Invasive Plants of California's Wildland

Spanish broom (Spartium junceum) is a perennial shrub more than head high, with cylindrical rush-like branches, green when young, turning brown as branches mature. Leaves are about half an inch long, entire, and elongated to lance-shaped. Leaves are present on the plant from February to early June, so this plant is leafless most of the year. Large (an inch or more), light yellow, pea-like flowers are prominently displayed on terminal racemes of outer canopy branches in late spring. Pods are abundant and dark brown when mature. Pods split when dry, and the two halves of the pod twist into a spiral shape, dropping the seeds.

In California Spanish broom is found in North Coast counties, the San Francisco Bay region, the Sacramento Valley, through South Coast counties to the Mexican border, in the western Transverse Ranges, and the Channel Islands. It is common in disturbed places, particularly eroding slopes, river banks, road cuts, and abandoned or disturbed lands, and can colonize post-burn chaparral and soft chaparral sites.

Spanish broom is native to the southern Mediterranean region of Europe, including Spain, Morocco, the Canary Islands, Madeira, and the Azores (Hickman, 1993). It was introduced into the California ornamental trade in 1848 in San Francisco (Butterfield 1964). Beginning in the late 1930s, Spanish broom was planted along mountain highways in southern California (Hellmers and Ashby 1958). In 1949 Spartium junceum had established naturalized populations in Marin County (Howell 1949). This species spreads by abundantly, producing seeds that are transported by any type of erosion or by rain wash.

Spanish broom rapidly colonizes disturbed habitats and develops thick shrub communities that prevent colonization by native soft or hard chaparral species. Plants grow to more than head height and form a tangle containing a large amount of dead wood. Mature stands of Spanish broom should be considered a fire hazard during the dry season. It is poor forage for native wildlife.

(click on photos to view larger image)

Spanish broom reproduces by seed after two to three years of growth. Flowers are pollinated by bees and produced in late March to early April. Pods mature and dehisce in late May through early June. Each inflorescence produces ten to fifteen pods containing approximately fifteen seeds each. One plant can easily produce 7,000 to 10,000 seed in one season. Seeds fall near the plant and are subsequently moved by erosion, rain wash, and possibly ants. Seed viability is at least five years, suggesting that a significant seedbank is present

Shoots of Spanish broom are initiated in late winter and early spring, but most rapid growth occurs in May. Shoots elongate quickly and produce leaves with long internodes. The shoots harden off in late spring and leaves drop. Photosynthesis by stems occurs all year. Although the leaves have twice the photosynthetic rate of stems (Nilsen et al. 1993), photosynthesis in stems provides most of the whole plant carbon gain because of their longer life span and larger surface area (Nilsen and Bao, 1990). Spanish broom flowers are popular in the ornamental trade and are used for yellow dye. Stems are used for fibers, which accounts for one of its common names, weaver’s broom. This species is also an effective stem sprouter.

Information about eradicating Spanish broom is meager because there has been little experimentation with this species compared with that for Scotch broom (Cytisus scoparius) or French broom (Genista monspessulana). Evaluation of the following mechanisms of renewal is therefore based on the biology of the species rather than on information derived from controlled experiments.

It is likely that the success of any control method will vary with topography, soil chemistry, and climate, age and density of plants in the stand, and the availability of human and technical resources. Since a large and persistent seedbank is predicted from reproductive characteristics, rapid regrowth of the stand is likely following fire or mechanical removal of above-ground plant parts.

A comprehensive monitoring of control effectiveness is critical because there is no scientifically based knowledge about control of Spanish broom. Experimental manipulations should be monitored at least annually. Each monitoring visit should determine the number of new plants and the size or age distribution of the recovering population. Significant attention should be placed on the proportion of new individuals coming from the seedbank or resprouting from old plants. Monitoring should continue for at least five years following control treatment.

Manual/mechanical methods: Hand pulling of Spanish broom plants will be practical when the stand is one to four years old. When plants are small enough, pulling should be an effective mechanism of control as long as roots are removed and follow-up treatment of seedlings is done. The optimal season for pulling may be July-September when plants are already experiencing water stress (Nilsen and Karpa 1994). When plants have matured to small tree size, they are not amenable to pulling technology such as the weed wrench.

Brush hogs: Use of these machines is not likely to be effective for several reasons. Many of the sites where Spanish broom has invaded are steep slopes that may be difficult to traverse with a brush hog. The trunks of Spanish broom also rapidly increase to a size outside the range of effectiveness for this technology. Because of their twisting motion, brush hogs are more effective in reducing resprouting than is saw cutting, but resprouting should be expected following cutting by brush hog.

Saw cutting: This method would remove individuals in more mature stands when the bases of plants are too large for pulling or for brush hog removal. However, the clean cut from a saw will allow for maximum resprouting. Spanish broom has a great facility for resprouting from a saw cut even when the cut is close to the ground. Among all mechanical methods, saw cutting is the least likely to be effective in preventing resprouting. In general, manual/mechanical removal may be effective only when Spanish broom populations are very young.

Prescribed burning: After low-temperature fires Spanish broom will be able to resprout vigorously from trunk bases and stem meristems. However, a hot fire that kills all above-ground stems and burns hot and close to the ground will completely kill standing individuals and most likely remove some of the seedbank. Seeds of this species are similar in structure to those of Scotch broom (Cytisus scoparius). In heterogeneous or low-temperature fires Scotch broom seedbanks were not effectively reduced. Under similar fire conditions it is unlikely that fire will effectively reduce seedbank regeneration of Spanish broom.

There are no USDA approved biological control agents for Spanish broom. In greenhouse situations plants are susceptible to mealy bugs and show significant evidence of viral depression of growth.

Spanish broom is sensitive to applied pesticides. In greenhouse situations only mild pesticides can be used without detrimentally affecting the plants. It is highly likely that application of chemicals such as glyphosate or triclopyr will drastically reduce population size. The ramifications of applying herbicides to a plant community must be carefully considered, because effects on non-target species are likely, especially when foliage spray methods are used.

A particularly effective control combination may be saw cutting followed by an application of 3 percent glyphosate (as Roundup®) to cut stems. This would remove above-ground plant parts to let light in for other species and also kill the root system, preventing root sprouting. However, if a substantial seedbank of Spanish broom already exists, the described combination will work effectively only if seedlings are also removed for several years, either by hand or by chemical treatments.