A hybrid between P. fraxinifolia and P. stenoptera, raised in the Arnold Arboretum, near Boston, Mass., from seeds received in 1879 from Lavallée’s collection of Segrez, where the cross had no doubt been effected by wind on trees growing together. As the seeds were received as P. stenoptera, that species was no doubt the mother plant. I saw the original hybrid in the Arnold Arboretum in June 1910, which was then 40 ft high and, owing to the faculty of producing sucker-growths from the root, forming by itself quite a grove. At least one of the parent species has the same faculty – rarely developed, however, unless the main stem is cut down. P. × rehderiana is intermediate between the parents. The common-stalk of the leaf has wings, but they are not so much developed as in P. stenoptera, and never toothed as they often are in that species. The wings of the fruit are shorter and rounder. In the Arnold Arboretum this hybrid has proved hardier and a better grower than either of its parents. Living plants were introduced to Kew in 1908 and have grown extremely well. One of them measures 72 × 91⁄2 ft (1972).
Other examples are: Westonbirt, Glos., 66 × 11 ft (1972); Wakehurst Place, Sussex, 48 × 71⁄4 ft (1968); Borde Hill, Sussex, 62 × 73⁄4 ft (1971). A young tree at Kew, seventeen years planted, measures 52 × 53⁄4 ft (1970).
At Borde Hill there are also some spontaneous forms of P. × rehderiana, raised in the 1920s from seed of P. fraxinifolia. The presumed seed-parent grows a few yards from two specimens of P. stenoptera, both of which flower in most years.