Scotch Broom, Cytisus scoparius, is a leguminous shrub native to Europe, first introduced to Australia in the early 1800s. Subsequent introductions were made for ornamental purposes and by 1901 it had spread significantly and was declared a noxious weed in NSW. C. scoparius is estimated to infest more than 200 000 ha in south-eastern Australia and has become an environmental weed in higher rainfall areas. It grows most successfully in cool temperate areas on moist, fertile soils. It is continuing to spread through both expansion of existing infestations and colonization into new areas. In some locations it has formed near monocultures. C. scoparius can grow to over 6 m but more commonly to 1-2 m. Flowering occurs in spring and seed set, which is the only method of reproduction, mainly occurs from late December to early February. Germination can occur at any time of the year if conditions are suitable.
C. scoparius readily invades sub-alpine grasslands and grassy woodlands as well as associated riparian habitats. Seedlings establish readily on sites where the soil has been disturbed or after fire. C. scoparius can also invade areas subject to minimal disturbance, including eucalypt forest and woodland, alpine grassland and pasture. After initial invasion of a site, C. scoparius competes strongly with native vegetation, reducing recruitment of seedlings and growth of understorey species in open forest areas. While seedling establishment can be suppressed in shade, massive seedling recruitment can occur after manual removal of C. scoparius cover by slashing, burning or herbicidal treatment of mature stands, at least in the absence of heavy mulching of the exposed soil surface. Grazing has been shown to increase the probability of recruitment from the soil seed bank by a factor of four. Fire can also stimulate seed germination resulting in dense infestations of seedlings.
Following invasion, C. scoparius can modify natural ecosystems in a number of ways. The establishment of C. scoparius can change the vegetation structure (for example by converting grasslands to shrublands), modify the microclimate, and alter the floristic composition. Thickets of C. scoparius provide habitat for rabbits, foxes and feral pigs that can increase soil disturbance. C. scoparius is highly flammable and can burn intensely under suitable fuel and weather conditions. Thickets increase the fuel loads in native vegetation, thus altering natural fire regimes and fire behaviour. Infestations can also impede access along watercourses and affect both native and exotic bird diversity, reptile density, and native mammal density.
The impacts of C. scoparius have been most thoroughly studied on the Barrington Tops Plateau, which has the largest infestation of the species in Australia. In this area C. scoparius forms a dense shrub layer, in contrast to the native shrub distribution that tends to be more scattered. The dense shrub layer reduces the recruitment of Eucalyptus pauciflora and modifies the native grasslands. C. scoparius has been identified as a contributing factor to the decline of the Broad-toothed Rat and disadvantages ground-feeding and ground-nesting bird species as a result of such habitat modifications.
Current control options include herbicide application, manual removal and biological control. Herbicide application is also likely to affect native species in the vicinity of infestations. C. scoparius can be effectively controlled by several herbicides although their use in natural areas can be limited and expensive, and unless residual herbicides are used or treatments are ongoing, the effect is only temporary. Mechanical control is ineffectual without follow-up treatments as the site is then left open to C. scoparius regeneration. Fire can be used as a management tool to reduce soil seedbanks, depending on the duration and intensity of the burn. Three biological control agents have also been released: Leucoptera spartifoliella, Arytainilla spartiophila and Bruchidius villosus.