SOME GARDENERS SWEAR that they grow great potatoes in old trash cans-either plastic or galvanized metal. Be sure to punch some drainage holes in the bottom. (A barrel, a wooden box, or any large container at least 18 inches deep with good drainage will work.) Put a layer of soil about six inches deep into the bottom. Place the seed potatoes inside, then cover with three inches of mulch or soil. When leaves appear and plants are six to eight inches high, add another layer of soil and continue hilling them up until they reach the top. To harvest, simply turn the can onto its side and shake out the crop!