Theory behind earthing up potatoes: (By Tim Keates - edited) The more soil you have above the seed potato the bigger the crop. Some market gardeners earth up twice in the growing year. Some people use grass clippings, others shredded paper, or cardboard. The idea is to exclude light so your tatties are not exposed to daylight and turn green. The earth (or other material) holds in moisture and the plant grows unchecked by hot weather. The more water, the bigger the crop.