Wait four to six months from the initial time of planting the slips, or newly germinated, red yams. Since yams do best when they are planted in the spring, wait until early autumn before looking for full-grown vegetables.
Look at the stalks that have sprouted above the soil. The stalks of ripened red yams should be thick and stiff.
Loosen the soil below the stalks and gently lift the red yams out of the soil. If there are about two or three yams at the root of the stalk, they are ready to be harvested. Use a rounded spoon to dig out the yams so as not to damage the vegetables.