Photo: King County, WA
Policeman’s helmet (Impatiens glandulifera), also known as jewelweed or Himalayan balsam, thrives in moist areas and riparian zones. Although sometimes sold as an ornamental, this native of Asia is highly invasive. In Britain, where the climate is similar to the Pacific Northwest, this plant is considered extremely invasive and is one of the “top 20” non-native weeds.
Policeman’s helmet is an annual that germinates from February through March and flowers from June to October. Growing up to 10 feet tall, the upright stems are hollow with a purple or reddish tinge; leaves are oblong to egg-shaped, with serrated edges, with white, pink or purple flowers resembling an old-fashioned English policeman’s helmet. A single plant can produce up to 800 seeds, which are viable for 18 months or more and can even germinate under water. Since the plant often grows along streams and ditches, seeds spread quickly downstream. When touched, the mature seedpods split and eject seeds up to 20 feet. This trait has earned the Impatiens family the name of “touch me not.”