Marsha Hanzi in Brazil sent the following about the sweet potato. “Our mentor / teacher / friend Ernst Gotsch says that sweet potato produces tubers when it is STRESSED. That is why in intercropping it produces more leaves and fewer tubers [Ed: because it is less stressed under the canopy.] The same is true in agroforestry systems where it can produce abundant leaves and be very vigorous, producing very few tubers indeed.
"To get more tubers in this situation, Ernst ‘stresses’ the plant, winding a long shoot around on itself (he calls it 'making a wig’), whereby it produces one large tuber instead of several tiny ones.”
Dr. Frank Martin told us of a custom in Paraguay where farmers would pull thorn trees over sweet potato patches to damage the leaves, with the goal of stressing the plant and thus increasing the yield of tubers.