This easily sprouts branches from lower down when cut back hard, which will also tend to encourage new shoots from the spreading roots.

I was attracted by your title 'Melianthus major care'. I planted one here 15 years ago between Douglas firs on a dry bank along the road. I have not done a thing to it, never even watered it since. Two years ago I harvested seed from it, and tried to grow it from seed with mixed results. Two of 9 seeds planted in a four inch pot germinated. When I picked out the seedlings I dumped the seed mix back into the five gallon container of seed mix. Last year I noticed one seedling(they are very distinctive)in a pot with other plants, which I picked out and grew it on. This year two more have shown up the same way. I still have some seed from 2009 which I will try different pretreatments on to see what is needed to have seed germinate in a predictable manor. Al

I plan to try the seeds sometime too, I actually am surprised there have been no volunteers yet. I started it in a pot because I was worried that it would spread too much, but in relatively dry soil it is very well behaved. I did a search for other comments on Melianthus, and the consensus seems to be that it should be cut back in late summer/fall when the seeds are mature.