If frost is likely to be a problem (and can be more of a problem if you plant
into pots on a patio) you can plant in slightly oversized pots. Line the
bottom of the pot with smallish pieces of old polystyrene. Add some earth, and
as you build earth up in the pot, add more polystyrene up the sides of the pot.
This will provice insulation and will limit the effects of most frosts.

Hope you find this helpful.

Dave.