Source: Idaho's Noxious Weeds by Robert H. Callihan & Timothy W. Miller
BACKGROUND
Scotch broom (Cytisus scoparius) is native to Europe and was likely introduced as an ornamental. It spreads by seed and inhabits well-drained sites over a wide range of precipitation regimes. Several commercial varieties of Scotch broom are not considered noxious. Check with your local weed control superintendent to determine if your plants are designated noxious.
DESCRIPTION
Scotch broom is a woody perennialPlant that lives for more than 2 growing seasons species up to 10 feet tall. Leaves are mostly trifoliateA leaf made of 3 leaflets; cloverlike with ½ inch long, alfalfa-like leaflets. Stems are strongly angled and dark green, with branches that spread only slightly from the main stem. Flowers are bright yellow, pealike, 1 inch in length, and borne in the leaf axilsThe angle formed between a leaf and stem during June. Brown seed pods are smooth (except for hair along the marginsmargin: edge of the leaf), flattened, and contain several beanlike seeds, which are thrown some distance as the pods snap open at maturity.
DISTRIBUTION
Scotch broom is widespread along both coasts and has been introduced in northern Idaho primarily.
CONTROL
Biological control agents (a twig mining moth, a seed weevil, and a shoot tip moth) are available for control of Scotch broom, but have not proved effective in Idaho. Herbicides are available that can control this weed.