Himalayan Balsam

This is an introduced species and is very invasive. It is particularly widespread on or near rivers and streams.
One reason why it spreads so rapidly and far can be seen from the seedhead up and to the left of the flower. The seeds are contained in the elongated pod which is built like a coiled spring. As the seeds expand they force the skin of the pod to break and the seeds are ejected at some distance beacuse of the way they are coiled inside the pod. If you come acroos these try it for yourself, gently break the skin of the pod and the seeds will be ejected BUT don't blame me if they invade and spoil your favourite stream or riverbank!