One of my clients has a melianthus, they do need to be in well drained soil, we have this one in a semi shade situation and it seems to thrive, they can get really big but once flowering is over you can cut back, by doing so new growth is produced, and it keeps the plant looking tidy.

Advice is to treat as a perennial rather than shrub in Britain - it will die back as winter approaches. Should be in poor, well drained soil, preferably in full sun, in a very sheltered spot. Protect the crown in winter by placing something over to keep it more sheltered - they're not entirely hardy here. Only flowers in very hot summers.