Answer: I suspect the seeds were not viable, due to lack of pollination. You can propagate Mandevilla by root division, stem cuttings or seed. If your vine is flowering, and there are insects around, the flowers should produce viable seed. If the plant is growing indoors, you can transfer pollen from flower to flower with a small paintbrush. Mark the flowers by loosely tying a piece of bright yarn on the vine (so you don't forget where they were) and then watch and wait until the petals fall and the swollen area at the base of the flower turns a tan color. This indicates the seeds are ripe and ready to harvest. Mandevilla seeds should be sown on the surface of moistened seed starting mix in individual peat pots. Just barely press the seeds into the starting mix and place the pots in a warm area. Seedlings should sprout in 14-30 days.
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