I would take a few twigs about 20 cm long, and stand them in a glass of water, changing it every few days. After a few weeks, it will grow roots on the submerged parts, and the cuttings can be transferred to pots of seedling compost. When planting them, I would prune back the top to remove about 1/3 of the leaves--that balances the top and the new root system, which won't have root hairs to make water absorption more efficient.

Thank you very much for you answer, what a beautiful plant this is.

Isn't it though?! It's one of my favorites.