A tree 40 to 60, sometimes 80 ft high; branches long, slender; the young parts so flexible that they can be bent double without breaking; young shoots shining green, perfectly glabrous or with minute brownish down. Leaves in fives, persisting for about seven years, often pointing forwards, or the youngest ones even appressed to the branchlet, 21⁄2 to 31⁄2 in. long, triangular in section, all three sides marked with three or four white lines of stomata; margins quite entire, apex finely pointed; leaf-sheaths 1⁄2 to 5⁄8 in. long, soon falling away. Cones 3 to 5 in. long, 11⁄2 in. thick before the scales open. Bot. Mag., t. 8467.
Native of the Rocky Mountains from Alberta and British Columbia southward to S. California, Arizona, and New Mexico; it was introduced to the Harvard Botanic Garden, Boston, by Dr Parry in 1861, from Colorado, and must have reached Britain soon after, since the trees at Kew came from Dickson and Turnbull of Edinburgh in 1871-2. The reputed introduction by Jeffrey in 1851 was really of P. albicaulis.
P. flexilis is easily distinguished from all the other five-needled pines, except P. albicaulis, by the absence of teeth on the leaf-edges in combination with the deciduous leaf-sheaths and the glabrous or very finely downy young shoots. From P. albicaulis it is most reliably distinguished by its cones, which are longer (sometimes 10 in. long on wild trees), shed their seeds as soon as they are ripe, and soon fall from the tree.
A tree at Kew, by the Isleworth Gate, pl. 1872, measures 54 × 43⁄4 ft (1970). There are smaller examples in the Edinburgh and Cambridge Botanic Gardens and in the National Pinetum at Bedgebury.