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Scotch broom, English broom, common broom

Sarothamnus scoparius, Spartium scoparius

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CalEPPC List A-1,CDFA C

Carla Bossard

Scotch broom (Cytisus scoparius)
is a perennial shrub six to ten feet tall. Its sharply angled branches generally
have five green ridges with hairs on them when young; as the branches mature the
hairs fall off, and the branches become tan and lose the distinct ridges. Pods
have hairs along the seams only. One or two golden yellow pea-like flowers
cluster between the leaf base and stem. About half the photosynthetic (green)
tissue is in the leaves and half is in twig tissue. Sometimes this species is
confused with French broom (Genista monspessulana), which has pods with
hairs all over them, stems that are not ridged or green, and more than
eighty-five percent of its photosynthetic tissue in leaf tissue (Bossard and
RejmÌÁnek 1994).

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Found along the California coast from
Monterey north to Oregon border, Scotch broom is prevalent in interior mountains
of northern California on lower slopes and very prevalent in Eldorado, Nevada,
and Placer counties in the Sierra Nevada foothills. It is also reported from Los
Angeles and San Bernardino counties. It is common in disturbed places, such as
river banks, road cuts, and forest clearcuts, but can colonize undisturbed
grassland, shrubland, and open canopy forest below 4,000 feet (

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Scotch broom is native to Europe and North
Africa. Its natural range is broad, from Great Britain to the Ural Mountains and
from Sweden to the Mediterranean. Introduced to California in the 1850s as an
ornamental in the Sierra Nevada foothills, it was later used to prevent erosion
and stabilize dunes (Geickey 1957, Schwendiman 1977). It spreads by prodigious
seed production. One medium-sized shrub can produce over 12,000 seeds a year.
After ballistic dispersal, seeds are further dispersed by ants, animals, or in
mud clinging to road grading or maintenance machinery. Scotch broom is also
readily dispersed by rain wash on slopes (Bossard 1991b). Plants can resprout
from the root crown after cutting or freezing and sometimes after fire (Bossard
and RejmÌÁnek 1994).

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Scotch broom currently occupies more than
700,000 acres in central to northwest coastal and Sierra Nevada foothill regions
of California (Barbe, pers. comm.). It displaces native plant and forage species
and makes reforestation difficult. It is a strong competitor and can dominate a
plant community, forming a dense monospecific stand. Scotch broom infestations
can attain a biomass of over 44,000 to 50,000 kg/hectare in three to four years
(Bossard and RejmÌÁnek 1994, Wheelor et al.1988). Seeds are toxic to ungulates.
Mature shoots are unpalatable and are not used for forage except by rabbits in
the seedling stage (Bossard and RejmÌÁnek 1994). Foliage causes digestive disorders in horses (Parsons 1992). Since Scotch broom can grow more rapidly than most trees used in forestry, it shades out tree seedlings in areas that are revegetated after tree harvest. Scotch broom burns readily and carries fire to the tree canopy, increasing both the frequency and intensity of fires (Parsons 1992). This species is difficult to control because of its substantial and long-lived seedbank.

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Scotch broom becomes reproductive at two to three years on reaching a height of two to three feet (60-100 cm). It flowers in late March to April inland, April to June on the coast. Flowers appear before leaves. Long-lived seeds are copiously produced (to 12,000+ seeds/mature shrub) and mature in June and July. Seeds initially disperse ballistically from the pod, with an audible pop, and are further dispersed by ants and rain wash on the ground (Bossard 1990b, 1993). Seeds are known to survive at least five years in the soil (Bossard unpubl. data) and possibly as long as thirty years (Carson 1998). The seedbank can build to over 2,000 seeds/sq ft.

Seeds germinate from November to June inland, January to July along the coast (Bossard 1993) when provided with disturbance that creates open mineral soil.

(click on photos to view larger image)

Germination may be enhanced by fire, but
relies less heavily on water, wind, or animal distribution than do some other
invasive plants, although its tough seed coat provides good protection from
abrasion associated with water transportation (Carson 1998). Seedlings can
tolerate even 90 percent shade. Approximately 35 percent of each seed crop
becomes part of a rapidly developed seedbank. Plants can resprout from the root
crown when cut, particularly during the rainy season (Bossard and RejmÌÁnek
1994).

Scotch broom is host to nitrogen-fixing bacteria, which assists
both its establishment on poor and disturbed sites and its ability to outcompete
native species. It tends to acidify the soil (although not as strongly as does
gorse, a relative). The period of most rapid vegetative growth is May to July,
with some dieback occurring during seasonal periods of drought (Bossard and
RejmÌÁnek 1994). Most photosynthate is moving upward in the shrub toward branch
tips during flowering, bud break, and seed set, which occur in late March to
mid-April, April, and May, respectively). Photosynthate starts moving down
toward roots after seeds are well grown but before seed release (Bossard,
unpubl. data). On dry, hot sites Scotch broom will drop its leaves in late July
or August. Its life span in California is longer than in its native range, with
some individuals surviving up to seventeen years (Bossard 1990a). Broom is
considered to be primarily an early serial colonizer that will be shaded out
once native species are established. There is, however, concern that its
vigorous and prolific growth, along with acidification of the soil, inhibits
establishment of other species.

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 non-target
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.

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Manual/mechanical removal: Pulling with weed wrenches is
effective for broom removal. The wrench removes the entire mature shrub,
eliminating resprouting. However, the resultant soil disturbance tends to
increase the depth of the seedbank (Bossard 1991, Ussery and Krannitz 1998).
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
area. Golden Gate National Park has had success in using volunteers to remove
broom with weed wrenches and then closely monitoring and removing broom
seedlings for five to ten years.

Ussery and Krannitz (1998) found significantly more trampling of
native species, and more soil disturbance and broom seedling regeneration, when
adult broom plants were removed by pulling rather than cutting in British
Columbia. Brush hogs, which twist off above-ground biomass, can be used for
broom removal. They are less labor-intensive, but heavily impact non-target
species and cannot be used on steep slopes. The twisting action is more
destructive to tissues that initiate resprouting than is clean cutting. However,
depending on the season of brush hog removal, resprouting can still be a serious
problem. Brush hog removal has been used with limited success in Redwood
National Park (Popenoe, pers. comm. 1997).

Saw cutting removes above-ground portions of shrubs, but
depending on the time of cutting, may result in high rates of resprouting. In
the Sierra Nevada foothills saw cutting undertaken at the end of the summer
drought period (August to October) resulted in a resprouting rate of less than 7
percent, whereas cutting done at other times resulted in resprouting rates of 40
to 100 percent (Bossard and RejmÌÁnek 1994). In British Columbia plants greater
than one-quarter inch (3 mm) in diameter cut below two inches (5 cm) from the
soil surface in July were found to have less than 1.5 percent resprout rate
(Ussery and Krannitz 1998).

Prescribed burning: Burning uncut broom has been used with some
success on Angel Island. Reburn of the removal site is usually necessary two and
four years after the initial burn (Boyd, pers. comm. 1997). For prescribed
burning of pretreated or cut broom see below under integrated
methods.

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Insects and fungi: Two USDA approved
insects, a stem miner, Leucoptera spartifoliella, and a seed beetle, Apion
fusciostre, were introduced in the 1960s as biocontrol agents, but have had
limited success in California. New insect biocontrol agents are being tested in
England and France for use on broom in Australia and New Zealand (Hoskings
1994). If proved safe and effective in California, these insects may ultimately
become available for use as biocontrol agents in California.

Grazing: Heavy grazing by goats during the growing season for
four to five years has been reported effective in New Zealand, and grazing by
llamas has been tried at a few sites in California (Archbald, pers. comm. 1997).
The disadvantage associated with using goats is that they are not selective, and
native species that start to revegetate the area are also eaten.

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Foliar sprayed until wet, 2 percent
glyphosate (as Roundupå¨) has been used to kill mature plants of Scotch broom.
Adding surfactant improves effectiveness (Parsons 1992). The foliar spray
impacts non-target species, and resprouting may occur. Triclopyr ester (25
percent) (as Garlonå¨) in Hastenå¨, Penevatorå¨, or other seed press oil (75
percent) applied with a wick in low volume (2-3 drops) to basal bark has also
proved effective (Bossard unpubl. data). This application technique does not
affect non-target species, but it is more time-consuming and may be impractical
for large infestations. Both of these chemical methods should be used during
periods of active growth after flower formation. Chemical removal alone results
in standing dead bio-mass that makes monitoring for and treatment of broom
seedlings difficult. The standing dead biomass also presents a major fire
hazard.

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The most effective removal treatment in a
project in Eldorado Forest in the Sierra Nevada foothills was found to be
cutting shrubs in September and October, allowing cut shrubs to dry on site, and
then burning dried shrubs in late May and early June. This killed any resprouts
and most of the seed within the top 2 in
(>6 cm) in ant nests, the reduction in the seedbank significantly decreased
the need for chemical or hand removal of new seedlings in succeeding years.
Follow-up monitoring and treatment using this same combination of methods in a
coastal area of Redwood National Park reduced the seedbank by only 52 percent
and did not significantly reduce the time spent in follow-up control. The
moister climate decreased the efficacy of this removal combination at the
Redwood National Park site (Bossard 1991a, 1993, and unpubl. data).

Because of broom‰Ûªs seedbank, monitoring removal sites to locate
and kill new seedlings is essential. Location and retreatment of resprouts is
also imperative. If any single removal technique is used the site should be
examined once a year, when seed germination ends in late spring, for five to ten
years. Using the combined removal treatments, monitoring should occur late
spring, yearly, for the first two years then again the fourth and sixth year
after removal.

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