The early/main/lates distinctions are on the length of time needed for tubers to form. Naturally potatoes are day length sensitive... they don't start making tubers until the nights start to shorten. Not a problem in Mexico but a real pig in N Europe as the frost can come early and destroy the tops... breeders have selected for earliness and therse give us the 1st and 2nd early groups etc.. Tim's answer was right but not very informative..
Pink Fir Apple are a late main, one of the very few still available and should be left in as long as possible..
;D
