Invasive Plants of California's Wildland

Common gorse (Ulex europaea) is a prickly evergreen shrub less than ten feet tall, with a profusion of yellow pea-like flowers from March to May. By May plants are covered with half-inch- to one-inch-long brown pods. The short, stout branches are densely packed and may appear leafless. Spines, approximately half an inch long, are located at base of leaves. The somewhat similar species, Scotch broom (Cytisus scoparius), is not prickly.

In California gorse can be found in all coastal counties and in the northern Sierra Nevada foothills. It invades infertile or disturbed sites, sand dunes, gravel bars, fence rows, overgrazed pastures, logged areas, and burned-over areas. It will grow on most soil types, from good silt to plain boulders. It has been recorded as growing on serpentine soils and, rarely, on highly calcareous soils.

Gorse is more tolerant of soil acidity than most legumes, and it readily invades soils of poor fertility. The only restrictions to soil quality seem to be adequate nutrition and availability of trace elements. It grows best in moist soils and on shaded slopes. Gorse can thrive in well drained soils and in areas with a high water table. It is intolerant of heavy shade, where it produces coarse foliage and few flowers.

Gorse is native to central and western Europe, where it has long been cultivated as hedgerows. It has naturalized in Australia and New Zealand, where considerable research has been done to control its spread. It has established along the Atlantic coast of North America, from Virginia to Massachusetts.

Gorse was introduced to the West Coast before 1894, and has been established in Mendocino County for 100 years. By the 1950s gorse had spread throughout western Washington and Oregon and northern California. It has been reported in every coastal county in California from Santa Cruz to Del Norte, and sparingly in southern California (Pryor and Dana 1952).

Gorse seeds are too heavy to be dispersed by wind, and usually fall within six feet of the parent plant. Seeds may be spread by ants, quail, water, and human activity.

Gorse may be slow in spreading and becoming established, but where it gains a hold, there are few other plants that will so completely dominate an area. Besides becoming a significant fire hazard, it can successfully outcompete native plants in part because of its association with nitrogen-fixing bacteria, which facilitate its colonization of nitrogen-poor soils. Gorse leaf litter acidifies and lowers the cation exchange capacity of moderately fertile soils by immobilizing the bases, making it more difficult for native species to establish. On San Bruno Mountain, San Mateo County, gorse is considered the most difficult exotic species to control, and it has caused considerable loss of valuable grassland habitat (Reid 1985).

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Gorse reproduces by resprouting from stumps and by seed. Seed reproduction is far more troublesome to control. Seeds are very small, averaging 60,000 seeds per pound. Seed counts in the upper inch of soil may be up to 2,000 per square foot (Zabkiewicz and Gaskin 1978a).

Seeds are impermeable to water, preventing immediate germination. They may remain dormant yet viable in the soil up to thirty years, with reports of up to seventy years (Zabkiewicz 1976). Seed germination may occur under suitable conditions at any time of year. Light is not essential, but few seeds germinate in the shade of established gorse. When dense gorse cover is removed, there is a flush of germination, because of either increased light or increased temperature. Heat stimulates germination, particularly at temperatures reached just below the soil surface during fire.

Gorse plants grow quickly, producing considerable dry matter. Year-old stands may contain 1,100 lbs/acre, with older stands producing 3,300 lbs/acre per year. Nitrogen in soils occupied by gorse can accumulate at an annual rate of 20-30 lbs/acre, surpassing the production of some well managed, fertilized pastures (Egunjobi 1971). Much of this production, with its high nitrogen content, ends up as litter, accumulating faster than any other temperate plant species.

Plants grow outward, forming a central area of dry, dead vegetation. A single plant can be up to thirty feet in diameter. Plants are typically medium-sized shrubs, but when exposed to constant wind they may be mat-like or cushion-like.

Roots tend to grow in the top few inches of soil, with only the tap root extending to greater depths. Extensive lateral roots are supplemented by a fine mat of adventitious roots that descend from the lower branches.

Gorse is a successful invasive plant because it grows on a variety of soil types, fixes nitrogen, and may impoverish soil of phophorus. It produces copious amounts of heat-tolerant seeds with long-term viability, and regenerates rapidly from seeds and stumps after disturbances such as brush clearing or fires.

Because of the longevity of buried seeds, gorse control efforts must be long-term to be successful.

Manual/ mechanical methods: Gorse seedlings and young plants less than five feet tall may be hand pulled, especially after rain has loosened the soil. It is important to remove the root system, which may resprout if left in the ground. Any piece of root left in the soil may produce a new plant. Hoeing is effective when plants are small. This method either cuts off the tops or exposes seedlings to the drying action of the sun. A claw-mattock is effective in pulling out large plants and their root systems.

Cutting of above-ground plant parts is only marginally effective, but it is a useful technique to prepare for other removal methods. Repeated cuttings may help to exhaust the reserve food supply in roots. Cutting is most effective when plants begin to flower. At this stage the reserve food supply is nearly exhausted, and new seeds have not yet been produced. After cutting or chopping, gorse will resprout in greater density if not treated with herbicides.

In 1983 the San Mateo County Department of Parks and Recreation manually removed dense gorse from San Bruno Mountain, a task requiring approximately 350 person-hours per acre. Gorse also was removed by chaining by bulldozers and with the use of a bulldozer-mounted rototiller. Herbicides were used as a follow-up treatment (Reid 1985).

Prescribed burning: Fire has frequently been used to eliminate gorse thickets, although burns may easily get out of control because of the high flammability of the plants. Fire may stimulate germination of buried seeds, so repeated burns may be necessary to exhaust the seedbank (Amme 1983).

Insects and fungi: There are no USDA approved insects for biocontrol of gorse. The gorse weevil (Apion ulicis) was accidentally introduced into the United States in 1953 from France, and by 1982 it had become established in California and Oregon. The weevil grub eats the seeds in the unopened legume. When the pods open, adult weevils are released to feed on spines and flowers, sometimes defoliating large plants. In California the weevil has been only partially successful in controlling gorse (Amme 1983). Plants often have enough food reserves to recover rapidly after serious injury. Additionally, the climate on the northern California coast is cool, delaying dehiscing of the pods and leaving the weevil larvae to die in the pod.

Other potential insect enemies of gorse exist but have not been tested for controlling gorse in the United States (Julien 1982).

Vegetative competition: Reseeding with native perennials after initial burning or chemical treatment of stumps may be productive. Once established, these species may displace gorse by competing for water or nutrients or by shading out lower-growing gorse plants. Amme (1983) has experimented with reseeding with native grasses in Jughandle State Reserve in Mendocino County.

Grazing: Goat grazing is effective in controlling gorse (Hartley et al. 1980, Hill 1955), as goats prefer woody vegetation to most grasses and herbaceous plants. Goats are less costly to use than mechanical and chemical methods. They are most cost-effective when used to clear or suppress young regrowth rather than to do the initial clearing of mature stands. A period of at least two years of goat grazing is required before there is any significant reduction in gorse (Hill 1955).

Chemical control of gorse has been well researched in New Zealand. The most effective chemical treatment was a combination of picloram (as Tordon®), which is not registered for use in California, and 2,4,5-T® which is not legal anywhere in the U.S. (Ivens 1979). Good results were obtained with picloram applied during summer months. Larger plants needed retreatment, and burned stumps showed a high degree of recovery (Ivens 1979). Both chemicals have distinct disadvantages, including persistence in soils, difficulty in being leached out of organic and clay soils, and damage to other plant species. Check with your county agricultural agent or a certified pesticide applicator to determine which herbicides are currently registered for use on gorse in California.

Gorse is difficult to eradicate with a single application of herbicide (Balneaves 1980). Greater success is possible with a combination of methods, including crushing, cutting, or burning.

Glyphosate (as Roundup®) is most effective with gorse seedlings in early summer. Plants began to die the following fall and winter as herbicide was carried to the roots.