Answer: Mandevillas are propagated by stem cuttings, not by the tubers. Tubers are not considered reproductive parts of a mandevilla. I suspect that they are storage organs that act as reserves for the plant. Similar tubers on plants such as the South African Asparagus densiflorus store both starches and water, imparting drought resistance to ride out long dry seasons in habitat. This enables the plant to be successful in coping with wildly eratic seasonal rainfall from year to year, and is a characteristic of many plants from seasonally dry subtropical climates. As Mandevilla also comes from parts of Brazil with seasonal drought, it may serve a similar purpose.
If you want to propagate mandevilla, stem cuttings may be taken in early spring. Choose side shoots, 2 or 3 inches long, and insert in sandy peat. They may be kept in a closed propagating case until roots form. The rooted cuttings should be potted individually in 3-inch pots. After a few days, pinch the tips of the main shoots to encourage side shoots to form. Once the side shoots grow, transfer them to 5-inch pots. They may be planted outdoors when they are well-rooted in these containers.
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