If they are in pots they will be more vulnerable to cold so would be best brought in to an unheated conservatory or greenhouse. If in the ground they would probably be OK left out, although penstemons do vary slightly in hardiness, depending on the species. A specialist grower could supply more information. As a precaution, you could mulch the crowns with a thick layer of leafmould or compost over the winter.
Answered on October 12, 2020
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