The very long viability of Scotch broom seed is one secret of its success -- that and the fact that not all seeds with germinate in the same year allows a substantial numbers of viable seeds to build up in the soil as the plant's literal "seed bank." Scotch broom tolerates cold but greatly prefers milder, more temperate coastal areas. Because it is a legume and capable of essentially manufacturing its own nitrogen fertilizer by "fixing" atmospheric nitrogen through its roots, brooms are very competitive in burned or other disturbed areas, such as along roadsides and in pastures.
Some cultivars of deciduous Scotch broom shrubs (Cytisus spp.) are less invasive than the fragrant, yellow-flowered plants that have become the scourge of cool-weather U.S. coastlines. The ideal time to prune both desirable and undesirable Scotch broom shrubs is immediately after they finish flowering in June, a practice that prevents spent flowers from developing and dispersing seeds. Early summer pruning also helps plants conserve plant energy and redirect it from seed production to foliage growth and development.
According to Montana State University Extension's website, cutting or mowing down entire stands of Scotch broom at the end of summer is an effective way to control spread and eventually eliminate them -- though consistency is the key. Late-summer pruning greatly reduces resprouting. Researchers warn against cutting stems off below the soil surface, which can "stir" banked Scotch broom seeds and stimulate their germination. Mowing with rotary motors twists stems off rather than cutting them and does not eliminate resprouting.
Mowing Scotch broom can also generate a "crop" of marketable stems for the floral industry, according to James Freed, special forest products coordinator for the U.S. Forest Service and Washington State University Cooperative Extension. Scotch broom stems are dark green, smooth and waxy, and make attractive accents for various arrangements. Pruning or mowing during the dormant winter season will rejuvenate woody old Scotch broom stands so they will produce new shoots the following summer. Free says you can make a small commercial stem harvest after the first summer growing season and a full harvest after the second summer.