapa) is very similar to Spotted Knapweed (Centaurea maculosa), Black Knapweed (Centaurea nigra), Creeping Knapweed (Rhaponticum repens) [as Acroptilon repens], Soldier Thistle (Picnomon acarna) and White-stemmed Distaff Thistle (Carthamus leucocaulos). It is also relatively similar to St. Barnaby's Thistle (Centaurea solstitialis), Maltese Cockspur (Centaurea melitensis) and Saffron Thistle (Carthamus lanatus) (Navie 2004).
Spotted Knapweed, Black Knapweed and Creeping Knapweed all have similar spineless stems and leaves to Star Thistle, however the purplish flower heads of these species are enclosed in several rows of spineless bracts (modified leaves), whereas those of Star Thistle end in spines. The margins of the bracts of Black Knapweed and Creeping Knapweed are fringed with black hairs up to 2.5 mm long, whereas those of Creeping Knapweed are papery only. The seeds of all three of these species are topped with a pappus (tuft) of bristles or hairs, although in Creeping Knapweed these can easily become detached. The seeds of Star Thistle have no appendages like bristles or hairs (Navie 2004).
Soldier Thistle differs from Star Thistle in having spiny, winged stems and spiny leaves. Its pinkish or purplish flower heads are similar as they are enclosed in several rows of spine-tipped bracts but the seeds of Soldier Thistle are topped with a pappus of numerous feathery white bristles (Navie 2004).
White-stemmed Distaff Thistle has spineless stems like Star Thistle but its leaves are spiny and the undersides are often without hairs (although the young leaves sometimes have a cobwebby appearance). Like Star Thistle, White-stemmed Distaff Thistle's purplish flower heads are enclosed in several rows of spine-tipped modified leaves but the outer rows are turned downwards (Navie 2004).
Like Star Thistle, St. Barnaby's Thistle and Maltese Cockspur, have spineless stems and leaves which have a covering of hairs on the undersides. Unlike Star Thistle, their flower heads are yellow rather than purple and the seeds may or may not be topped with a pappus of numerous bristly hairs (Navie 2004).
Saffron Thistle also has spineless stems like Star Thistle but generally there are no hairs on the undersides of its spiny leaves (although the young leaves sometimes have a cobwebby appearance). The flower heads are yellow rather than purple and are enclosed in several rows of spiny bracts, the outer rows of which are turned downwards. The greyish-brown coloured seeds of Saffron Thistle are topped with a pappus of numerous bristles or narrow scales unlike the seeds of Star Thistle which are whitish and have no pappus at all (Navie 2004).