This genus of brooms, on first acquaintance, appears quite out of keeping with New Zealand vegetation; yet species are found in many types of habitat throughout the country. Thirty-eight species are in fact endemic, and there is only one other species in the genus, C. exsul, which is found in Lord Howe Island.
The plants are mostly broom-like shrubs which are almost leafless. When leaves are present, they occur on young plants or on plants growing in the shade, and are small and insignificant. Stems on the other hand are mostly flattened, green and, in some species, up to half an inch wide; they therefore take the place of leaves. Habits of the species differ greatly. Some are bushy plants, some mat plants, while others are almost tree-like with bushy heads. Flowers are mostly small, purplish, are single or borne in simple or branched racemes and, since the genus belongs to the Papilionaceae, they form pods. These present some of the main diagnostic features separating the species, many of which are difficult to identify, a difficulty that is added to by possible hybridisation.
C. arborea is the tallest growing species reaching heights of 15 ft. It has narrow compressed branchlets about one-tenth of an inch across. Confined to the South Island, it occurs in shrubland or along forest margins mainly west of the divide. A species with showy flowers and half-inch-wide branchlets is C. williamsii, confined naturally to a few spots in the North Island but now widely cultivated. It is an attractive garden plant. Several species are found in forest openings along stream and river banks or lake shores. Some occur widely, while others have local distributions only. Thus C. aligera is principally a North Auckland species found along forest margins. C. sylvatica is known only from the Waipoua River in Waipoua Forest. Five peculiar species are almost mat-forming plants growing in dry situations. Four of these occur on dry river terraces and in tussock grassland east of the South Island Main Divide, while the fifth, C. orbiculata, grows on the Volcanic Plateau of the North Island.
Three closely related genera of native brooms are Corallospartium, Notospartium, and Chordospartium. Corallospartium crassicaule is known as the coral broom because of the deeply grooved branchlets, while N. torulosum or pink broom is cultivated for its prolific flowering habit.
by Alec Lindsay Poole, M.SC., B.FOR.SC., F.R.S.N.Z., Director-General of Forests, Wellington.