- Name also: Red Deadnettle, Purple Deadnettle (USA)
- Family: Mint Family – Lamiaceae (Labiatae)
- Growing form: Annual herb.
- Height: 10–30 cm (4–12 in.). Stem erect, base branched, lateral branches ascending and rooting, often reddish, 4-edged, soft-haired along edges–almost glabrous. With unpleasant fragrance.
- Flower: Corolla irregular (zygomorphic), (purplish) red (rarely white), 10–20 mm (0.4–0.8 in.) long, fused, bilabiate, long-tubed. Upper lip convex, 4–6 mm (0.16–0.24 in.) long; lower lip approx. 2 mm (0.08 in.) long, lateral lobes very small or absent, central lobe obcordate. Calyx almost regular (actinomorphic), 5-veined, 5-lobed, lobes roughly same length as calyx-tube, branching after flowering. Stamens 4, of which 2 long and 2 short. Pistil a fused carpel, stigma 2-lobed. Inflorescence composed of spike-like, often conical, dense, spike-like, axillary whorls.
- Leaves: Opposite (decussate), stalked, stalk wingless. Blade triangularly ovate, with cordate–flat base, net-veined, smooth-haired, evenly shallowly large-toothed. Upper leaves often reddish. Subtending bracts like stem leaves.
- Fruit: 4-parted schizocarp. Mericarps slightly bristly, brown.
- Habitat: Gardens, soil heaps, rubbish tips, pastures, arable land, wasteland.
- Flowering time: May–October.
Red dead-nettle is an annual, although it often overwinters. It likes airy soil, e.g. vegetable patches that have been well dug over. It doesn’t usually survive in the wild among other plants, but it can be very abundant on arable land. It does quite well as a weed: it can stand weed-killers quite well, it flowers for an exceptionally long time and it produces an abundance of seed. Its red flowers can often still be seen after light autumn frost, and as long as the soil doesn’t freeze it can flower right into the middle of a mild winter.
There are many names for dead-nettles in Finland, and many of these cover all the species because they can be difficult to tell apart. Red dead-nettle is very like cut-leaved dead-nettle (L. hybridum). Red dead-nettle’s leaves are evenly shallow-toothed while cut-leaved dead-nettle is unevenly rough-toothed. Northern dead-nettle (L. confertum) differs from other red-flowered dead-nettles in that the lobes of its calyx are longer than the calyx-tube. The uppermost leaves of henbit dead-nettle (L. amplexicaule) are clearly amplexicaul.
Dead-nettles in Finland are usually uninvited guests in gardens, but they can also be cultivated as ground cover. The most common species in Finland is spotted dead-nettle, which sometimes escapes into the wild.