- Name also: Brittlestem Hempnettle (USA)
- Family: Mint Family – Lamiaceae (Labiatae)
- Growing form: Annual herb.
- Height: 20–70 cm (8–28 in.). Stem branching, clearly swollen at joints, 4-edged, purple edges, abundantly rough-haired along edges, usually upper part also with glandular hairs, glandular hairs’ head red.
- Flower: Corolla irregular (zygomorphic), light purplish red, occasionally white, 15–20 mm (0.6–0.8 in.) long, fused, bilabiate, long-tubed, hairy. Upper lip convex; lower lip usually with dark markings, base with yellow spots, with broad, pale margins, 3-lobed, central lobe narrowly square-like, only slightly longer than broad, with flat, blunt or rounded tip, lacking notch, base with 2 glands. Calyx campanulate (bell-shaped), 5-lobed, lobes rigid, sharp-pointed. Stamens 4, of which 2 short and 2 long. Gynoecium composed of 2 fused carpels. Inflorescence lower part with long gaps, upper part denser, spike-like group formed of dense axillary whorls terminating stems and branches.
- Leaves: Opposite, stalked. Blade ovate, tip often long-tapered, feather-veined, hair flush with surface, margin large-toothed. Inflorescence’s subtending bracts like stem leaves.
- Fruit: 4-parted schizocarp. Mericarps elliptic, slightly flattened, almost glossy, brown.
- Habitat: Arable land, gardens, waste ground, logging clearings, rocky outcrops.
- Flowering time: July–September.
Common hemp-nettle is apparently an established alien in Finland which grows commonly in the southern half of Finland and in quite a small area in western Lapland. In between lies a large gap where it grows only rarely. Old southern stands have clearly accompanied agriculture, and it has probably arrived in the north on the shores of the Arctic Ocean, where it grows among rotting piles of seaweed. In Finland it is wisest to search for the plant in nitrogenous places: cow-shed yards, behind outdoor summer toilets and in refuse heaps. Common hemp-nettle spreads by attaching to passing people and animals by means of its hard and sharp-tipped calyx lobes.
Common hemp-nettle’s colloquial Finnish names have been inspired by its calyx spines and long-tubed flowers, and its close relatives, which look very similar, have all been similarly named. It can be very difficult to differentiate common hemp-nettle from bifid hemp-nettle (G. bifida) – for a long time botanists have regarded these two as mutations or subspecies of the same plant, although they differ with regards to e.g. their corollas. The central lobe of bifid hemp-nettle’s lower lip has notched tips and is clearly longer than broad; common hemp-nettle’s lack notches and are quite square. The internodes on bifid hemp-nettle’s stem are usually rough-haired throughout and it might have white-headed glandular hairs below the nodes. Common hemp-nettle’s stem is often sparsely haired or glabrous on the lower half of the internodes, and the glandular hairs beneath the nodes are red-headed.