In late spring and summer, Scotch broom (Cytisus scoparius) bursts into bloom with scores of fragrant, cheery yellow blooms. The upright shrub, which may grow up to 8 feet tall, requires minimal care to produce its striking floral display. However, Scotch broom is considered an invasive species in California, Oregon, Washington, New York, North and South Carolina, Massachusetts, Delaware, Tennessee, Georgia and Virginia. If ingested, all parts of the shrub, especially the seeds, may cause mild stomach pain.
Plant in Full Sunlight
Scotch broom is hardy in U.S. Department of Agriculture plant hardiness zones 5 to 8, where it does best in full sunlight. The shrub will also tolerate light shade. Scotch broom is a tough shrub that will grow in polluted urban conditions, and can handle maritime exposure. Scotch broom should be planted in its permanent position; the shrub dislikes root disturbance. If planting multiple shrubs, provide at least 4 feet of space between plants.
Well-Draining Soil Is Best
Scotch broom isn't overly picky about soil, though soil should have good drainage. The plant does poorly in shallow, chalky soils, though it will do well in slightly acidic or neutral soil; it can grow in rocky or dry, sandy soil. Water newly planted shrubs weekly. Once established, Scotch broom is notably drought tolerant, though the shrub will benefit from occasional irrigation during dry periods. The shrub will suffer in consistently wet soils. Expect the shrub to reach its final mature height in 10 to 20 years. Scotch broom does not require fertilization.
Pruning and Pests
Keep your shrub looking tidy by pruning with disinfected clippers right after flowering, clipping the old shoots from the previous year's growth right down to the base. The shrub is very tolerant of hard pruning and grows quickly afterwards. Scotch broom generally doesn't fall prey to any serious pests or diseases, although gall mites may sometimes cause raised bumps on leaves or swellings on twigs. Though aesthetically unappealing, gall mites don't usually warrant control. Prune and throw away infested leaves, and avoid the use of broad spectrum insecticides in the garden, which kill beneficial predators.
Propagation Is by Seed
After flowering, Scotch broom produces long, flattened seed pods which mature in late summer or fall. Seeds may be scattered directly in the garden, or sown in a cold frame in biodegradable pots. At 68 degrees Fahrenheit, seeds generally germinate in 4 weeks. Potted seedlings should be removed from the cold frame and planted directly in the garden when large enough to handle in either late summer or late spring. Scotch broom often self-sows freely in the garden, which may be a nuisance if multiple shrubs are not desired.