The velvety, green leaves are serrated with no stalks and oppose each other on the stem. They are lanceolate, 5 to 10 cm long and 2 to 3 cm wide with a tapered end.
Spikes of hooded flowers are a good source of nectar mainly visited by bees, and appear in whorls with about six per whorl, from June to October. They are typical in shape for members of the Lamiaceae, varying from pink to dark purple and the lower lip has darker speckles.
The underground stems, or creeping stolons, are white and fleshy with swollen lumps (like the Michelin Man); more mature ones can have a bright yellow hue near the end. They are brittle so it can be difficult to remove them completely.
The green parts have been used in poultices to stem bleeding for centuries and scientific analysis has found antiseptic properties in extracts from the plant. One of the common names, Clown's Woundwort, was given to it by the herbalist Gerard after his talents were rejected by a workman who preferred to use Marsh Woundwort to heal a deep cut - it worked in a quarter of the time Gerard's remedy would have, but he regarded the man as a clown at first for rejecting his help. It is likely that it was introduced to many areas for use of its healing properties and it is often found close to older dwellings.
It does not withstand repeated close cutting and can be dug up, but some of the brittle tubers may be missed - the tiniest portion will produce new shoots and spread rapidly.
See also Common Hemp-nettle and Hedge Woundwort which have similar leaves and flowers. There is also a Hybrid Woundwort Stachys x ambigua, between S. palustris and S. sylvatica which is usually sterile. The main distinguishing feature is the leaves on the upper stem which have a short stalk; S. palustris leaves have no stalk and S. sylvatica leaves have a stalk that is about a quarter of their length. The feel of the hybrid leaves is closer to that of S. sylvatica.
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