Answer: According to the American Horticultural Society's book, "Plant Propagation," the best way to propagate a mandevilla by rooting is to use softwood and greenwood cuttings in early summer with a bottom heat of 68-77 degrees F.
Softwood cuttings are of new growth that is not yet firm. They should be about two inches long, with two-three pairs of leaves at the top of the stem. Insert the cuttings into the planting medium, just up to the lowest leaves. Rooting hormone may be used, but is not essential. They also recommend pinching off the growing tip, which helps force more roots. If possible, place the cuttings in a covered environment (a mini greenhouse) and provide bottom heat. Greenwood cuttings are handled the same say but the new growth is starting to firm. It is sometimes easier to handle. I hope this info helps--good luck!
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