Anytime it's not freezing or about to freeze. Leave the stems on, though you can shorten them a bit to make things easier.You need a sharp spade and just get in there and cut out a chunk of whatever size you want. Gather up the stems in a bunch and lift and slide the corms onto an old compost bag or similar to aid transportation. Replant at the same depth and only water if you are somewhere really dry. It's as tough as old boots and mine grows really fast. It has already been divided several times over the years and needs doing again. We've had snow and ice since New Year's Day. It finally began to thaw onTuesday and I cut it all back in readiness, we've had 2 days of grass and lots of rain and now we're back to snow and ice. Heigh-ho, that's gardening for you!The biggest clump will be divided into 3, but the smaller ones are just being split once. A lot will have to get thrown out as I can only make space for 2 more clumps before it becomes a Lucifer garden with no room for anything else!Mine always flops and tries to smother everything round it. It needs something quite substantial to keep it upright and it is hard to conceal that in the border. The height and the colour make it ummissable, but as a plant I prefer Emberglow, shorter, but a lovely deep red and it has thicker stems which support it better. I think my favourite though is Carmin Brilliant, quite short and dainty and with really pretty flowers of pinky crimson with yellow centres.