Browse our selection of maincrop seed potatoes. Buy from Ireland's leading supplier of seed potatoes. What is a maincrop seed potato? Maincrop potato varieties produce larger spuds for baking and roasting. For best results, 'chit' the tubers and plant up from March to mid-May. The potatoes will be ready to harvest approximately 20 weeks after planting. Maincrop potatoes are best grown in the ground, where they will have more space. 'Sarpo' potatoes offer exceptional resistance to blight, a common problem in potatoes.