With its tall, purple flower spike marsh woundwort (Stachys palustris) is mistaken by some as an orchid but it far from related to orchids in the general scheme of things. On closer inspection it quickly becomes obvious that it is a member of the mint family. The tubular flowers arranged around a square stem and the hairy and mildly serrated leaf are all classic features of the mints (or deadnettles).
There are five woundworts altogether and they are vaguely similar but only this and the hedge woundwort are at all common and likely to be seen in Dorset. The field woundwort is now, sadly, very uncommon having once been a frequent weed of cultivation. The marsh woundwort is most likely to be found in wet places; ditches and stream sides are the most frequent habitats for it.
Also known as the marsh hedge-nettle this is a plant very popular with insects, especially bees. Once pollinated the seed capsules fall in to the water and float away and when they reach a suitable spot germinate to form a new plant.