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A deciduous shrub or small tree 20 to 40 ft high, with a trunk 12 in. or more thick; branchlets downy. Leaves pinnate, 6 to 12 in. long, with downy stalks; leaflets fifteen to twenty-five, oval to slightly ovate, 1 to 13⁄4 in. long, 1⁄2 to 2⁄3 in. wide, with a bristle-like tip; stipules spiny, ultimately 1 in. long. Racemes 2 to 3 in. long, 2 in. wide, the stalk covered with brown shaggy hairs. Flowers 3⁄4 to 1 in. long, pale rose, each on a hairy stalk 1⁄4 in. long; the standard petal large, the calyx glandular, shaggy, with slender teeth. Pods 3 or 4 in. long, 1⁄3 in. wide, covered with gland-tipped bristles 1⁄8 in. or more long. Bot. Mag., t. 7726.
Native of Colorado, New Mexico, Arizona, and S. Utah, in places at 7,000 ft above sea-level. First discovered by Dr Thurber in 1851; introduced to Kew in 1887. It flowers prettily every year in June, and frequently a second time in August. It differs from R. pseudacacia in its bristly pods, and from R. viscosa in the young twigs not being viscid. The larger of two examples at Kew measures 50 × 6 ft (1968).
R. luxurians was at one time confused with R. neomexicana A. Gray, a related species from New Mexico, which makes a shrub to about 6 ft high; its leaves have not more than fifteen leaflets and its pods are hairy but not glandular-bristly.
The specimen at Kew, probably from the original introduction of 1887, measures 70 × 51⁄4 ft (1981). In the University Botanic Garden, Cambridge, pl. 1958, it is 48 × 33⁄4 ft (1982).
R. × holdtii – Mr Bean’s statement that this hybrid grows as vigorously as R. pseudoacacia ought to have been amended. Two specimens in the Kew collection, over eighty years planted, measure only 48 × 33⁄4 ft and 52 × 23⁄4 ft (1981). The cultivar ‘Britzensis’, however, planted at Borde Hill, Sussex, in the 1930s, has reached 62 × 41⁄4 ft (1984).