Are dividing existing clumps ? if so they are very easy to do and now is a good time to put them in, younger plants put them in now, remember to give them a good start by adding some decent well rotted compost and add some mulch to a good depth round the younger ones, if you have younger plants then it may take a couple or so years for them to flower once they have bulked up.

if they are still in pots any time as long as the ground isn't frozen. I tend not to plant after November as the soil is cooling down and roots tend not to make much root growth.

I try to get all my herbaceous planted in september or october. This gives them a bit of time to establish whilst the soil's warm and damp before they die down.