Article from Bean's Trees and Shrubs Hardy in the British Isles
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'Clematis ligusticifolia' from the website Trees and Shrubs Online (treesandshrubsonline.
This little-known species, climbing to 20 ft, is a native of western N. America, and in the American floras is grouped with C. virginiana. It is easily distinguished from that species (which has trifoliolate leaves) by its five leaflets, the terminal one of which is deeply three-lobed or occasionally three-foliolate. It is, perhaps, more likely to be confounded with our native C. vitalba in foliage, but the plant is much less vigorous, and does not flower freely with us. Moreover, male and female flowers are confined to separate plants. They are white, fragrant, 3⁄4 in. across, and produced in corymbose panicles, the sepals downy. Leaflets ovate, pointed, coarsely toothed, of firm texture, bright green and glabrous. Seed-vessels with feathery tails. Flowers in September.
var. californica S. Wats., has leaves downy or even velvety beneath.