- Latin synonym: Lamium purpureum var. hybridum
- Written also: Cut-leaved Deadnettle
- Family: Mint Family – Lamiaceae (Labiatae)
- Growing form: Annual herb.
- Height: 10–30 cm (4–12 in.). Stem erect, base often branching, branches ascending and rooting, 4-edged, sparsely soft-haired along edges. With unpleasant fragrance.
- Flower: Corolla irregular (zygomorphic), red, 10–20 mm (0.4–0.8 in.) long, fused, bilabiate, long-tubed. Upper lip convex, 3–4 mm (0.12–0.16 in.) long; lower lip approx. 2 mm (0.08 in.) long, central lobe obcordate, lateral lobes with very small, triangular teeth. Calyx almost regular (actinomorphic), 5-veined, 5-lobed, lobes about same length as calyx-tube, straggly after flowering. Stamens 4, of which 2 short and 2 long. Gynoecium composed of 2 fused carpels. Inflorescence terminal, spike-like, comprised of dense, axillary whorls.
- Leaves: Opposite, stalked, upper leaves’ stalk short, winged. Blade broadly triangularly ovate–quite round, with cordate–flat base, net-veined, smoothly haired, dark, with irregularly and deeply large-toothed or lobed margins. Inflorescence’s subtending bracts like stem leaves.
- Fruit: 4-parted schizocarp. Mericarps slightly bristly, brown.
- Habitat: Gardens, soil heaps, rubbish dumps, roadsides, arable land and waste ground.
- Flowering time: May–October.
Cut-leaved dead-nettle thrives mainly as a weed in ploughed vegetable fields and cultivated land, and it is rarely seen growing in the wild. Like many other herbs it flowers as long as the ground isn’t frozen –and in exceptionally mild spells it can even flower in the middle of winter. It mainly self-pollinates and is not dependent on insect activity. Like its relatives, its seeds have an oily elaiosome to attract ants, although their impact is quite small in arable land.
Cut-leaved dead-nettle is highly reminiscent of red dead-nettle (L. purpureum), even to the extent that they have not always been seen as different species. Nowadays however it is seen as an independent hybrid species, with one parent red dead-nettle and the other open to question. The easiest way to differentiate between the species is to look at the margins on the leaf blade: red dead-nettle’s blade is regularly shallow-toothed while cut-leaved dead-nettle’s is irregularly large-toothed or lobed, and neighbouring teeth are often of different sizes. Northern dead-nettle (L. confertum) is different from other red-flowered Finnish dead-nettles in that its lobes are longer than its calyx tube, and the uppermost leaves on henbit dead-nettle (L. amplexicaule) are amplexicaul.
Dead-nettles can be differentiated from hemp-nettles ( Galeopsis spp.) by examining the leaves: the former has clearly net-veined leaf blades while the latter’s are feather-veined. The structure of their flowers is also different in that the lateral lobes on the lower lip of dead-nettles’ corollas are small, fused with the central lobe, or entirely missing. Hemp-nettle corollas are clearly three-lobed. Dead-nettles are soft-haired and hemp-nettles are bristly.