• TAXONOMY:

    Cytisus scoparius (L.) Link (Sarothamnus scoparius [L.] Wimmer); Scotch Broom. Fabaceae.

  • NATIVE RANGE:

    Scotch broom is native to Europe from the Ukraine to Ireland and from Spain to southern Sweden (Peterson and Prasad 1998). It is often considered a weed in its native range as well (Aguinagalde et al. 2002).

  • DESCRIPTION:

    This cultivated shrub grows 3-5 m high. It produces yellow flowers in the spring and early summer (Newcomb 1977, Gleason and Cronquist 1991) that are strongly reminiscent of pea flowers and contain both male and female reproductive parts (i.e. perfect) (Peterson and Prasad 1998). These grow on an elongated flower cluster called a raceme or are arranged singly or in pairs along the central stem in the axils of the leaves (Gleason and Cronquist 1991). Other garden varieties may have red or purplish flowers. The leaves are deciduous and composed of three leaflets on the same petiole (leaf stalk), though the upper leaves may have only one leaflet. They grow from the stem in an alternate pattern along the stem, which is typically green and strongly angled. Typical of legumes, the plant produces a 2-5 cm long pea-pod-like fruit that is green when immature and black upon maturity and has hairs along the margin (Newcomb 1997, Gleason and Cronquist 1991, Peterson and Prasad 1998). As the pod matures, the two halves split and curl backward, launching the seeds away from the plant (Peterson and Prasad 1998).

  • ECOLOGICAL THREAT:

    Scotch broom was apparently first introduced to the United States on the east coast as an ornamental. It was subsequently planted on the west coast, where it more quickly became a problem along roadsides and along the inland valleys (Hoshovsky 1986). In Canada, Scotch broom is known as a competitor of commercial conifer seedlings and in the Pacific northwest of the United States, the failure of regeneration of Douglas-fir stands has been attributed to shading by Scotch broom b 12v (Peterson and Prasad 1998). It is an invasive weed in imperiled Garry oak (Quercus garryana) ecosystems in British Columbia and the northwest United States (Peterson and Prasad 1998, Ussery and Krannitz 1998). In California, Scotch broom had infested over 250,000 hectares of rangeland by 1991 (Bossard 1991).

  • DISTRIBUTION IN THE USA:

    On the East Coast, Scotch broom is naturalized from Maine (and in fact as far north as Nova Scotia) to Virginia (Gleason and Cronquist 1991). On the Pacific coast, it is naturalized from British Columbia to central California (Hitchcock and Cronquist 1973). In California, it is common in non-desert, disturbed areas below 900 m (Stuart and Sawyer 2001). In Massachusetts, it is naturalized in Norfolk, Bristol, Plymouth, Barnstable, Dukes, and Nantucket counties (Sorrie and Somers 1999).

  • HABITAT IN THE USA:

    The species appears limited by cold temperatures in the eastern United States, but stretches as far north as Nova Scotia and Prince Edward Island, though it has been reported to suffer severe dieback as the result of a cold winter (Peterson and Prasad 1998). It can occur in dry moisture regimes (Peterson and Prasad 1998) and sandy soils (Newcomb 1975, Gleason and Cronquist 1991), such as recent alluvial (flood) deposits and disturbed areas (Stuart and Sawyer 2001). It can be found with Douglas-fir (Pseudotsuga menziesii), gorse (Ulex europeus), and Himalayan blackberry (Rubus discolor) in the Pacific northwest, and with Ponderosa pine (Pinus ponderosa), white leaf manzanita (Arctostaphylos viscida) and native grasses in California.

  • BACKGROUND:

    The plant is reported to have been introduced from Europe primarily as an ornamental, and possibly with the intent of using the thick stems to make brooms (Peterson and Prasad 1998, Aguinagalde et al. 2002). Peterson and Prasad (1998) report the additional vector as ship ballast soil, and also that the seeds were roasted and made into a hot beverage and the shoots were used to make beer. More recently, the species has been used for soil stabilization (Peterson and Prasad 1998).

  • BIOLOGY & SPREAD:

    Scotch broom grows rapidly and produces monospecific stands, shading out all other species, something that has been attributed to its invasion success (Peterson and Prasad 1998). As a legume, Scotch broom is a nitrogen-fixer when associated with Rhizobium spp. (Peterson and Prasad 1998). Given the dependence of broom on bacterial symbionts, invasion success may be limited to bacterial symbiont activity. Parker et al. (2006) tested growth rates of seedlings in inoculated and uninoculated soils and found that inoculation doubled biomass, and that in uninoculated soils, plant biomass and nodule formation was correlated with the proximity of a native legume Showy Tick Trefoil (Desmodium canadense) but not correlated with the proximity of the legume Groundnut (Apios americana).

    The stems of Scotch broom are green and contain chlorophyll, allowing the plant to photosynthesize effectively during periods of defoliation. Bossard and Rejmanek (1992) found that stem photosynthesis contributed to somewhat under 40% of the total photosynthates produced during their study and allow the plant to continue to photosynthesize under severe herbivory and loss of leaves due to drought. Herbivory on young shoots has been reported by deer and elk in California, as well as by phytophagous insects in its native range, but overall herbivory apparently has very little effect on the reproduction and growth of the plant (Bossard and Rejmanek 1994).

    Scotch broom reproduces primarily by seed; each plant produces a large number of seeds per year and over 50% of those remain viable (Bossard and Rejmanek 1994). When the pod is mature, the two sides curl back allowing the seeds to be catapulted away from the mother plant (Peterson and Prasad 1998). Ants have also been reported to disperse the seeds (Bossard 1990).

    Buckley et al. (2003) found that broom from invasive populations in Australia, New Zealand, California, and Chile had significantly heavier seeds than native populations, which they suggest can lead to higher seed survival. They suggest that this is due to adaptations that have occurred since introduction, and given that seed collected from nurseries was no heavier than seed collected from wild populations, they suggest it is not the result of founder effects. They propose selection for larger seed size came from a release from herbivory or from increased seedling competition in the introduced range. Individual broom shrubs can live up to 17 years (Bossard and Rejmanek 1994) and can contribute large quantities of seed to the seed bank in those years. Seeds typically disperse within 1 meter of the plant (Smith and Harlen 1991, Rees and Paynter 1997). It has been suggested that broom lives longer in introduced regions than in native regions (Rees and Paynter 1997).

  • MANAGEMENT OPTIONS:

    The Nature Conservancy recommends that establishment of broom infestation be prevented by keeping soil disturbance low, and that observation of management in infested regions be conducted to determine which methods succeed (Hoshovsky 1986).

    Mechanical: Ussery and Krannitz (1998) compared the effects of uprooting versus cutting Scotch broom in British Columbia and found that trampling of native plants was a significant problem when broom was pulled in the spring, but less so when broom was pulled in the late summer because the plants trampled then were of lesser conservation importance. They also found that pulling broom created a significant relationship between the amount of broom removed and the amount of soil that was disturbed and that more seedlings sprouted the following spring in those plots where significant disturbance occurred. Cutting broom resulted in resprouting of only 9.3% of the total cut stems, and resprouting was more likely to occur on young stems though no new shoots survived longer than 1 year. They suggest that cutting is a better approach than uprooting Scotch broom and that resprouting can be minimized by cutting during the late summer when the plant is drought-stressed. Bossard and Rejmanek (1994) found that less than 10% of the broom they cut in experimental treatments during the month of August resprouted and cutting at the soil surface resulted in a numerically lower rate of resprouting that was not statistically significant. Seeds will continue to sprout after the initial removal, and continual follow-up treatments are necessary (Ussery and Krannitz 1998). The Nature Conservancy (Hoshovsky 1986) recommends that cut vegetation be piled for wildlife habitat if cut prior to seed set, however mechanical chippers and burning is also an effective means of disposing of cut material.

    Chemical: The most widely used herbicide for Scotch broom is 2,4,5-trichlorophenoxy acetic acid applied to the foliage or the stump (Peterson and Prasad 1998). Ketchum and Rose (2003) reported that hexazione, which works by interfering with the photosynthetic pathway, applied to the soil prior to broom seed germination significantly retarded seedling growth and establishment, and sulfometuron resulted in a significant reduction in dry weight, but did not significantly affect survival. Metsulfuron also reduced survivorship as well as the dry weight. With all, increasing application rates decreased broom survival and growth; the rates tested ranged from .56 to 3.36 kg a.i./ha.

    Biological: Twig mining moths (Leucoptera spartifoliella) and seed weevils (Apion fuscirostre) were introduced to California from Europe in the 1960’s (Hoshovsky 1986). A study in Eldorado National Forest, however, found no evidence that herbivory by Apion fuscirostre, a seed weevil, impacts broom reproduction (Bossard and Rejmanek 1994), which the authors suggest could be due to a late snow during one of the study years, or differences in climate between the native and introduced range that allow broom to set seed earlier in the season to escape predation by the seed weevil. They found that less than 9% of early-season seeds were damaged by Apion, compared to 22-91% of seeds at the end of the reproductive season. Redmon et al. (2000) took a different approach to biological control when they considered that broom in the eastern part of the United States is not considered an invasive pest to the same degree that it is in the western part. They suggest that eastern populations are kept under control by the weevil Bruchidius villosus which lays eggs on the seed pod and whose larvae predate the seeds upon hatching. They found that seed destruction in oviposted pods was 82%.

  • GENETIC STUDIES:

    Scotch broom is a diploid species with the subspecies C.s.scoparius from Europe containing 23 pairs of chromosomes (2n=46) (Cubas et al. 2001). The genus, however, has a high variability of chromosome numbers and variation may exist in nonnative U.S. populations. Studies conducted in Europe reveal high chloroplast DNA diversity using PCR-RFLP techniques. A total of 14 different haplotypes were identified, 10 of which are unique to a single population (Aguinagalde et al. 2002). To date, no phylogenetic studies have been conducted with introduced populations, though researchers in China and Australia have developed microsatellite markers for this purpose (Kang et al. 2007). They tested twelve markers and found that 8 of them were polymorphic with a range of 4 to 23 alleles, which individuals showing one to four alleles each suggesting tetraploidy.

  • ECONOMIC THREAT:

    As broom populations in Massachusetts are not generally considered a threat, the costs of large-scale removal or biological control are likely to not be cost-efficient in terms of controlling this species. The cost-benefits of removal should be considered on a site-by-site basis. In New Zealand, Jarvis et al. (2006) estimated that the benefits of removing broom would outweigh the costs of removal by almost 3:1 and would benefit the country by almost $6 million once biological control agents were established. This estimate took into account the cost of programs for removal including biological and chemical control by local municipalities, the forestry industry, and private landowners, given that a 1991 Act in the country mandated the removal of broom infestations from property boundaries. The estimate also took into account the costs of removal of a significant pollen source to the beekeeping industry and the potential threat to a similar native legume that could be at risk of damage by broom control measures.

  • LOCAL (MASSACHUSETTS) IMPACTS:

    In Massachusetts, Scotch broom is naturalized in Norfolk, Bristol, Plymouth, Barnstable, Dukes, and Nantucket counties (Sorrie and Somers 1999), however no bans or restrictions exist on its sale. Populations of broom on the east coast are considered stable (Redmon et al. 2000).

  • SUGGESTED ALTERNATIVES:

    If the desired aesthetic effect is a shrub with yellow flowers, spring-flowering forsythia may prove a suitable alternative to Scotch broom. The New England Flower Society (Cullina 2003) suggests California lilac (Ceanothus spp.), Western Mockorange (Philadelphus lewisii), and Red Blueberry (Vaccinium parvifolium) as alternatives for Gorse (Ulex europaeus), which is a similar-sized shrub to Scotch broom and also has pea-shaped yellow flowers.

  • AUTHOR:

    Renee Eriksen, University of Massachusetts Boston, Boston, MA.

  • REFERENCES:
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