Where are the plants at present? surprised they are putting much growth on at this time.

I bought a collection of Penstemon as plugs this summer, too. I've potted them into 5 inch pots (gradually) and they are overwintering in an unheated cold frame. They are lovely plants and will go out in the garden in late spring.

thank-you. I have the indoors in an un-heated room on a windowsill. I understood they must be kept at 65f.

dreeny

I think this is too warm, and also why they are growing and need potting on.I think you would be better putting them somewhere cooler to slow them down as low light levels and heat will give spindly growth .They can be cut back in spring to promote bushy growth but if you intend to keep them indoors moving them up a pot size wont harm but these conditions arent ideal.

Its no wonder they are doing so well, you can trim penstemons to keep there shape, you can very easily make new plants from the cuttings as well. Did they come in a little enclosed container? I bought some like that a few years ago and I still use that to strike cuttings in, I fill each side with a sandy compost put the small cutting in the middle after dipping in rooting powder, close it , leave in the greenhouse (cold) over winter and pot on new plants in the spring.
Put your plants in a cold frame if you have one they will be fine or a cold greenhouse.