Answer: You can leave the tops alone until frost kill the stems, then remove any browned portions. Next spring, trim them back again. Campanulas are very cold hardy, container plants are always more susceptible to winter damage because there is less insulating soil around their roots and crowns. If you have a place to plant them right in the soil, that is the best option. If you opt to keep them in the planters, shelter them from wind and insulate the roots by wrapping the sides of the containers or settling the pots into the ground. Once the ground freezes you could also lay some evergreen boughs or similar light and fluffy mulch over top of them to prevent damage from freeze/thaw action in the soil. Snow cover provides even better protection. Best of luck!
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