Posted: 31/08/2014 at 09:50
Not a daft question at all. I'm trying to remember what I used to do because it is such a long, long, time since I grew tender plants (in a different garden, elsewhere) and my penstemons in my present garden were only planted this year.
I would use the pots only as a temporary measure if the weather turns very cold. Penstemons vary in temperature tolerance. I shall treat mine as if they cannot tolerate being frozen. While I keep my garden tidy to avoid slugs and disease, I do tend to allow any top growth to remain on tender plants as it acts like a protective mulch.
I use terracotta pots when I can as they afford more insulation than plastic and are more likely to stay in position. Also, terracotta pots tend to have only small holes in the centre which would allow some ventilation. The hole can be blocked in severe weather simply by placing a rock on top (this would also keep plastic pots in place). If you leave pots on in warm weather you might create a problem with mould, pests, and other disease.
It's not an exact science, I'm afraid. It's just something I once did to keep the chill off and my plants alive.