A deciduous shrub, 2 to 4 ft high, with round, erect, hairy branches. Leaves trifoliolate, with a main-stalk about 1⁄2 in. long; leaflets obovate or elliptical, very hairy beneath, ultimately glabrous above, 1⁄2 to 1 in. long. Flowers in a terminal cluster or umbel 2 in. across, each flower nearly 1 in. long, with bright yellow petals; calyx tubular, 1⁄2 in. long, very hairy. Pod 11⁄2 in. long, 1⁄4 in. wide, covered with shaggy, spreading hairs.
C. supinus is representative of a group of brooms that have their centre in the lands around the Black Sea and in the Danube basin but extend westward through much of Central and S. Europe as far as Spain; it is itself the most widespread of the group, being found over much of this area. All are characterised by a terminal umbel of flowers, a hairy, tubular calyx and hairy leaves and pods. C. austriacus and C. albus (leucanthus) also belong to this group and both have by some botanists been united with C. supinus into a large aggregate species with several subspecies and varieties. C. austriacus is very near to C. supinus but differs in having the hairs on the shoots and pods appressed; C. albus may be distinguished by its whitish or creamy flowers.
In the plant described, all the flowers are borne at the ends of the current season’s growth, but in some forms there is an additional flowering in spring on short axillary shoots from the previous year’s growth. Such plants, in their spring state, might be taken for C. hirsutus, but in that species all the flowers are borne in this way, while in C. supinus the spring flowering is only the prelude to the main flowering at the ends of the new growths.
C. supinus has some value in the garden, bearing its flowers in July and August and then intermittently until the frosts come. Whatever pruning is necessary should be done in spring before growth recommences, when the last-made shoots may, if desired, be cut back almost to the old wood.