At the moment mine are in small 4" pots. they will stay in them until April only because I havent got the final place sorted. I do have some in the ground from last year and they have survived the winter. If you plan to put them in the soil wait until the ground is unfrozen.

They are pretty hardy - all our runners were potted up and left sitting outside over winter & are all ok. As SBG says, whenever the soil is unfrozen - anytime from now on to plant out once your ground is prepared.

If you want to leave them in pots (which quite a lot of people do!) then give them plenty of room as they have a very extensive root system. Feed them with tomato feed in summer.