Gorse [Ulex europaeus L.][ULEEU][CalEPPC: A-1][CDFA list: B] Photographs Map of Distribution Biocontrol

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SYNONYMS: prickly or thorn broom, furze, whin

GENERAL DESCRIPTION: Spiny evergreen shrub with yellow pea-like flowers, to 3.5 m tall. Gorse typically forms dense impenetrable thickets that exclude desirable vegetation and increase fire hazard risks. Mature plants contain ~2-4 % flammable oils. Older shrubs develop a central mass of dead material and are particularly flammable. Abundant leaf litter acidifies upper soil layers. Introduced from Western Europe as an ornamental or hedge shrub. The gorse seed weevil (Apion ulicis) and spider mite (Tetranychus lintearius) are introduced biocontrol agents currently established in California. The seed weevil reduces seed production, but cannot kill established stands. Heavy mite infestations can kill branches and are apparent by the dense webbing that covers the foliage.

SEEDLINGS: Cotyledons sessile, oblong, 5-7 mm long, bases slightly tapered, tip rounded, leathery, glabrous. Stalk below cotyledons (hypocotyl) +/- woody, glabrous. First leaves mostly simple, alternate, sessile, +/- lanceolate, 2-5 mm long, tips acute, leathery, covered with stiff unicellular hairs. Subsequent leaves usually compound with (2-)3 leaflets, ~ 5-10 mm long, otherwise similar to first leaves. Juvenile plants typically lack lateral branches and have compound leaves with 3 leaflets.

MATURE PLANT: Stems highly branched, interwoven, stiff, spreading, longitudinally ridged. New stems green, covered with hairs. Branchlets terminate as thorns. Leaves alternate. Unlike juveniles, mature shrubs have simple leaves modified into stiff, curved awl-like spines, 5-30 mm long. Stipules lacking.

ROOTS and UNDERGROUND STRUCTURES: Taproot typically poorly developed. Lateral roots extensive, branched, mostly shallow, but a few penetrate the soil more deeply. Lower branches resting on soil may develop adventitious roots at the base. Roots associated with nitrogen-fixing bacteria.

FLOWERS: January-July. Flowers axillary, solitary or in few-flowered clusters. Petals yellow, 15-20 mm long, persistent. Calyx (sepals as a unit) yellow, membranous, deeply 2-lipped, 10-15 mm long, upper lip 2-toothed, lower lip 3-toothed, covered with short hairs. Stamen filaments fused into a tube.

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FRUITS and SEEDS: Pods ovoid to oblong, 1-2 cm long, slightly flattened, dark brown, covered with tawny spreading wavy hairs, partially enclosed by the calyx. Seeds 2-6, shiny green to brown, smooth, +/- triangular, flattened, ~ 3 mm long, with a straw-colored appendage.

HABITAT: Disturbed sites, sand dunes, fields, pastures, logged areas, burned sites, particularly where winters are mild and some moisture is available year round. Frost-damaged plants can resprout from the crown. Does not survive severe cold winter or arid climates. Grows best on acidic (pH 4.5-5.0) soil. Tolerates most soil types, including serpentine, but seldom grows in high calcium (calcareous) soils.

DISTRIBUTION: North Coast, western North Coast Ranges, northern and central Sierra Nevada foothills, San Francisco Bay region, eastern South Coast Ranges, South Coast, Peninsular Ranges; to Washington. To 400 m (1300 ft).

PROPAGATION/PHENOLOGY: Reproduces by seed. Plants produce abundant quantities of seed. Most seeds are ejected to within 5 m of the parent shrub when pods snap open at maturity. Some seeds may disperse to greater distances with human activities, water, soil movement, animals, and ants. Seeds are hard-coated and can survive for more than 30 years under field conditions. Large soil seed banks often accumulate. Scarification or heating often stimulates germination. Optimal temperature for germination is between 15-19º C. Germination can occur year round when conditions are favorable. Shrubs may live for up to ~ 30 years.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL: Plants cut or burned to ground level may resprout from the crown. Burning often stimulates a flush of seedling germination after the first rain. Browsing and trampling by goats can greatly reduce seedling establishment and crown re-growth.

SIMILAR SPECIES: Unlike gorse, the brooms (Cytisus, Genista, Spartium, Retama species) lack spiny foliage.

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