Spring when it is first emerging or, in warmer climates, early fall.

when its not in bloom.. or about to bloom ...

or july and august.. if you dont want it to suffer too much ... and not in drought ...

and not on the third thursday of any month ... thats just bad juju ... lol ...

ken

Which one? I've never had one of newer ones come back after winter in zone 6a, with or without division.

The threadleaf "Zagreb " was divided last fall and came back. I dug up more around May 1 and water regularly and it is doing fine. If you don't want to baby a plant don't divide when the weather is hot.