Invasive Plants of California's Wildland

Bridal veil broom (Retama monosperma) is a large shrub, ten feet tall or more, and when full grown may be twenty feet across. Plants are gray-green, almost entirely leafless, with slender branches drooping in weeping willow fashion. Young plants are usually wispy, with a single stem and strong taproot. Young plants are most visible in late summer and early fall when annual grasses are brown. Flowers are small, white, and pea-like, appearing in short clusters from the stems. There is usually one seed per pod, as indicated by its specific epithet. Spanish and Scotch broom can be easily distinguished from this plant by their larger golden yellow flowers. Spanish and Scotch broom also have more seeds per pod, four to six and five to twelve, respectively.

Bridal veil broom is known to occur in the wild in California only on and adjacent to the Naval Ordnance Center, Pacific Division, Fallbrook Annex in San Diego County. It occurs in granitic sandy loam soils that are well drained. Associated plant communities are Diegan coastal sage scrub, southern perennial grassland, and disturbed annual grassland. It occurs mostly in disturbed annual grassland and disturbed Diegan coastal sage scrub. The plant is suited to mediterranean climates on rocky slopes and sandy banks. Planted as an ornamental species in south and central coastal California, it may escape and become established elsewhere. It is unknown at this time if other coastal habitats are threatened by this plant.

Bridal veil broom is native to the Mediterranean region, chiefly Spain (Iberian Peninsula) and North Africa, where it occurs on sandy soils near the coast and at border marshes. In its native habitat it is a minor component of the coastal scrub community (Valdesi et al. 1987). It was cultivated as an ornamental in England as early as 1690 and has been known to escape from cultivation there. There are a few records indicating some cultivation of this plant in California since 1916, with one record from San Diego in 1917 (McClintock, pers. comm.). A new invasive exotic in San Diego County, it was first noted six years ago on the Naval Ordnance Center near the city of Fallbrook. At that time it was growing in a few scattered patches covering less than ten acres. When eradication measures were implemented in 1996, it occupied over 2,000 acres of the Annex. It is believed that this plant escaped from a nearby nursery. Bridal veil broom spreads primarily by seed. Each plant produces hundreds, and on larger plants thousands, of seeds. It appears that ants and birds are the primary seed dispersers. At present, little is known about its physiology and reproductive mechanisms.

Bridal veil broom crowds out other vegetation and can dominate grasslands and disturbed habitats. It is a threat to the habitat of the federally endangered Stephen’s kangaroo rat and the threatened California gnatcatcher. As it becomes dominant in the landscape it reduces the availability of vegetation to support sensitive species. No nesting or foraging in and among these plants has been noted. There is no direct evidence of allelopathy at this time. It is unknown whether this plant increases fire risk. It has been in the natural landscape for such a short time that there has been no large die-off of bridal veil broom or large build up of dead litter on the ground in infested areas. However, its growth form of large, finely branched limbs suggests that plants would burn readily.

Specific data regarding reproduction is unavailable. However, peak flowering is February-April, and plants have been observed flowering year round. Each plant produces large numbers of seed. Seeds are dispersed from the pods ballistically and then probably further dispersed by ants since they have elaiosomes (Bossard, pers. comm.) and rain runoff. Seeds remain viable in the soil for several years. Bridal veil broom is a shrub, but detailed data regarding growth patterns is unavailable. Rapid growth is apparent during peak growth periods in spring and early summer. Annual growth rates are not known, but up to three feet (1 m) of growth has been noted within one year following cut-stump treatment. Young plants appear to grow one to two feet in the first year. The plant develops deep taproots. Bridal veil broom spreads readily and can resprout from the crown. Large stands of over 200 plants per acre have been noted. Where solid, homogeneous stands had not developed prior to eradication efforts, it appeared likely that they would develop in short order.

The plant appears to readily decompose upon cutting, and there is no direct evidence that it spreads by shoot fragments.

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A preferred method of control has not yet been developed. This species is known in California from only one population, and current eradication methods appear to be eliminating it there. Continued monitoring and control efforts will be necessary until the seedbank is depleted. Annual monitoring in summer following chemical treatment is recommended. Young plants are most easily seen in summer, and sensitive species such as the California gnatcatcher are not nesting at that time.

Mechanical removal: Cutting stems off near the ground with saws (brush saw and chain saw) or loppers will stress the plant. Cut plants resprout vigorously, so cutting alone will not kill the plants (Jacobsen, unpubl. data).

Prescribed burning: Burning has not been investigated for this species, but may be useful for clearing the seedbank as for other brooms (Bossard 1993).

No USDA approved biological controls are available. However, some plants were killed by gophers eating the roots. At this time nothing is known about insects or fungi that could be used to control bridal veil broom. No browsing by cattle has been noted.

Foliar application of glyphosate (as Roundup®, 2.5 percent) will kill young plants less than about two feet tall. Medium (up to six feet) and large (twenty feet in diameter) plants will be stressed but not killed. Foliar application of triclopyr (as Garlon 4®, 10 percent) on young and medium-sized plants will achieve approximately 50 percent kill. Basal bark application of triclopyr (as Garlon 4®, 25 percent) yielded the same results but is less likely to impact non-target species.

A combination of cutting with chain saws and applying 10 percent triclopyr (as Garlon 4®) solution to stump surfaces severely stressed the plants but killed only 50 percent of them. A subsequent summer application achieved approximately 90 percent mortality.