all depends on variety, but essentially once u have 1/2"-3/4" of shoots you are good to go. A word of caution, with regard to potato planters, if the temp gets down to -2 or more, it really can set them back, even if you fill the planter right to the top from the start. A sheltered place surrounded by other pots is the way I go about it, in the ground, as long as they are earthed up, should be fine.