Well i would say as soon as the ground warms up a bit ..... March/April here but i can see you're a lot further north than me so i'd wait to see what some other members, from a region near you, say.

thank you for the ansewers to my plastic spuds
stoker

depending on the variety and your local weather I would say like louise, as soon as the soil warms up. if it is an early variety as soon as poss. if a later one then wait awhile until you can see growth.

thanks