Purple Loosestrife

Ter Yde, Belgium (July, 2015).
It is a herbaceous perennial plant, that can grow 1-1.5 m tall. The stems are reddish-purple and square in cross-section. The flowers are reddish purple, with six petals (occasionally five) and 12 stamens, and are clustered tightly. They have three different flower types, with the stamens and style of different lengths, short, medium or long; each flower type can only be pollinated by one of the other types, thus ensuring cross-pollination between different plants