French Broom
HOW DO I RECOGNIZE IT?
Distinctive features: 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 usually 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).
Description: Fabaceae. Shrub, usually <10 ft (3 m), but occasionally to 16 ft (5 m). Stems: twigs silvery silky-hairy. Leaves: alternate; stipules <0.1 in (2 mm); deciduous; leaflets of trifoliate leaves 0.3-0.5 in (10-15mm), oblanceolate to obovate with length about twice width, upper surface glabrous, lower surface with appressed or spreading hairs hairiness; petioles <2 in. (5 mm). Inflorescence: 4-10 flowers in subcapitate racemes (on axillary short shoots); terminal or central flower usually opening last; pedicels <0.1 in. (1-3 mm).
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.
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).
The period of most rapid vegetative growth is April-July. As in other brooms, most photosynthate is moving up in the shrub toward branch tips during flowering, bud break, and seed set. Photosynthate starts moving down toward roots of this broom after seeds are well grown but before seed release (Bossard et al. 1995). French broom retains much of its foliage in coastal areas, and is more deciduous in inland areas. Its life span is typically ten to fifteen years
Common Names: Montpellier broom
Genus: Cytisus
Species: monspessulanus