Scotch Broom (Cytisus scoparius)
Scotch Broom can be considered a beautiful addition to the landscape or it can be considered an invasive weed that should never have been brought from the British Isles to the Pacific Northwest. In either case, it is here to stay. This plant grows to about 3 m tall. The densely packed leaves on upright branches are trifoliate (three leaflets) and clover-like. The leaves are lost in winter, and the plant becomes a cluster of dense, hard branches, not nearly so attractive as in spring, when each plant may become covered with yellow pea-like flowers, about 2 cm in length.
The flowers are pollinated mostly by bees, both honeybees and bumblebees. When the bee enters the flower, it opens explosively and dusts the insect with much of the pollen contained. Then the bee is forced to go elsewhere to continue foraging and will eventually visit another plant, thus mixing the genes.
The pollinated flowers are followed by flattened black seed pods about 2-3 cm long. As the pods dry, they burst open with an audible snap and expel their seeds to some distance, a mechanism for avoiding the immediate shade and nutrient and water capture of the parent plant.
Like other legumes (members of the pea family), Scotch Broom fixes atmospheric nitrogen through symbiotic cyanobacteria in nodules along its roots. The nitrogen is converted to nitrates, which are important nutrients for plant growth. Thus the broom is actually improving the soil for itself and other plants. But because it grows so densely, not much else can grow in its presence.
Scotch Broom is a popular ornamental plant and has been planted in freeway medians for years for this reason. It spreads rapidly to disturbed and deforested lands and competes with the seedlings of native or plantation trees. The Puget Sound prairie ecosystem is threatened by the rampant growth of Scotch Broom. Biological control of it has had some success. It is also poisonous to livestock, and its pollen is allergenic to some people.