Black horehound

Ballota nigra has a very strong characteristic smell reminiscent of mould or damp soil, and can be recognised by its clusters of hairy, reddish-purple flowers. It can grow up to 1 m (3 ft 3 in) tall.

Stem and root. It has erect herbaceous stems, wooden and branched at bottom, covered by downy hairs. The plant has a taproot system.

Leaves. The leaves are opposite and decussate, and range from oval-lanceolate to heart-shaped, with crenate or dentate border. They are dark green and usually pubescent, measuring 2–8 cm by 2–6 cm, and have a 1–3 cm petiole. Upper face is wrinkled, with a net-like vein pattern.

Flowers. The flowers are arranged in verticillasters, subspherical to one-sided, with 15–30 flowers. Each verticillaster consist of two condensed dichasial cymes in the axils of normal leaves. Each flower has an actinomorphic calyx, length 9–10 mm, width 7 mm, made up of five sepals fused together in a tube with five teeths; and a labiate corolla of 12–14 mm, ranging from pink to pale purple to withish. The corolla consist of a tube of about 6 mm and two lips; the upper one slightly concave (like a hood) and externally hairy; the lower one glabrous, with two minor lateral lobes and a major central bifid lobe. There are four didynamous stamens, running parallel under the upper lip, with glabrous filaments and yellow anthers. …

Ballota nigra is a nitrophilous plant; it grows in ruins, fallows, and hedges at elevations up to 1,300 m (4,300 ft). It prefers loose, calcareous (alkaline) soils. It tolerates temperatures as low as -5°/-10 °C.

Usually the plant is used dry and harvested when blooming. Syrups can be made from fresh plants.

Biochemistry. Ballota nigra contains diterpenoids like marrubiin, ballonigrin, ballotinone, ballotenol and 7-acetoxymarrubiin.

The common name comes from the Old English words har, meaning "downy or hoary", and hune, meaning the plant itself. This name refers to the hairs that give the herb its distinctive appearance. In modern times, some alternative medicine practitioners have referred to the plant as "seed of Horus" and claiming that horehound takes its name from Horus, the Egyptian sun god.