They may not be hardy in your area, take them out in a month or so and plant them out in a herbacious border and plant the containers up with winter pansies etc.

I've had some success but last winter they all died

I put a few in a large pot last winter, and had it in a frost-free greenhouse, where they flowered some more, had a rest and then flowered again in the summer. At least they were cheeful companions out there!

They're only hardy down to about -9 deg. C, and they don't like snow.