Corn Mint (Mentha arvensis)
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.
The Corn Mint flowers from May trough to September and occurs mainly in woodlands and sometimes in meadows, often where the ground is damp rather than wet.
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.