I cut mine back about a month ago and I already have new growth about four or five inches tall. Have you been able to get yours to bloom? The tag when I bought it said it should have blooms, but mine did not last year.

Got mine too late in the season last year to hope for any flowers in our extremely mediocre summer weather :-) They need more heat than we often generate. I've decided to uncontainerize mine and plant in the ground this year so we'll see what happens. And I do intend to cut it back considering how much and how fast it grew after I got it in August - still plenty of time to regrow!! Thanks for confirming what I had been thinking.

Melianthus is a winter-growing plant in the wild, generally. It does not need heat and seems to grow faster when it's cool as you have observed. The best plant I have seen was on the Oregon Coast where the temperatures are consistently cooler all the time. To get it to bloom the stems need to reach a certain height perhaps 8 - 10' plus on average, though I think I have seen it bloom a little smaller than that occasionally.

Having said all that... I think you could cut it back and it would regrow just fine, but you would be giving up your chances for flowers this year and possibly in the future if next winter is colder. I wouldn't miss the flowers too much though; the foliage is the main event in my opinion.