A tall herb. It keeps growing from year to year from a tuberous rootstock. It has few branches. The stems are slender and have white down. The leaves are simple and opposite. They are broad at the base and taper to a blunt point. The flowers occur in several groups at the top of the plant. The flowers are small and 5 mm long and dark purple. The fruit is a narrow follicle. The seeds are flat with tufts of hairs at the end.