Not a lot to do in winter. You can keep a bit on the dry side over winter. You can divide after 3 years or so, but I'm guessing yours aren't that old.

You might also like to dig in some well rotted manure in the vicinity of the plants. They like a rich, moist soil so anything else (compost, etc) which will add to the condition and help retain moisture over the warmer months will be appreciated. You may as well mulch when all the leaves have died off. I try to remember to put in some sort of marker before the leaves die off completely just so I know where not to dig.

Aren't they the most beautiful flowers? Have you seen Intrigue - a gorgeous mauve?

I'd use a good quality potting mix with perhaps added organic matter (like spent mushroom compost) and some Osmocote (or similar). Some water holding crystals probably wouldn't go astray either.

But I wouldn't repot until they start to come into growth in spring - if that's what they do up your way. I have very clear cut seasons and won't see any growth until the soils warms up again.

I'd still keep the pots on the dry side until new growth appears.

But perhaps you could check at a local nursery. As I said, I'm not an expert on pot culture.