I just give mine a prune back to a smaller neater mound, cutting off all the growth hanging over the path/wall. It usually grows it all back again next year.

Best done after the first flush is over roundabout June/July.

Thanks both:-) If I had cut it back in July after main flush, would it have flowered again? Is it too late to cut it back now? I'm so impressed with it I've ordered two more helianthemums, georgeham and sulphureum plenum, hope they're as good:-)

Mine hasn't so far.

Yes, it would have flowered again - its quite difficult to time the haircut if you don't keep a watch - miss it and loads more flowers arrive and it just keeps getting bigger. I'm slightly worried about cutting it hard now in case we get an early, arctic blast again this year - prior to last winter, I'd have said yes, do it now, but think I'd go for a light trim and then cut it hard next year at the right time (think I usually do it late June, early July), just in case, but its up to you.

Thanks again both, mine will have a light trim then just in case:-))