Water-Plantain - Alisma Plantago-Aquatica
Water-plantain is often found at the edges of shallow waterways including ditches and ponds. It is a tall (up to 1m) perennial with small pale lilac flowers which open from midday to evening. They tend to do well where the ground is occasionally disturbed.
Sow the seed at any time of the year direct in final location. The seed is long lasting in the soil so if the area becomes dominated by other plants then ground clearance will usually result in re-establishment.