Self-heal, Prunella vulgaris, is establishing itself on my mother-in-law's lawn, particularly on the front lawn on a north-facing banking alongside the pavement.
It doesn't grow this tall on my Mum's lawn, which gets mown to within an inch of its life at frequent intervals. My's Mum's self-heal is flowering at the moment but it's a bonsai version of the blooms on Barbara's Mum's lawn, with tiny flower-heads the size of small currants. The plant spreads by runners across the turf.
The leaves contains tannins which act as a phenol, an aromatic hydrocarbon, to stop bacteria from multiplying. Carbolic acid is an example of a phenol which is used, in dilute form, as a disinfectant.
The leaves, which also contain bitters and an essential oil can be made into a tisane to soothe sore throats when used as a gargle, although David Bellamy warns, in Blooming Bellamy, that too much tannin can have to opposite effect.
Here's what Culpeper has to say:
Culpeper's Complete Herbal, 1653
I should warn you not to try this at home! - it sounds like powerful stuff. Although I might just try a sip of self-heal tea, just to see how bitter it is.
Medieval herbalists used the doctrine of signatures as a clue to what use a medicinal herb might be put: the upper lip of the self-heal flower is sickle-shaped and this was taken, rightly it seems, to suggest that the plant could be used to treat wounds. At harvest and hay-cutting time sickle wounds were a common.