Cytisus scoparius (L.) Link

Habit: Shrub

Cytisus scoparius has been planted for a variety of reasons including livestock forage, erosion control, and as a landscape ornamental. Its dried twigs can be tied together to make brooms, providing its common name, Scotch Broom. It has also been used medically as a diuretic, and to treat hemophelia and arthritis (NMCD, 2009). Broom contains alkaloid compounds and can be toxic. It is also used in basketmaking, for thatching materials, and as a source of fiber (ILDIS 2005).

More info for the terms: competition, cover, density, fern, fire exclusion, fire frequency, fire intensity, fire management, fire regime, fire suppression, forb, forbs, frequency, fuel, fuel loading, low-severity fire, natural, nonnative species, prescribed fire, presence, presettlement fire regime, restoration, severity, shrub, shrubs, top-kill

Fire adaptations: Scotch broom sprouts from the stem after top-kill from fire [17,130,137] or mechanical removal [13,58]. Its ability to sprout seems to vary with season and severity of damage, although this relationship is unclear and deserves further investigation (see Fire Effects and Physical/mechanical control).

Several reports indicate a postfire flush of Scotch broom germination from the soil seed bank [17,85,97,138]. Several studies also indicate increased germination of Scotch broom seeds following heat treatments in the laboratory [14,107,121,130]. These results suggest that seeds of this species are well adapted to postfire germination. 

FIRE REGIMES: There is no information available on the FIRE REGIMES in which the brooms evolved in their native range. However, Scotch broom and Genista florida, a close relative of French broom, were early successional species following fire in their native range in Spain [52].

It is unclear how the presence of brooms may affect FIRE REGIMES in invaded communities. In general, in ecosystems where broom replaces plants similar to itself (in terms of fuel characteristics), brooms may alter fire intensity or slightly modify an existing fire regime. However, if broom invasion introduces novel fuel properties to the invaded ecosystem, they have the potential to alter fire behavior and potentially alter the fire regime (sensu [20,31]). A review of Scotch broom in Australia [88] suggests that presence of Scotch broom creates a fire hazard in forest areas in Australia and California, although the source of this assertion is not given. Where Scotch broom invades subalpine eucalypt woodland in Australia it forms a dense shrub layer, overtopping and depleting the grass layer, thus altering fuel structure such that fire intensities fueled by shrubs in the invaded community would likely be higher than those fueled by grasses in an uninvaded community [36]. Scotch broom invasions are also said to increase fire intensity and frequency in invaded Oregon white oak communities [23,138]. According to Tveten [138], where Scotch broom has invaded prairie and Oregon white oak woodlands on Fort Lewis in western Washington, it forms dense stands and increases fire hazard by creating extensive areas with large amounts of dead wood.

Scotch broom and Portuguese broom occur in a variety of ecosystems in North America that represent a range of historic FIRE REGIMES. In many areas where brooms occur, historic FIRE REGIMES have been dramatically altered due to fire exclusion and to massive disturbances associated with human settlement. The historic FIRE REGIMES of native communities in which brooms sometimes occur range from high frequency fires in grasslands to high frequency, low-severity fires in open ponderosa pine forests; and moderate frequency, high-severity fires in California chaparral. Brooms did not occur in these communities at a time when historic FIRE REGIMES were functioning, but has established since fire exclusion and habitat alteration began. It is unclear how historic FIRE REGIMES might affect broom populations.

It is also unclear how the use of fire to control broom in these communities might impact native species. Plant adaptations to fire are usually to a particular fire regime, or combination of fire frequency, intensity, extent, and season. When fire is used to control nonnative species, the frequency, intensity, and season of burning must be carefully chosen to avoid damaging native species. Prescribed fire may have undesirable effects if introduced into an ecosystem that has undergone shifts in species composition, structure, and fuel characteristics outside a natural range of variability in these attributes [1]. When the natural fire regime is altered, even highly fire-adapted plant communities may be vulnerable to competition from nonnative species [63].

According to Swezy and Odion [129], fire is an effective management tool for French broom, but is used primarily in mixed evergreen forest and grassland communities in California, where repeated annual burning for broom control "appears to have no unwanted side effects." Prescribed fire is used less frequently in chaparral communities where frequent burning or burning outside the natural fire season may have adverse effects on native communities.

Herbaceous communities dominated by nonnative annual grasses and forbs of Mediterranean origin occur throughout the Coast Ranges and foothills of the Cascade Range and the Sierra Nevada. A review by Keeley [63] indicates that much of the nonnative annual grassland in the Coast Ranges of central and southern California derives from a fire-induced type conversion of shrublands. The herbaceous communities that have long dominated these landscapes were largely created by anthropogenic burning by Native Americans, and were further maintained by intensive land use with fire and livestock grazing by European-Americans. In recent decades, however, grazing has been eliminated in some areas and anthropogenic fires reduced such that woody vegetation is reestablishing. Along with native shrubs, nonnative shrubs such as Scotch broom, French broom, and gorse colonize these sites. Nonnative shrub colonization of grasslands may decrease fire frequency but increase fuel loads and alter fire behavior ([63] and references therein).

In the Puget Trough of Washington and adjacent parts of British Columbia, native plant communities were once a mosaic of Oregon oak woodlands, wetlands, and fescue prairies. This mosaic is said to have been maintained by Native Americans who used fire to maintain conditions favorable to the growth of common camas and bracken fern (Pteridium aquilinum). These frequent fires removed shrubs and killed small Oregon oaks and Douglas-fir, maintaining a low density of woody species. A plethora of impacts following in the wake of Euro-American settlement, including fire suppression, grazing by livestock, introduction of nonnative species, landscape fragmentation, recreation, and other management impacts, have changed the structure and composition of these plant communities ([23,107,144] and references therein). Exclusion of fire from these communities has changed the regeneration pathways of Oregon white oak and increased densities of Douglas-fir, ponderosa pine, and understory shrubs. The natural vegetation associations of Oregon white oak are threatened by these interrelated conditions [1]. Scotch broom has become an important nonnative species in Oregon white oak habitats. It forms dense canopies 3 to 9 feet (1-3 m) tall, interfering with native species. Altered fuel structure and increased fuel loading result from invasion of Scotch broom and other nonnative species such as colonial bentgrass [23,134,137].

The relationship of Oregon white oak communities and fire is critical to any restoration effort [106]. If fire is used to reduce the occurrence and spread of Scotch broom in these ecosystems, consideration must be given to presettlement fire regime characteristics. Oak woodlands and associated prairies evolved with frequent, low-severity surface fires [1]. "Seasonal burning" in lowland prairies in Washington reportedly discourages Scotch broom invasion [108]. Spring burning on a 3- to 5-year rotation in Weir Prairie, Washington, causes little change in native prairie vegetation and maintains open Oregon white oak stands. Conversely, 50 years of annual burning in one area has eliminated Scotch broom and restricted Douglas-fir establishment, but has changed the native perennial bunchgrass prairies to introduced forb and annual grassland. On the other hand, fire suppression is more harmful to prairie vegetation than excessive burning, allowing Douglas-fir and Scotch broom to invade prairies and Oregon white oak woodlands. Closed stands of these species eliminate nearly all native prairie species. Sampled plots indicate that no native prairie species remained after 12 years of closed Scotch broom cover. Fall burning is recommended to remove Douglas-fir and Scotch broom from heavily infested areas, along with follow-up fires to kill dense Scotch broom postfire seedling establishment [138].

Reintroducing fire to these communities as a means of rehabilitation and restoration is complicated by the increased fuel loads associated with long-term fire exclusion and nonnative species invasion. On oak-prairie margins, fire used to control Scotch broom can pose risks to oaks unless it is used frequently enough to prevent excessive accretion of fuel [23]. High fire severities associated with high fuel loads increase mortality of Oregon white oak seedlings and saplings. Thysell and Carey [134] describe areas at Fort Lewis where mature oaks appear to have been killed by severe fire that was fueled by Scotch broom. Mechanical removal of Scotch broom and Douglas-fir before burning may reduce the potential for negative effects on oaks [23,134,137].

A single intense fire can reduce Scotch broom cover, but is likely to encourage germination of Scotch broom from the seed bank [17,85,97,138] and at least temporarily reduce cover of native perennials such as Idaho fescue. A second fire is necessary to kill broom seedlings within 2 to 3 years, before Scotch broom seedlings are reproductively mature (see Fire Management Considerations) [17]. Many native plants in these native communities thrive after a single low-severity fire, but may be adversely affected by repeated burning [1]. Additionally, nonnative species may be favored over native species if fire is too frequent [23,137]. Spot treatments using a flame thrower in the winter, when grasses are green and fire will not spread, can remove residual Scotch broom plants missed in previous fire treatments [1].

A prescribed fire program has been used to manage prairies and oak woodlands in some areas of Fort Lewis since the 1960s and 1970s. Fires are mostly set in February and March, and occasionally in the fall, on a 3 to 5 year rotation. The primary objective of the program is fuel reduction. While the program has been successful at maintaining some of the "best prairie and Oregon white oak woodland vegetation in western Washington," it has not completely stopped prairie encroachment by Douglas-fir and Scotch broom. Spring fires often fail to burn under dense Scotch broom or young Douglas-fir. So whenever possible, heavily invaded areas are burned under drier conditions than are open prairies. Even under drier conditions, however, spring fires often fail to burn through stands of dense Scotch broom or young Douglas-fir, leaving clusters of these species to reinvade burned areas [138].

The complex relationships among oak woodlands, wetlands, prairies, Douglas-fir forests, introduced nonnative plants, and intensively developed urban, suburban, and agricultural areas suggest that both a comprehensive set of conservation objectives and a comprehensive assessment of techniques for promoting indigenous species and techniques for controlling nonnative species are needed [23].

The following list provides fire return intervals for plant communities and ecosystems where Scotch and/or Portuguese broom may be important. It may not be inclusive. If you are interested in plant communities or ecosystems that are not listed, see the complete FEIS Fire Regime Table.

*fire return interval varies widely; trends in variation are noted in the species review

More info for the terms: competition, frequency, natural, resistance, tussock

General landform/land use type: Scotch broom is well adapted to dry hillsides (often on steep slopes), pastures, and forest clearings [29,88,116]. In eastern North America, Scotch broom typically occurs near areas where it was purposely planted [9,47,104,124,146], in open woods [104,111], old fields and pastures [19,111], along roadsides, and in other disturbed areas [104,111,159]. Its occurrence is also noted on sandy substrates such as dunes and beaches [47,116].

In western North America, Scotch broom occurs on similar sites [29,58,100,102]. It is invasive on coastal sites from Monterey, California, to Washington state [77], especially in areas where it was planted for dune stabilization [61]. In California, Scotch broom has spread extensively in grassland areas on open hills and invades chaparral and lower montane habitats in the San Bernardino Mountains [122]. It also invades upland areas in the Central coast. Scotch broom is also common in riparian areas, including riparian sites in the Sierra Nevada [38], and cobble bars on the Olympic Peninsula, Washington [33]. It is also invasive in undisturbed prairie remnants in western Washington (see Succession: Disturbance) [84].

Scotch broom does not grow well in forested areas but invades rapidly following logging, land clearing, and burning [74]. Scotch broom has become a serious pest in logged areas replanted with conifer seedlings [29], and is commonly found on clearcuts and on open sites disturbed and opened by logging, roads, or fire. It also invades natural meadows and open forest [33,97,100].

In New Zealand and Australia, Scotch broom invades similar land types, including pastures and cultivated fields, dry scrubland and "wasteland," native grasslands, previously forested hill country, rocky sites, roadsides, dry riverbeds, other waterways [96,157], and both disturbed and undisturbed woodland and open forest [59]. Regression analysis of aerial map data from a national park in Australia indicates that sparse vegetation and flat terrain are among the variables contributing most to expansion of Scotch broom populations [79].

The limited available literature on Portuguese broom seems to indicate similar site tolerances as Scotch broom; however, Portuguese broom's distribution is more limited [4].

General climate: Scotch broom inhabits maritime to submaritime cool mesothermal climates with dry summers in moderate to high rainfall areas of humid temperate regions. As elevation and continentality increases the frequency of Scotch broom decreases ([97] and references therein). Distribution of Scotch broom indicates that it is well-adapted to the coastal climate and the drier climate of the Sierra foothills. Scotch broom thrives in coastal areas due, in part, to mild winter temperatures that allow it to photosynthesize and fix nitrogen into the winter [155]. Optimum temperatures for stem photosynthesis range from 68 to 77 °F (20-25 °C), while lower temperatures cause a decrease in stem photosynthesis [76]. Scotch broom has some drought resistance, but does not survive in arid regions [34,58].

Scotch broom is native to the Mediterranean, where its southern distributions are limited by drought and its northern by winter cold ([96], and references therein). Several reviews [29,96,97] indicate that the brooms do not tolerate extreme high or low temperatures. Scotch broom does not do well in areas with very cold winters. Seedlings and young plants are especially sensitive to frost, while mature plants can tolerate fairly severe frosts. Frost appears to have little direct effect on its total height growth as the damaged tips are replaced by growth from lateral buds, but plants may experience considerable dieback after very severe winters [96,97]. Some large Portuguese broom individuals can be killed by an unusually long freeze, perhaps limiting its geographic distribution (Alvarez, personal observation in [4]).

The distribution of Scotch broom in North America is limited in the north and inland by cold winter temperatures. In eastern North America, severe winter conditions and more intense competition from local flora may limit the growth and distribution of Scotch broom more than in western North America. On the Atlantic coast of Canada, Scotch broom occurs on disturbed sites in coastal areas south of 46 °N latitude [97]. In Nova Scotia, Scotch broom does not seem to persist in colder regions inland or northward [111]. On the Pacific coast, Scotch broom occurs in disturbed areas in the southwest coastal British Columbia mainland and adjacent islands. Except for the latter, Scotch broom generally does not occur north of 51 °N in western Canada [97]. The altitudinal limit of broom in New Zealand also appears to be limited by winter cold or winter drought [96].

With sparse leaves and photosynthetic stems bearing sunken stomata beneath thick epidermal wax, Scotch broom is adapted to the high-sunlight, seasonally droughty, Mediterranean environment. Scotch broom is considered to be drought tolerant and can have a water potential of -0.259 Mpa or lower without showing any apparent signs of stress (Bannister 1986, cited in [88]). Scotch broom can tolerate some level of drought stress and invade harsh riverbed environments with mid-summer drought in New Zealand and have the ability to set seed over a wide altitudinal range and under conditions of moisture stress [96]. Nonetheless, Scotch broom seedlings may die in sunny locations during rainless periods, even in cool and humid environments. In Australia, Scotch broom occurs mainly in cool temperate areas. In drier climates, it is restricted to the edge of watercourses and along drainage lines. Scotch broom also invades and persists in treeless vegetation such as subalpine grassland and cleared pasture land. In open areas, tussock grasses protect Scotch broom seedlings from drought and grazing [59].

In North America, brooms occur on sites with moderately dry to very dry moisture regimes (Klinka and others 1989, cited in [97]), but do not survive in extremely arid regions in the southwestern United States [34]. Scotch broom is occasional east of the Cascade Range, where precipitation is above 20 inches (500 mm) per year. In California, water-use efficiency of Scotch broom remained unchanged between spring and summer seasons [76], and daily carbon gain remained constant throughout the year [77]. Although Scotch broom is difficult to grow in the hot valleys of California [74], its occurrence in dry habitats and value for stabilizing sand dunes [61] implies a certain degree of drought resistance [58].

Soils: Literature reviews [17,34,58,59,96,97] indicate that brooms can survive under a wide range of soil conditions and have few constraints to growth on almost any medium. They seem to prefer coarse textured, seasonally dry, well-drained soils and a pH range of 4.5 to 7.5. They do well on sites with low to moderate fertility, but grow only rarely on highly calcareous soils. In the San Francisco Bay area Portuguese broom is particularly common on noncalcareous soils [4].

There is very little empirical information on site tolerances of Scotch broom, and some of the observational or anecdotal information is contradictory. In general, Scotch broom seems to be invasive in dry sandy soils in full sunlight [17,61,148,157], and strongly drained, water-shedding sites (Klinka and others 1989, cited in [97]). However, it may grow best on moist, fertile soils [59]. In Australia, broom occurs on soils derived from a wide variety of substrates, particularly basalt, and is rarely found on undisturbed skeletal sandy soils [59].

Scotch broom also seems to tolerate acid soils, but it is unclear whether is tolerates lime soils. In Europe it is found on moderately leached heathland soils, acidic grasslands, and in sand dunes (Bicher and Larsen 1958, cited in [58]). At 1-year-old mine spoil sites in Kentucky, Scotch broom was among several woody species tested for revegetation. Scotch broom was an average of 3.4 feet (1 m) tall after 4 growing seasons at Site 1 but did not survive at Site 2. Differences in site conditions were as follows [99].

Scotch broom can survive on nitrogen-deficient soils and those with very low levels of inorganic phosphorus, but also responds rapidly to increases in phosphorus [16,17,89,96]. Scotch broom prefers nitrogen-medium soils at sites in British Columbia (Klinka and others 1989, cited in [97]).

Elevation: Elevation ranges reported for Scotch broom are as follows:

Portuguese broom is found on sites less than 980 feet (300 m) in California [54].

Altitudinal limit of Scotch broom in New Zealand appears to be determined by winter cold or winter drought. Scotch broom grows more rapidly at low altitudes in New Zealand. Growth (cumulative shoot elongation) was highest on sites at 2,800 feet (854 m) and decreased with increasing elevation. Scotch broom plants flowered as high as 4,600 feet (1,400 m) [96].