If you live in a warm climate, the good news is that the purple yam (Dioscorea alata) will have enough warm days to produce its famous large, purple-fleshed tubers. The bad news is that purple yam can become an annoying weed if you don't hem it in. If you live in U.S. Department of Agriculture plant hardiness zones 9 to 12, take the good -- plentiful harvests of hefty yams, along with ornamental foliage -- while minimizing the bad -- that pesky invasive potential -- by growing it in large pots and removing the small "air spuds" that act as self-seeders.
1 Plant a sprouted tuber slip in a large container set in front of a sturdy trellis or other support structure. If desired, set additional containers at 10-foot intervals along the support structure for additional yam harvests.
2 Water the yams at least once a week -- and check soil moisture between watering sessions. The top 2 to 3 inches should be moist to the touch. Potted soil dries out more frequently than garden bed soil.
3 Feed plants two to four weeks after planting. Apply a 1-inch-high band of balanced fertilizer, such as 30-30-30, in a narrow ring that is 6 inches from the base of each yam plant.
4 Prune vines if they threaten to outgrow their support structure.
5 Feed purple yam plants a second time when they are 3 or 4 months old. Apply a 1-inch-high band of balanced fertilizer, such as 30-30-30, in a ring about 6 inches from the base of each yam plant.
6 Monitor plants for aerial tubers, or bulbils, growing from the vine's leaf axis. These tiny tubers, once fallen, are the main way in which the vines can reproduce and become a nuisance.
7 Remove and dispose of bulbils them as you find them in order to decrease the incidence of the bulbils falling onto the ground and potentially rooting.
8 Dig up tubers when the foliage dies back, which occurs when the day length shortens 8 to 10 months after planting, depending on the variety.
9 Reserve one tuber per container and re-bury it to produce a new crop of purple yams for next year's harvest, if desired.