Vegetable · Dioscoreaceae
Yam
Dioscorea
Give it heat, a tall trellis, and a long season, and the yam rewards you with heavy, storable tubers.
Full sun
Every 5–7 days
68–86°F · 20–30°C
240–330 days
Overview
The true yam is a twining tropical vine grown for its starchy underground tubers, and it has fed people across Africa, Asia, and the Caribbean for thousands of years. It belongs to the genus Dioscorea, a group unrelated to the sweet potato that often borrows its name. West and Central Africa grow the great majority of the world's yams, much of it the white Guinea yam.
These are big, hungry plants. The vines climb 7 feet and can reach far higher on a good support, so a tall, sturdy trellis goes in at planting time. Below ground the tubers can grow enormous, with the winged yam occasionally pushing past 100 pounds, though most kitchen yams are far smaller and easier to handle.
Yams need warmth and patience. They grow as a warm-season crop through a long season of 8 to 11 months, so they suit frost-free climates or a head start indoors where the season is short. Cooked, the tuber is a wholesome staple, but it must never be eaten raw.
Care
The short version: full sun, warm ground, and a tall trellis set at planting. Keep the deep, loose, well-drained soil evenly moist all season, feed monthly with plenty of potassium, then ease off the water as the vines yellow ahead of harvest.
Light
Yams crop best in full sun, which fuels the long climb and the tubers swelling below. They will take some partial shade, but expect lighter growth and a smaller harvest where the light runs short.
Water
Keep the soil evenly moist through the long growing season while the vines and tubers bulk up, watering roughly every 5 to 7 days. Ease off as the foliage yellows ahead of harvest, and never let the ground stay waterlogged, which rots the tubers.
Soil
Plant in deep, loose, well-drained sandy loam rich in organic matter so the tubers can run without forking. A slightly acid pH of 5.5 to 6.5 suits them, and raised mounds or ridges help drainage on heavier ground.
Temperature
These are tropical plants that thrive between 68°F and 86°F (20°C to 30°C). They have no tolerance for frost, so the soil must be warm before setts go in.
Humidity
Yams like it humid, around 60% to 90%, in keeping with their tropical roots. Pair that moisture with airy spacing and good trellising, since damp, crowded foliage is what invites anthracnose.
Feeding
Feed monthly through the growing season with a balanced fertilizer high in potassium, which is what drives strong tuber development. Steady feeding over the long season matters more than any single heavy dose.
Pruning
There is little to prune. Train the twining vines onto a tall, sturdy trellis or stakes set at planting, then just remove dead or diseased foliage and keep the vines up off the ground.
Size & spacing
The vines climb from 7 to 30 feet (2 to 9 m) on their support, while each plant spreads about 2 to 4 feet (60 to 120 cm) at the base. Give them that room so air moves freely and disease has less of a foothold.
When to plant
Yams run on heat and a long calendar. Where summers are short, pre-sprout the setts indoors a few weeks ahead, then plant them out only once frost has passed and the soil has warmed. The tubers are ready 8 to 11 months later, when the vines yellow and die back.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: In cool climates, pre-sprout seed-tuber pieces (setts) in warm, shaded conditions about 4-6 weeks before planting out.
- Plant: Plant setts outdoors after all danger of frost once soil is warm (roughly mid-to-late spring); install a tall trellis or stakes at planting time.
- Harvest: Harvest tubers 8-11 months after planting, when the vines yellow and die back.
Hardiness
USDA hardiness zones: 9–11
Temperature: down to -7°C
Troubleshooting
Most yam trouble shows up either on the foliage during humid weather or in the tubers at harvest. Catch the leaf diseases early and fix the soil before you plant, and you head off the worst of it. Match what you see below, then act while it still helps.
Dark, blighted spots spreading across the leaves
This is yam anthracnose, the worst foliar disease of water yam, and it takes hold when warm weather and long leaf wetness meet a crowded canopy. The dark lesions climb from the lower leaves upward and can run into the stems. Strip off the badly hit foliage, open the vines up with wider spacing and tidy trellising, and lean on tolerant cultivars and clean planting material to stay ahead of it.
Fix:
- Remove and destroy badly infected foliage
- Improve airflow by spacing and tidy trellising
- Plant tolerant cultivars and apply an appropriate fungicide where warranted
Small, forked or rotten tubers when you lift the crop
Misshapen and rotting tubers usually mean wet or compacted ground, sometimes with root-knot nematodes chewing on the developing roots. Heavy, soggy soil rots the flesh while stony or shallow soil forks and distorts it. Grow in deep, loose, well-drained mounds, harvest as soon as the vines die back, and cure the tubers somewhere dry and airy before storing.
Fix:
- Harvest promptly once vines die back and cure tubers in a dry, airy place
- Discard rotted tubers
- Rotate out of infested ground and improve drainage before replanting
Chewed, holed leaves with pests clustered on the vine
Ragged foliage points to leaf-feeding beetles, while sticky clusters in the leaf axils mean mealybugs or soft scale drinking the sap. Check the leaf undersides and stem joints to tell which one you have. Hose off or hand-pick light cases, treat stubborn ones with insecticidal soap, and keep the plants vigorous so they shrug off the damage.
Fix:
- Hand-pick or hose off light infestations
- Treat persistent outbreaks with insecticidal soap or an appropriate insecticide
- Encourage natural predators
Toxicity
Cooked yam is a safe, everyday food, but the raw tuber is not. Raw flesh holds bitter saponins and skin-irritating crystals, and wild or bitter species carry the toxic alkaloid dioscorine. Cook every yam before eating, keep raw tubers and trimmings away from pets and children, and wash your hands after handling cut raw flesh.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Caution
A raw tuber or a mouthful of vine can leave a cat drooling and off its food for a while. Cooked yam in small amounts is the only part worth offering.
Dogs Caution
Chewing the raw tuber or foliage may bring on mild drooling and an upset stomach, since the raw flesh carries bitter saponins and irritant crystals.
Horses Caution
The raw tuber and the vines are best kept out of reach. A horse that grazes them can end up with mild digestive upset.
Rabbits Caution
Raw yam is too sharp and saponin-rich for a rabbit's gut. Stick to its usual greens and leave the tubers and foliage out of the hutch.
Livestock Caution
Cattle, goats and sheep may nibble the foliage or stray raw tubers, which can cause a passing stomach upset, especially with bitter wild types.
Birds Caution
Backyard poultry can react to raw tuber scraps and trimmings, so keep peelings out of the run and cook anything you share.
People Cooked tuber safe
Cooked yam is a wholesome staple eaten the world over. Never eat the tuber raw, leach and cook bitter or wild types well, and wash hands after handling cut raw flesh, which can irritate skin.
Always cook yams before eating and never eat the raw tuber. Wild and bitter yams contain dioscorine and may need soaking/leaching plus long cooking to be safe. Handling cut raw tubers can irritate skin.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Yam covers a whole genus of edible species rather than a list of garden cultivars. These are the main ones you will run into.
Dioscorea alata (water / winged / greater yam)
The most widely grown yam in the world, recognized by its squarish winged stems and very large tubers that can run from white to purple inside.
Dioscorea rotundata (white Guinea yam)
The dominant food staple across West and Central Africa, the region that grows the great majority of the world's yams.
Dioscorea esculenta (lesser yam)
A spiny vine that carries smaller, tender tubers with a sweeter taste than the bigger yams.
Dioscorea bulbifera (air potato)
Grown mainly for the aerial bulbils on its vines, though many forms are bitter and toxic and it has turned invasive in the southeastern US.
How to propagate
Yams are grown from the tubers themselves, not from seed, which makes starting a new crop straightforward once you have clean planting stock.
Bulbs & cormsModerate
Spring, after frost danger has passed and soil is warm · 3-6 weeks to sprout
Yams grow from setts, which are pieces of seed tuber or small whole tubers of roughly 100 to 300 grams, set into warm soil in spring once frost has passed. Pre-sprouting them in warm, shaded conditions first gets them going faster, and they take about 3 to 6 weeks to sprout.
Bulbs & cormsModerate
Spring · 3-6 weeks to sprout
Some species such as D. alata and D. bulbifera also make aerial bulbils in the leaf axils. Collect these and plant them in spring like small tubers, and they sprout on the same 3 to 6 week timeline.
Myths
Yams and sweet potatoes are the same thing.
True yams belong to the genus Dioscorea in the family Dioscoreaceae and are not related to the sweet potato, which is Ipomoea batatas in the morning glory family. Most produce sold as "yam" in US supermarkets is really sweet potato.
You can eat a yam tuber raw like a carrot.
Raw yam holds bitter saponins and skin-irritating crystals, and wild or bitter species carry the toxic alkaloid dioscorine. Yams must be cooked first, and some wild types need soaking and leaching before they are safe to eat at all.