- Name also: Common Knapweed, Black Knapweed, Hardheads
- Family: Daisy Family – Compositae, subfamily Carduoideae
(formerly Aster Family – Asteraceae)
- Growing form: Perennial herb. Strong rootstock.
- Height: 30–80 cm (12–32 in.). Stem unbranched–short-branched at top, capitulum thick underneath, sparsely short-haired, slightly rough.
- Flower: Flowers form 2–3 cm (0.8–1.2 in.) wide, single flower-like capitula surrounded by involucral bracts. Capitulum lacks ray-florets; disk florets bluish red, tubular. Stamens 5. Pistil of 2 fused carpels. Involucre barrel-shaped, involucral bracts overlap in many rows, basal part green, appendage black, narrowly elliptic–ovate, ciliately lobed (comb-like). Capitula usually solitary, terminating stem and branches.
- Leaves: Alternate, stalkless. Blade oval–elliptic, with entire margins–sparsely toothed, sometimes shallow-lobed, with short greyish hairs.
- Fruit: Elliptic, flattish achene with short bristles on tip.
- Habitat: Harbours, railway embankments, roadsides, waste ground, lawns. Casual alien.
- Flowering time: July–September.
The genus Knapweed is large and is well represented, especially around the Mediterranean. Only 5 species have established themselves to any great extent in Finland’s nature, but a few more grow as established aliens. Lesser knapweed arrived in Finland in the 19th century in ballast soil deposits from sailing boats, and the species spread inland, also in connection with wartime convoys. Many of these old stands have disappeared, and the species has not been able to establish growing places. It is rarely cultivated as an ornamental in Finland. Lesser knapweed likes mild winters, so global warming might help it put down solid roots in Finland.
With regards to its inflorescence, lesser knapweed differs sharply from most other knapweed species because it lacks the large, funnel-shaped ray-florets, which advertise the smaller, tubular-shaped disk florets to pollinators. The second feature that is typical for knapweeds is also connected to pollination. The stamens respond to touch: when an insect arrives at the flower to suck the nectar and take pollen with it, a tube comprised of merged stamen’s anthers withdraws downwards and a whorl of hair under the stigma lobes, which are still closed, eventually brushes the pollen away. The stigma lobes then open and the flower awaits its next insect guest and pollen from another flower. The formation of seeds requires visits from various insects.
The central features for identifying knapweeds are the dry-tipped form and colour of the capitulum’s involucral bracts. Lesser knapweed’s dark extension of the involucre is ciliately lobed like the teeth of a comb. Apart from its ray-florets, lesser knapweed resembles our more common wig knapweed (C. phrygia) and brown knapweed (C. jacea; also known as brownray knapweed). It also cross-breeds with brown knapweed: in many stands there are individuals whose extensions of the involucral bracts are irregularly or shallowly lobed and are light brown in colour.