Scotch broom, Cytisus scoparius, also known as Scots broom, is an attractive but invasive plant. Because Scotch broom exhibits spreading growth, it's often classified as a weed, but many gardeners find it to be an attractive addition to their outdoor spaces. Keep your Scotch broom plant trimmed regularly to keep it from spreading all over the garden.
Scotch broom shrubs may grow up to 10 feet tall and 6 feet wide when left to their own devices, and growth may be spindly and unattractive. The shrubs grow in USDA hardiness zones 5 to 8. The flowers, which are usually a vivid shade of yellow but may also be white or red, bloom in late spring and early summer. Scotch broom grows in dry, nutrient-poor soil ideally but it will also grow in dense, heavy clay.
Prune Scotch broom shrubs after the flowers have blossomed and wilted for the season, in mid to late summer. After you've pruned the plant, the foliage will change colors normally in the fall but no more flowering will occur until the following spring. Do not wait until late winter or early spring to prune Scotch broom, as this may prevent flowering.
Gardeners prune Scotch broom for two reasons: health and size. Remove dead and dying branches to keep only healthy growth, improving the overall look of the bush. Prune damaged branches all the way back to healthy growth, or back to the ground. Scotch broom also requires pruning to maintain its size. Trim back all overly long or twisted branches, keeping only those that are straight and true.
It's important to remove growth that makes Scotch broom plants unhealthy or untidy, but gardeners must also practice care not to remove too much. Never cut away more than a quarter of the plant at once to avoid doing damaging shock. Never prune just the tips of growth on the Scotch broom, as this practice will result in dense growth that prevents light and air flow.