Article from Bean's Trees and Shrubs Hardy in the British Isles
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'Clematis × durandii' from the website Trees and Shrubs Online (treesandshrubsonline.
A robust climber, growing 9 ft high, with stout stems. Leaves undivided, ovate, pointed, tapering or slightly heart-shaped at the base; 3 to 6 in. long, about half as wide, with three or five longitudinal veins, firm-textured, shining green, almost or quite glabrous; stalk 1 to 2 in. long. Flowers 3 to 41⁄2 in. across, the sepals usually four (occasionally more), obovate, 11⁄2 in. wide, wavy at the margins, dark blue-violet; stamens yellow. Seed-vessels with long silky tails.
A beautiful hybrid between C. integrifolia and (probably) C. × jackmanii, raised by Durand Frères of Lyons about 1870.