common broom (Invasive Plants of Maryland)

common broom

Cytisus scoparius

Summary ⁷

Cytisus scoparius, the common broom or Scotch broom, syn. Sarothamnus scoparius, is a perennial leguminous shrub native to western and central Europe. In Britain and Ireland, the standard name is broom, but this name is also used for other members of the Genisteae tribe, such as French broom or Spanish broom, and the term common broom is sometimes used for clarification. In other English-speaking countries, the most prevalent common name is Scot

Impacts and control ⁸

More info for the terms: cover, density, fire management, formation, interference, invasive species, natural, nonnative species, presence, restoration, shrub, shrubs, tree

Impacts: Rankings - Portuguese broom is listed by the California Invasive Plant Council as one of the most invasive wildland pest plants with regional distribution in California, and Scotch broom is listed as one of the most invasive wildland pest plants with widespread distribution in the state [22]. Scotch broom is considered a "serious, documented threat to sensitive species or ecosystems" especially in the South Coast, Central Coast, North Coast, and Sierra Nevada bioregions [38]. Scotch broom is classified as a "Category 3," widespread, nonnative species by the USDA, Forest Service, Eastern Region. These plants are often restricted to disturbed ground, and are not especially invasive in undisturbed natural habitats [139].

Little information is available in the literature regarding impacts of Portuguese broom invasion. A review by Alvarez [4] indicates that it displaces native herbaceous and woody plant species, its seeds are toxic to ungulates, and mature shoots are unpalatable [4]. Impacts of Scotch broom invasion are suggested or described by several authors. These include interference with native plants and animals, interference with desirable species in forest plantations, and impacts on soil properties and processes. Scotch broom invasion may also increase fire hazard under some conditions, although the details of these conditions are not entirely clear (see Fire hazard potential). Additionally, Scotch broom invasion along roadsides causes problems and economic impacts for road maintenance crews [61].

Scotch broom invasion displaces native plant species including many threatened and endangered species (see Habitat types and Plant Communities). Scotch broom can dominate a plant community, forming a dense monospecific stand (see Growth habit and stand structure). Conditions under broom thickets are shaded and more humid than uninvaded stands and most understory vegetation dies out beneath large, mature broom shrubs ([59] and references therein), which leads to loss of herbaceous plants and tree seedlings on sites where broom is abundant [121]. In a cutover Douglas-fir forest in British Columbia, a 71% reduction in photosynthetically active radiation was measured under Scotch broom stands [102]. In an invaded prairie at Fort Lewis, Washington, a dense stand of mature Scotch broom intercepted 96% of the incident light and all native prairie vegetation was absent under these stands except colonial bentgrass and nonvascular cryptogams [138]. Scotch broom invasion in Australia appears to permanently change the structure, floristic composition and ecology of woodlands at one site [59].

Scotch broom is a major forestry problem in the U.S., Canada, and New Zealand, particularly in reforestation after logging, as it can quickly overtop commercial crop trees ([59,97] and references therein). Scotch broom establishment after logging interferes with conifer establishment in the Douglas-fir region [30,61,102]. Scotch broom may occupy 90% canopy cover and intercept 65% of the intermittent light in Douglas-fir plantations. In Oregon and Washington, there have been complete stand failures of Douglas-fir regeneration because of Scotch broom infestations ([97] and references therein). Studies in Monterey pine (Pinus radiata) plantations in New Zealand suggest that Scotch broom competes with Monterey pine for light and water, reducing growth and biomass of the trees and affecting their resource allocation [151,152].

Scotch broom invasions may also impact native insects and animals. For example, Wilson and Carey [158] found a negative relationship between Scotch broom and deer mice, suggesting that Scotch broom areas have diminished value as wildlife habitat in the Oregon while oak communities in the Puget Trough, Washington. The small areas of remaining Oregon silverspot and Fender blue butterfly habitat have been invaded by Scotch broom and other nonnative species, changing plant community composition and structure and adversely affecting the quality of habitat for the threatened butterflies [98]. See Habitat Types and Plant Communities for more information on impacted species.

Broom invasion may also impact soils in invaded habitats. According to Bossard [17], Scotch broom "tends to acidify the soil," although the source of this assertion is not given. Results present by Haubensak and others [50] suggest that invasion by brooms (Scotch broom or French broom) can increase the total amount of nitrogen and the way in which nitrogen cycles in invaded ecosystems. Scotch broom is capable of fixing nitrogen throughout the year in regions with mild winters [155]. Nitrogen enrichment is unlikely to benefit native plants and may reduce species diversity in ecosystems where nitrogen-fixers are rare, although this has not been tested with regard to invasions by Scotch or Portuguese broom [32,50].

There have been reports of livestock loss due to ingestion of toxic quinolizidine alkaloids produced by Scotch broom. Human consumption of flowers and seeds of Scotch broom and related species results in nausea and vomiting [82]. Scotch broom is listed as an unsafe herb by the U.S. Federal Drug Administration (1993) [97].

Control: Scotch broom is difficult to control because of its substantial and long-lived seed bank. The best method for removal of a Scotch broom infestation depends on the climate and topography of the site, the age and size of the infestation, the relative importance of impact to nontarget species, and the type and quantity of resources available to remove and control broom at a given site. All methods require appropriate timing and follow-up monitoring [17].

Portuguese broom is difficult to remove because of the large size of individual plants, deep roots, and the long-lived seed bank. Removal should be followed by 5 years of monitoring and follow-up treatments to achieve control. With limited resources, it is particularly important to determine the primary direction in which the population is expanding and start removal efforts there. Prevent seed dispersal into uninvaded areas by removing widely dispersed individuals from the main population center. Focus on preventing seed set and dispersal from all mature individuals each season so that no net increase in the seed bank can occur [4].

This is a general review on control methods for Scotch broom and Portuguese broom, and includes information that may be applicable for better understanding of its fire ecology. Given their similar biology and lack of information on all but Scotch broom, much of this information is generalized to apply to all brooms. More research is needed, however, to understand how this information applies to Portuguese broom. The reader is referred to other reviews [4,17,58,58,103] and the Weed control methods handbook for more details on control methods.

Prevention: The most effective method for managing invasive species is to prevent their establishment and spread. Some methods of prevention include limiting seed dispersal, containing local infestations, minimizing soil disturbances, detecting and eradicating weed introductions early, and establishing and encouraging desirable competitive plants [118].

Alternatives should be used for horticultural and landscape purposes. McClintock [72] indicates that there are several ornamental brooms that are not invasive. An effective step in preventing further introduction and spread of the weedy brooms may be in asking nurseries to carry only the nonweedy species of broom [72].

Integrated management: Brooms are best controlled by an integrated vegetation management program including monitoring, prevention, biological control, uprooting, cutting, controlled burns, competitive planting, and spot treatments with herbicides as a last resort. The choice of specific methods, timing, and combinations depends on the site conditions and the nature of the infestation [160].

Hoshovsky [58] sites a personal communication with a manager who is controlling thickets of both broom and gorse on San Bruno Mountain in southern California, by mechanical disking followed by burning. Seedlings and sprouts are then spot treated with 2 applications of glyphosate. He reports that all phases and tools of the program are working well and native species are returning. See the broom Element Stewardship Abstract for more information [58].

Physical/mechanical: A variety of mechanical controls has been used in the control of Scotch broom, with some methods having the undesired effect of actually increasing spread and growth [97]. Manual methods of broom removal, such as hand-pulling and removal with hand tools, have the benefit of being highly selective, thus allowing removal of weeds with minimal damage to surrounding desirable vegetation. Hoshovsky [58] suggests the Bradley method, as described by Fuller and Barbe [44], as a sensible approach to manual control of brooms. He also suggests some logistics for securing and managing volunteers for manual control programs. Manual removal must be repeated regularly, since broom seedlings continue to establish from the seed bank after removal of adult plants [58]. See the broom Element Stewardship Abstract [58] for more information. Physical and mechanical control methods applied to adult broom plants are often followed by sprouting from remaining root crowns or aboveground stems, and emergence of numerous broom seedlings [103]. Therefore follow-up monitoring and treatments are requires for several years.

Hand-pulling can sometimes remove broom seedlings or plants up to 5 feet (1.5 m) tall. It is most easily done in coarse-textured soil after rain, when the soil is moist and loose, so that the root system may be more easily removed and the chances of sprouting minimized. Plants should be pulled as soon as they are large enough to grasp but before they produce seeds [4,58]. Manual pulling is a successful and popular method of removal of young shrubs in urban and park areas. Pulling can disturb the soil, however, possibly damaging desirable species [97]. Hand-pulling controls Scotch broom on preserves in northwest Oregon [108].

Hand tools: Small broom plants can be killed by hoeing, either by cutting off the tops, or by stirring the surface soil and exposing seedling roots. Care must be taken to avoid damage to roots of desirable vegetation. Broom plants with a large tap root may not be effectively removed by hoeing and are likely to sprout; a claw mattock is effective for removing plants up to 13 feet (4 m) tall. Hand digging may be more effective; however, it is suggested that any piece of root that breaks off and remains in the soil may produce a new plant. Additionally, digging disturbs the soil and may trigger germination and establishment from broom seeds in the soil seed bank [58].

Pulling with weed wrenches is effective for broom removal [4,17,136]. The wrench removes the entire mature shrub, eliminating sprouting from the stump. If broom is densely branched, long-handled loppers or a pruning saw can be used to remove the lower limbs before pulling shrubs [4]. Wrench removal is labor-intensive, but can be used in most kinds of terrain and allows targeting of broom plants with low impact on desirable species in the areas. However, the resultant soil disturbance tends to increase the depth of the broom seed bank, prolonging broom presence on the site [17]. Additionally, Ussery and Krannitz [143] found substantially more trampling of native species when adult broom plants were removed by pulling rather than cutting. Golden Gate National Recreation Area has "had success" using volunteers to remove broom with weed wrenches and then closely monitoring and removing broom seedlings for 5 to10 years [17].

Cutting: Brush cutters, power saws, axes, machetes, loppers and clippers can be used to remove top growth. Cutting is best done before plants set seed. According to a review by Hoshovsky [58], about half of the remaining broom roots can be expected to sprout following this treatment. Several other authors (e.g. [4,97,103,108,143]), however, indicate minimal sprouting from cut Scotch broom plants, especially when cut down to ground level or below the soil surface during or prior to periods of moisture stress. Saw cutting at the end of the summer drought period in the Sierra Nevada (August-October) resulted in a sprouting rate of less than 7%, whereas cutting at other times resulted in sprouting rates of 40% to 100% [13]. For additional insurance against sprouting from cut stems, peel bark back to the ground, or split the stump into shreds with a hand axe. The Golden Gate National Recreation Area's Habitat Restoration Program has achieved 60% to 80% mortality of Portuguese broom by hand cutting alone during late summer and early fall in the Marin Headlands [4].

In Oregon white oak savannas on the southeastern tip of Vancouver Island, British Columbia, Ussery and Krannitz [142,143] compared the effectiveness of uprooting versus cutting of Scotch broom at the base as control methods for Scotch broom. They also compared the relative site disturbance of each method (disturbed soil, exposed soil, and trampled vegetation) during 2 different seasons (May, when the shrub was in flower, and July, just prior to seed dispersal). Greater numbers of Scotch broom seedlings emerged in plots where adult brooms were uprooted, although there was considerable variation within and between study sites. Sprouting was observed in only 2 of the 60 Scotch broom stems cut during broom removal, and there was no difference in rate of sprouting between plants cut in May and July. Uprooted plants did not sprout. Cutting resulted in less overall site disturbance than did uprooting. Trampling was higher in July than in May, but in July the trampled plants were nonnative grasses, whereas in May they were native common camas. The authors suggest that a preferred strategy for removal of Scotch broom from Oregon white oak savanna is to cut Scotch broom shrubs at the base after native herbaceous species have set and distributed seed. This approach will minimize damage to native vegetation and reduce the amount of broom seedling regeneration [142,143].

Other mechanical methods of broom removal such as power tools, bulldozers, and backhoes are less selective and more likely to damage associated vegetation, and are more effective on gentle topography with few obstacles. Dense patches of Scotch broom have been eliminated by bulldozing and repeated disc cultivations over 2 years [88]. Brush hogs, which twist off aboveground biomass, can be used for removal of aboveground broom biomass, and are more destructive to regenerating tissues than is clean cutting. However, depending on the season of treatment, sprouting can still be a problem [17]. Equipment operators should also be aware that seed-contaminated soil may become embedded in tools, tires, and machinery, and be transported to new, uninfested sites [97]. These methods are rarely appropriate for wildland settings, as they are likely to adversely impact native species and create conditions that favor soil erosion.

Brush cutters and chainsaws are also effective for removing Portuguese broom. However, where it is difficult to cut shrubs flush with the ground, sprouting is likely. Mowing has had mixed results on Portuguese broom [4].

Mulching with 3 to 4 inches (8-10 cm) of straw (certified weed-free) during winter or spring (before seedlings are over an inch tall) may prevent broom seedling emergence. A controlled study by the Habitat Restoration Team in California, demonstrated that mulching with rice straw was 99% effective in preventing French broom seedlings from emerging through straw throughout the germination period from December to April. Mulching also increased the mortality of brush-cut French broom in the same study when applied during winter (Alvarez unpublished data, cited by [4]).

Fire: See the Fire Management Considerations section of this summary.

Biological: Biological control of invasive species has a long history, and there are many important considerations before the implementing a biological control program. Tu and others [136] provide general information and considerations for biological control of invasive species in their Weed control methods handbook. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.

Biological control of Scotch broom in North America began in 1960. Three insects were purposefully introduced as biological control agents, and numerous accidental introductions of natural enemies have also been reported. The insects purposely introduced include Scotch broom bruchid (Bruchidius villosus) [26], Scotch broom seed weevil (Exapion fuscirostre) [27], and Scotch broom twig miner (Leucoptera spartifoliella) [28]. The latter 2 species are specific to Scotch broom, while the former also attacks Portuguese broom, Spanish broom, and French broom. See Coombs and others [29] for more information on these insects, their distribution, and effects. Scotch broom is also unique in that it is heavily attacked by a number of native arthropods, particularly aphids and other sucking insects. Additionally, many Scotch broom plants in older infestations are dying due to a fungal pathogen suspected to be Selenophoma juncea, which is widespread and seems to be the most important mortality factor. This pathogen has not, however, been approved for distribution [29].

Experimental analysis by Parker [84] suggests that a seed-eating insect would need to reduce Scotch broom seed production by as much as 99.9% to halt population expansion in the fastest growing population and by 70% to stop population expansion in the slowest-growing population. In Australia, Sheppard and others [119] compared the impact of seed predators among nonnative pastures and native grasslands and suggested that a seed loss of 62% was sufficient to suppress Scotch broom in native grassland, whereas >97% seed loss was required for suppression in nonnative pasture.

Biological control agents have not been reported for Portuguese broom [4].

Use of "bioherbicides" whereby native pathogens are employed to control Scotch broom and gorse is being investigated [102].

Grazing: Heavy grazing by domestic goats during the growing season for 4 to 5 years has been reported effective in New Zealand, and grazing by llamas has been tried at a few sites in California (Archbald, personal communication in [17]). A study in Australia indicates that domestic sheep had minimal impact on Scotch broom vigor in infested pastures. Goats had a major impact when Scotch broom density was low (4% ground cover) and no impact when broom density was at 10% cover. Goats also stripped bark from Scotch broom plants in winter. Both sheep and goats prevented seed production by eating stem and flowering points, and both ate new Scotch broom shoots in summer [57].

The disadvantage associated with using domestic goats is that they are not selective, and native species that start to revegetate the area are also eaten [17,59]. Because cattle do not graze broom, it is becoming an increasing problem in Australia in areas where cattle are the only grazing animals. In these areas broom stands increase in size and the pasture is covered with dense broom stands [59].

Chemical: Herbicides are effective in gaining initial control of a new invasion (of small size) or a severe infestation, but are rarely a complete or long-term solution to invasive species management [21]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur. See the Weed control methods handbook [136] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals and adjuvants. Also see the broom ESA [58] and other reviews [59,88,97] for more detailed information on chemical control.

The main chemicals used to control broom are picloram, triclopyr, glyphosate, fluroxypyr and metsulfuron-methyl [59,88]. Foliar sprayed glyphosate has been used to kill mature Scotch broom plants [88]. The foliar spray impacts nontarget species, and Scotch broom sprouting may occur after spraying. Triclopyr ester applied to basal bark is also effective at killing Scotch broom (Bossard unpublished data in [17]). Both of these chemical methods should be used during periods of active growth after flower formation. Chemical removal alone results in standing dead biomass that makes monitoring for and treatment of broom seedlings difficult [17].

Herbicides have not been used on Portuguese broom in the Marin Headlands, and there are no published accounts of their use elsewhere, but the treatments used on Scotch broom are likely to be effective on Portuguese broom [4].

Killing large amounts of broom with herbicides may leave a large amount of standing dead biomass that may present a major fire hazard [17]. Cultural: Sowing native plant species that have the potential to interfere with broom may be a preventive method of weed control. In some cases later successional plants may be encouraged to take root among the unwanted vegetation. Williams [157] suggested that broom stands provide a good environment for establishment of some native broadleaved shrubs and trees, and that these native seedlings should be looked for in broom stands, and encouraged. Williams also recommends that seeds of taller growing plants be sown among the broom, as they may eventually shade out the broom [157].

Taxon biology ⁹

Cytisus scoparius (Broom; syn. Sarothamnus scoparius) is a species in the pea family Fabaceae. It is native to much of Europe, from the British Isles east to southern Scandinavia, south to Iberia, and east to Belarus and Romania. Further northeast, its range is limited by its lack of tolerance of severe winter cold, with temperatures below around -25° to -30°C killing the stems. It is a woody shrub with green photosynthetic shoots, and small caducous leaves present only in spring and early summer. The leaves are simple or trifoliate, 5-15 mm long. Young shoots remain green for several years, silky-hairy at first, and have up to five small longitudinal ridges. Older stems have finely flaky to stringy grey-brown bark. The flowers are bright yellow, 1-2 cm long in bud opening to 2-3 cm long, with the typical pea flower structure; they are produced in mid spring to early summer and are pollinated by bees. The seeds are 3-4 mm diameter, produced in a 2-5 cm long pod, green ripening black. Seed dispersal starts with explosive pod splitting in hot sunny weather, and is continued further by ants, which feed on the small fleshy peduncle at the base of the seed. The seeds are long-persistent in the soil (up to 20-30 years); this can enable the species to survive periodic bush fires, and also to survive in colder regions of northeastern Europe (southern Scandinavia, Poland, etc.) where periodical severe winters may kill the entire adult population.

There are two subspecies, which differ mainly in growth habit:

* Cytisus scoparius subsp. scoparius (Common Broom). An erect shrub, growing to 2-3 metres (rarely 4 m) tall; shoots thinly hairy at first, soon becoming glabrous. This is the common form, occuring throught most of the species range.

* Cytisus scoparius subsp. maritimus (Rouy) Heywood (Prostrate Broom). A prostrate, ground-hugging shrub, not exceeding half a metre in height; shoots densely silky-hairy. It is restricted to the Atlantic coasts of southern Ireland, west Wales, southwestern England, and northwestern France.

Broom (primarily subsp. scoparius) is widely cultivated as a garden plant, and for wildlife benefit. The Andreanus Group cultivars are particularly popular, selected for their bright orange-red to pink flowers. It is also naturalised, and sometimes an invasive weed species, in parts of Australia, New Zealand, India, and North America.

The English name derives from its historical use in the manufacture of brooms, as the harvested twigs retain a degree of flexibility in use without becoming brittle. A number of other English names, some of them offensive, have been applied to the species outside of its native range.