Do it as soon as the pointy buds start to show. It makes it easier to cut up the roots and see that each clump has growth points. They get away quickly too as at this time of year they are full of the urge to grow. I did it in autumn once with a clump I decided was too big and they all sulked.
"We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
A chap at a nursery (not a garden centre) took a division off a big hosta (I forget its name) for me (in April) and popped it in a pot with some dry compost. I'll give it a good drink when I get home said I. No!, says he, don't do that for at least a week, it's got enough to cope with as it is.
I always water before and after splitting and re-potting as I believe it helps reduce damage to the finer roots but I do also make sure they can drain and not sit in a puddle.
"We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
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What do others think about this?