Where would you expect to find a flower species called corn mint (Mentha arvensis)? Arvensis means 'of the field' so it is, on the face of it, likely to be weed of cultivated ground is it not? Such are the vagaries of English common names, however, that I have only ever encountered this flower in damp areas of woodland, often on or by pathways. My book says it also occurs in damp fields and grasslands but it adds that as something of an after thought.
It is less well known than its close relative water mint, but corn mint is, none the less, a fairly common flower in Dorset flowering from May through to September.
Unmistakably a labiate with square stem, pointed leaves and tubular flowers but it is quite different to water mint despite its similarities. The flowers are blue, often a pale blue, and the occur in whorls around the stem just above where the leaves occur. The leaves are green rather than purple. It also lacks the pleasant smell of mint of its cousin having a rather sickly or acrid smell. It can grow in similar places to water mint and there are known to be hybrids of the two so that can make identification difficult.
It grows in many areas of the world and it is used medicinally for various purposes but in Europe it was mainly used to treat flatulence, digestive problems, gall bladder conditions and coughs.