- Written also: Northern Deadnettle
- Family: Mint Family – Lamiaceae (Labiatae)
- Growing form: Annual herb.
- Height: 20–40 cm (8–16 in.). Stem often limp–ascending, branched from base, branches ascending, 4-edged, soft-hairy–almost glabrous along edges. Virtually scentless.
- Flower: Corolla irregular (zygomorphic), red, 15–20 mm (0.6–0.8 in.) long, fused, bilabiate, long-tubed. Upper lip convex, 4–5.5 mm (0.15–0.22 in.) long; lower lip approx. 4 mm (0.15 in.) long, central lobe obcordate, lateral lobes very small, with triangular teeth. Calyx almost regular (actinomorphic), 5-lobed, 5-veined, lobes clearly longer than corolla tube, straggly after flowering. Stamens 4, of which 2 short and 2 long. Gynoecium composed of 2 fused carpels. Flowers in whorls forming an interrupted spike.
- Leaves: Opposite, stalked, uppermost stalkless. Blade kidney-shaped–almost cordate, net-veined, smoothly haired, dark, margin quite regularly with big and broad teeth. Subtending bracts like stem leaves.
- Fruit: 4-parted schizocarp. Mericarps elongated, slightly bristly, brown.
- Habitat: Gardens, heaps of earth, arable and waste ground.
- Flowering time: May–September.
All four red-flowered dead-nettles that grow in Finland are small field and waste ground plants. They demand a rich, open habitat. They mainly grow as an annoying weed near people. The sharp-eyed weeder might be able to differentiate between the similar-looking dead-nettles at first glance based on identification markers on the flowers and leaves.
Northern dead-nettle is assumed to have come into being as a hybrid of red dead-nettle (L. purpureum) and henbit dead-nettle (L. amplexicaule). It became independent when its chromosomes doubled and it could no longer cross-breed with its parent plants. Plants becoming their own species in this way can happen quite quickly.
Northern dead-nettle is the largest of the red-flowered dead-nettles, but it can be confused with large red dead-nettle plants. Perhaps the surest identification marker is in the calyx, whose lobes are longer than the corolla tube. In classifying the species one can also follow one’s hunch: northern dead-nettle is almost scentless while red dead-nettle and cut-leaved dead-nettle (L. hybridum) have an unpleasant smell.