Plants can grow from one to five feet tall with numerous rust-colored branches that are woody at the base. In autumn, infestations are easy to spot by the remaining rust-colored branches. Leaves are opposite, attached directly to the branch, elliptic to oblong and generally not more than an inch long. A diagnostic characteristic of St. Johnswort is the presence of tiny, transparent glands on the leaves, which are observable when held up to the light. Flowers are bright yellow with five petals.
Black glands can occur along the margins in an open, flat-topped group.
Even with some biocontrol agents in place,St. Johnswort can infest significant amounts of rangeland. It likes well-drained gravelly or sandy soils and sunny exposures. St. Johnswort on the National Bison Range is an example of the problem. A few years ago, it covered about 9.000 of the reserve's 18,500 acres.