Biological Flora of Britain and Ireland: Cytisus scoparius*

Broom or Scotch Broom. Fabaceae. Cytisus scoparius (L.) Link (Spartium scoparium L., Sarothamnus scoparius (L.) W.D.J. Koch, S. bourgaei Boissier, S. oxyphyllus Boissier, Genista scoparia (L.) W.D.J. Koch, Genista scoparia (L.) Lam., Genista scoparia var. vulgaris Rouy nom. Illegit., Genista vulgaris Gray nom. illegit., Sarothamnus vulgaris Wimm. nom. illegit., Sarothamnus vulgaris var. scoparius (L.) Timbal-Lagrave, Cytisogenista scoparia (L.) Rothm.) is a multi-branched non-spiny shrub, typically up to 60 cm but reaching 200(–250) cm high, as wide as tall, branches numerous, erect, ascending or procumbent, twigs green, usually leafy, deeply 5-angled, pliable, glabrous after the first year, or sericeous when young. Leaves deciduous or semi-evergreen, alternate, usually 3-foliolate, but 1-foliolate and sessile on young twigs; stipules absent; leaves subsessile or petiolate to 7 mm, glabrous or hairy; leaflets 6–20 × 1.5–9 mm, green on the upper surface, paler beneath, narrowly elliptic-oblong to obovate, narrowed or cuneate at base, more or less acute apex, glabrous or with silky appressed hairs on both surfaces. Flowers axillary, zygomorphic, solitary or in pairs on 1-year-old stems, 15–20 mm; pedicels up to 10 mm, twice as long as calyx, slender, glabrous. Calyx 6–7 mm, green, 2-lipped, the lower lip with 3-min lobes, the upper lip with 2-min lobes, usually glabrous. Corolla 15–20 mm, golden-yellow or with dark red to mauve areas; standard 16–18(–20) mm, the limb ovate, emarginate at apex, rounded to a subcordate base, with a short, narrow claw; wings with oblong limb, rounded at apex, with short, very broad lobe at base and a narrow claw; keel with limb broadly oblong, slightly upturned at the rounded apex, with a short, broad lobe at the base and a short, narrow claw. Stamens 10, monadelphous, filaments pale, anthers alternately long and attached at or near the base, short and versatile. Single style, long, spirally coiled, stigma capitate. Fruit an oblong dehiscent legume 25–50(–70) × 8–10(–13) mm, oblong, strongly compressed, 2-valved, with long brown or white hairs on the sutures, otherwise glabrous; green when immature, dark brown or even black at maturity, seeds dispersed by explosive dehiscence, valves coiled after dehiscence, remaining on the parent plant. Seeds, 2.0–3.5 mm, ellipsoid, flattened, dark green to red-brown, hard water-impermeable seed coat with an elaiosome.

Cytisus contains around 30 species, mainly distributed in Spain and Portugal (Sell & Murrell, 2009). Currently, there are four recognized subspecies of Cytisus scoparius (Auvray & Malécot, 2013). Two of the subspecies are native to Britain and Ireland. Subsp. scoparius (L.) Link has erect or arching branches, 150–200(–250) cm tall, leaves and young twigs glabrous or sparsely sericeous. It is usually a calcifuge of heaths, dunes, sandy banks, open woodland, mountain slopes and rough ground and occurs through most of the species' range, including British and Ireland. Subsp. maritimus (Rouy) Heywood (Heywood, 1959) (Genista scoparia var. maritima Rouy, Sarothamnus scoparius subsp. maritimus (Rouy) Ulbr., S. scoparius subsp. prostratus (C. Bailey) Tutin) is procumbent ‘looking as though someone has sat on it’ (Sell & Murrell, 2009) with occasional erect or ascending branches from the centre of the plant, up to 40(–50) cm tall, leaves and young twigs densely, silky sericeous. It occurs on maritime cliffs exposed to wind but not to heavy salt-spray, occasionally on shingle, and occasionally on rocky outcrops up to a kilometre inland (Pearman, 2023; Stroh et al., 2023), in north-west Europe, including Britain and Ireland. Plants of this subspecies from Britain, Ireland and the Channel Islands retain their characteristics in cultivation although those with extreme morphology on shingle at Dungeness, east Kent, do not come true from seed (Stace, 2019). Some low-growing individuals of subsp. scoparius occur, but these lack the intensely prostrate habit of subsp. maritimus and are only phenotypically dwarfed (Stroh et al., 2023). The other two subspecies are subsp. reverchonii (Degen & Hervier) Rivas Goday and Rivas Mart., found across central and southeastern Spain; and subsp. insularis (S. Ortiz & Pulgar) (Auvray & Malécot, 2013) endemic to Island de Ons and Island Salvora in the north-west of the Iberian Peninsula (Ortiz et al., 2001).

A variant occurs in Jylland, Denmark, with ascending branches up to 50–60 cm tall with sericeous, 1-foliolate leaves on young twigs. It remains constant in cultivation and may deserve recognition as a further subspecies (Tutin et al., 1968). More than 60 varieties of C. scoparius used in horticulture have been described (Meyer, 2008).

In the following account, Cytisus scoparius and Broom are taken to refer to subsp. scoparius unless otherwise stated.

Native. A pioneer species on sandy acidic soils, on heaths, open woodland, railway banks, stony riversides and, particularly, on roadside banks and verges where it may often be planted, forming dense thickets in cooler areas. Introduced into many parts of the world where its prolific seeding and spread have led to it being often considered a noxious and invasive species.