Source: California Invasive Plant Council
URL of this page: http://www.cal-ipc.org/ip/management/ipcw/pages/detailreport.cfm@usernumber=52&surveynumber=182.php
Invasive Plants of California's Wildland
|Scientific name||Genista monspessulana|
|Additional name information:||(L.) L. Johnson|
|Common name||French broom, soft broom, canary broom, Montpellier broom|
|Synonymous scientific names||Cytisus monspessulanus, C. racemosus, C. canariensis, Genista monspessulanus, Teline monspessulana|
|Closely related California natives||0|
|Closely related California non-natives:||4|
|Listed||CalEPPC List A-1,CDFA C|
HOW DO I RECOGNIZE IT?
French broom (Genista monspessulana) is an upright, evergreen shrub, commonly to ten feet tall. The round stems are covered with silvery, silky hair, and the small leaves are ususally arranged in groups of three. About eighty-five percent of the photosynthetic tissue of French broom is in leaf tissue. The small (less than half-inch) yellow flowers are pea-like and clustered in groups of four to ten. The mostly inch-long pods are covered with hairs.
This species sometimes is confused with Scotch broom (Cytisus scoparius), which has pods with hairs only at the seam, green stems that are five-angled and ridged, flowers that are golden yellow and larger than half an inch, and only about fifty-five percent of total green tissue as leaves (Bossard and Rejmánek 1994).
Flowers: shaped like pea flowers; calyx silky-hairy, 0.2-0.3 in (5-7 mm); banner 0.3-0.5 in (10-15 mm), corolla yellow to light yellow. Fruit: a pod, 0.5-1 in (15-25 mm), covered in dense silky hairs, dark brown or black at maturity. Empty seed pods curled. Seeds: 3-8 seeds per pod, brown to black, shiny, round to oval, with a cream to yellow eliaosome (description from Hickman 1993 and pers. observation).
|WHERE WOULD I FIND IT?||
French broom is found primarily in central coastal counties from Monterey County north to Mendocino County and inland in Lake, Solano, and Contra Costa counties. It is also known from Del Norte County, northern Sierra Nevada foothill counties to 800 meters, and in Kern, San Bernardino, and San Diego counties.
This broom is common on coastal plains, mountain slopes, and in disturbed places such as river banks, road cuts, and forest clearcuts, but it can colonize grassland and open canopy forest. It is found growing in varied soil moisture conditions, but prefers siliceous soils. Unlike other broom species in California, it grows reasonably well on alkaline soils with pH 8. It is competitive in low-fertility soils because of mutualistic relationships with nitrogen-fixing bacteria found in small nodules on roots. While Scotch broom is a problem species in many parts of the world, French broom is especially problematic in California and Australia (Partridge 1989, Parsons 1992). French broom seedlings are less tolerant of frost than are those of Scotch broom and consequently are less often found at higher elevations.
|WHERE DID IT COME FROM AND HOW IS IT SPREAD?||
Native to countries surrounding the Mediterranean and in the Azores, French broom is thought to have been introduced to the San Francisco Bay Area in the mid-1800s as an ornamental. It spreads via prodigious seed production. A medium-sized shrub can produce over 8,000 seeds a year (Bossard unpubl. data). After pods open explosively, flinging seeds up to 4 m, the seeds are further dispersed by ants, birds, and animals and in river water and rain wash (McClintock, pers. observation), in mud, and on road grading or maintenance machinery (Parsons 1992). It resprouts readily from the root crown after cutting, freezing, and sometimes after fire (Bossard et al. 1995).
|WHAT PROBLEMS DOES IT CAUSE?||
French broom currently occupies approximately 100,000 acres in California (D. 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 dense monospecific stands. In an experiment in New Zealand French broom had a higher growth rate than any other broom species found in California, reaching an average height of more than 4.5 feet (141 cm) in two growing seasons. Since it can grow more rapidly than most trees used in forestry, it shades out tree seedlings in areas that are revegetated after harvest.
French broom foliage and seeds are toxic, containing a variety of quinolizidine alkaloids, especially in young leaves (Montlor et al. 1990). In some livestock, ingestion of plant parts can cause staggering followed by paralysis (McClintock 1985). Foliage can cause digestive disorders in horses (Parsons 1992). Infestations of broom degrade the quality of habitat for wildlife by displacing native forage species and changing microclimate conditions at soil levels. French broom is believed to be responsible for reducing arthropod populations by one-third in Golden Gate National Recreation Area (Lanford and Nelson 1992). It burns readily and carries fire to the tree canopy layer, increasing both the frequency and intensity of fires. French broom along roadsides obstructs views, requiring expensive ongoing road maintenance. This species establishes a dense, long-lived seedbank, making it difficult to eradicate.
|HOW DOES IT GROW AND REPRODUCE?||
French broom becomes reproductive at two to three years of age, on reaching a height of one and a half to two feet (45-60 cm). It flowers in late March-May inland, March-July on the coast. Flowers appear just prior to new leaves. Long-lived seeds are copiously produced (Hoshovsky 1995) and mature in June-July. Seeds are known to survive at least five years in soil (Bossard unpubl. data). French broom seedbanks have been found to contain 465 to 6,733 seeds per square meter (Hoskings 1994, Parker and Kershner 1989). Seeds germinate December-July (Bossard unpubl. data). Cheng (in press) reports that heat treating seeds with temperatures of 65 degrees C improved germination of seed in some populations but not in others. Seedlings can tolerate up to 80 percent shade (Bossard unpubl. data). Plants can resprout from the root crown after cutting. Once seedlings are taller than approximately eight inches (20 cm), their rate of resprouting after cutting can be over 90 percent, particularly if cut in the rainy season (Bossard unpubl. data).
|HOW CAN I GET RID OF IT?||
As with other broom species, the best method for removal of a French broom infestation depends on climate and topography, age and size of the infestation, importance of impact to non-target species, and type, quantity, and duration of resources available to remove and control broom at the site. All methods require appropriate timing and follow-up monitoring. Because of the seedbank, monitoring removal sites to locate and kill new seedlings is essential. Location and retreatment of resprouts is also necessary. Sites should be examined once a year, when the seed germination period ends in late spring, for five to ten years and every two years thereafter.
Manual/mechanical removal: In general, when using hand removal or mechanical methods it is best to start in areas with small infestations and many desirable species that will reseed naturally. Desirable species should be given some assistance by hand weeding of French broom. Next work on areas with an intermediate degree of infestation (Fuller and Barbe 1985). Finally, tackle larger areas and dense concentrations of French broom using other techniques (fire, chemicals) to augment or replace hand pulling.
Pulling with weed wrenches is effective for broom removal in
small infestations or where an inexpensive, long-duration labor source is
dedicated to broom removal. The weed wrench removes the entire mature shrub,
eliminating resprouting. However, the resultant soil disturbance tends to
increase depth of the seedbank and prolong the need for monitoring. Wrench
removal is labor-intensive, but can be used on slopes. It also allows targeting
of broom plants while minimizing impact on neighboring species. Golden Gate
National Recreation Area has had success in using volunteers to remove broom
with weed wrenches and closely monitoring and removing broom seedlings for five
to ten subsequent years.
Brush hogs, which twist off above-ground plant material, can be used for broom removal. Although less labor-intensive than weed wrenches, they damage neighboring 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 problem. Brush hog removal has been used with limited success in Redwood National Park (Popenoe, pers. comm.).
Saw cutting: Archbald (1996) reported success with saw and/or brush cutter following four steps: (1) cut shrubs at or below ground level in late July or August, after broom has gone to seed and soil moisture is at a seasonal low; (2) move cut broom plants to sites appropriate for disposal or burn in spring after plants dry (use tarps to avoid spreading mature seed to uninfested areas while moving sawed broom); (3) the following summer, after grasses are dry and have dispersed their seed, destroy new French broom seedlings by mowing as low to the ground as possible with a heavy-duty brush cutter with a four-pointed metal blade; and (4) repeat for the next five or six seasons or until the seedbank is exhausted. Timing and height of cutting are critical in using this technique. Cutting French broom in June in Mendocino County at 5-8 cm above soil surface resulted in extensive resprouting (Bossard et al. 1995).
Mulching: A 10 cm deep wood bark mulch significantly decreased seedling emergence of French broom in experiments conducted by Cheng (in press) in the San Francisco Bay Area. This suggests that mulching could be used to suppress regrowth from the seedbank after removal of mature shrubs.
Prescribed burning: Using fire to remove uncut French broom in
late spring or early summer has had some success at Mt. Tamalpais State Park in
Marin County (Boyd 1994). Reburning of the removal site is usually necessary two
and four years after the initial burn (Boyd, pers. comm.). Reburnings are most
effective in killing resprouts and seedlings if there are either naturally
occurring or reseeded grasses to carry the fire.
There are no USDA approved biocontrol agents for French broom. The distribution and effects of the native pyralid moth, Uresiphita reversalis, on French broom were investigated by Montllor et al. (1990). While this insect may defoliate some French broom shrubs, plants grow new leaves after the larval stages undergo metamorphosis. Other insect biocontrol agents are being tested in England and France for use on Scotch broom in Australia and New Zealand (Paynter 1997). Some of these agents may use French broom as a host as well. However, the insects known to feed on and impact mature Scotch and French brooms (some Sitona sp.) are likely to feed on Lupinus species as well and consequently would not be appropriate for release in California (Paynter 1997).
Grazing: Heavy grazing by goats for four or five years during the growing season has been reported as effective in New Zealand and has been tried at a few sites in Marin County in California (Archbald, pers. comm.). The disadvantage is that goats are not selective, and native species that may start to revegetate the area are also eaten.
A solution of 3 percent glyphosate sprayed on foliage until wet has been used to treat mature French broom shrubs. Adding surfactant improved effectiveness (Parsons 1992). However, the foliar spray impacts non-target species, and resprouting often occurs. Triclopyr ester (25 percent), in Hasten® or Penevator® oil (75 percent) in one spot, low-volume basal bark application with a wick has proved effective in killing French broom (Bossard et al. 1995). Dye should be added to the herbicide solution to help avoid missing stems. It was necessary to spot only the main stem with 2 or 3 drops of herbicide, within 8 cm of the ground surface, to obtain a 99 percent kill of the eight-year-old French broom plants in this experiment conducted in Mendocino County. Soil analyses showed no contamination by the triclopyr, even in plots that were later burned. However, killing the mature shrubs was not sufficient to remove the infestation of French broom because of its well developed seedbank (Bossard et al. 1995). This application technique does not impact non-target species, but it is time-consuming if the site is large. Both of these chemical methods should be used during periods of active growth after flower formation and seed set but before seed dehisces.
The herbicide 2,4-D, alone or with additives such as diquat, picloram, dicamba, and sodium chlorate, has been used to control French broom. Not all of these herbicides are registered for use in California. French broom seedlings are least resistant to auxin-minimizing herbicides such as 2,4-D at the four- to six-inch (10-15 cm) size. Chemical removal alone results in standing dead biomass, which makes monitoring for and treatment of broom seedlings difficult. Standing dead biomass also presents a fire hazard.
|An integrated approach||
The most effective removal treatments in a project in Jackson State Demonstration Forest conducted by the CalEPPC broom committee (Bossard et al. 1995) was a combination of treatments that began in early July with low-volume basal bark application of triclopyr ester (as Garlon®) (25 percent) in Hasten or Penevator oil (75 percent) and a purple dye in a low-volume basal bark application (2-3 drops in one spot <8 cm from the soil surface) with a squirt bottle on mature dense stands of French broom. After four weeks all broom shrubs were dead and were cut down, left on site, and burned. This flushed the seed from the seedbank by increasing germination rate with the next rains. French broom seedbanks in burned plots were reduced to less than 5 percent of their original size three years after prescribed burns. Seedbanks of unburned plots otherwise treated the same were reduced to 15.5 percent of their original size, and control plots exhibited no significant decrease in seedbank size. For the next two years, in July, seedlings in plots were treated with either 2 percent glyphosate (as Roundup®, label-recommended strength) or cut with a four-blade gasoline powered brush cutter. Glyphosate was applied with a backpack sprayer, and non-target vegetation was avoided. Brush-cut plots had 1.6 resprouts per square meter, whereas plots in which glyphosate was applied to seedlings had 0.2 resprouts per square meter. Mean percent cover by French broom was reduced from 87 percent to less than 0.2 percent in plots treated with basal bark triclopyr, cut, burned, and seedlings treated with glyphosate.