Water Mint (Mentha aquatica)
It is very common in Dorset where its preferred habitat exists flowering from July right through until October.
It is easy to identify this species as being mint, just take a leaf and have a sniff, all that is missing is the roast lamb!
Water Mint has this purple tinged green in the leaves like many of the labiate (or mint) family. The flowers are pale mauve and are formed around the top of the stem.