Vegetable · Dioscoreaceae
Yam
Dioscorea
Give this twining vine heat, a tall trellis, and a long season, and it swells heavy, storable tubers below.
Full sun
Every 5–7 days
68–86°F · 20–30°C
240–330 days
Overview
The true yam climbs as a twining vine, its stems winding upward while heart-shaped leaves ride the length of it. Underground it fattens a starchy tuber, the part grown and eaten across Africa, Asia, and the Caribbean for thousands of years. It is a Dioscorea, a genus apart from the sweet potato that keeps borrowing its name. West and Central Africa grow the great majority of the world's yams, much of it white Guinea yam.
This is a large plant with real reach. The vines run up to 7 feet and press higher on a tall support, twining as they go. Below the soil the tubers thicken into heavy masses, and a winged yam now and then swells past 100 pounds, its flesh ranging from white to purple within. Most kitchen yams stay far smaller and easier to lift.
Heat and a long season shape everything about it. The vine runs green through a warm-season stretch of 8 to 11 months, then the foliage pales and yellows as the tubers finish below. Cooked, the tuber turns into a dense, starchy staple. Raw, it is never safe to eat.
Care
The short version: full sun, warm soil, and a tall trellis for the twining vines to climb from the start. Keep the deep, loose, well-drained soil evenly moist all season, feed monthly with plenty of potassium, and taper the water as the leaves yellow toward harvest.
Light
In full sun the vine stretches long and leafy, driving the tubers to swell fat below. Partial shade is tolerated, but the growth thins and the crop comes up smaller.
Water
Through the long season the soil stays evenly moist, watered about every 5 to 7 days as the vines lengthen and the tubers fill out. The watering tapers as the leaves yellow toward harvest. Standing wet ground turns the firm tuber flesh to rot.
Soil
The tubers drive down cleanly through a deep, loose, well-drained sandy loam dark with organic matter, running straight instead of forking against stones. A slightly acid pH of 5.5 to 6.5 suits them, and raised mounds or ridges keep heavy ground from staying sodden.
Temperature
This tropical climber holds its deep green between 68°F and 86°F (20°C to 30°C). Frost blackens it outright, so the soil must be warm to the touch before the setts go in.
Humidity
Yams thrive in humid air of 60% to 90%, the thick warmth their tropical home gives them. That damp needs open, airy trellising around the leaves, since crowded wet foliage is where anthracnose blackens them.
Feeding
A monthly balanced fertilizer high in potassium feeds the swelling tubers through the season. Steady feeding, rather than one heavy dose, keeps the underground flesh packing on weight.
Pruning
Little cutting is needed. The twining vines wind up a tall, sturdy trellis or stakes set at planting, and the work is just stripping dead or diseased leaves and lifting the vines off the ground.
Size & spacing
The vines climb a long 7 to 30 feet (2 to 9 m) up their support, while each plant spreads a broad 2 to 4 feet (60 to 120 cm) across the base. That open span lets air slip between the leaves and starves disease of a foothold.
When to plant
Yams move on heat and a long calendar. Where the summer is short, the setts sprout indoors in warm shade a few weeks early, then go out once frost has cleared and the soil warms. Some 8 to 11 months on, the leaves pale and die back, and the swollen tubers are ready to lift.
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
Yam trouble shows on two surfaces: the leaves in humid weather, and the tubers pulled at harvest. Catching the leaf disease early and setting up good soil beforehand stop most of it. Find the symptom below and act while it still turns.
Dark lesions blackening up the leaves
This is yam anthracnose, the harshest leaf disease of water yam, striking when warm wet air settles over a crowded canopy. Dark sunken lesions spread from the low leaves upward, their margins blighting brown, and the blackening runs on into the stems. Strip the worst foliage, spread the vines wide on clean trellis, and lean on tolerant cultivars and clean planting material.
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 rotted tubers at the lift
Twisted or rotting tubers usually come out of wet or compacted ground, sometimes with root-knot nematodes galling the roots. Soggy soil softens the flesh to rot, while stony, shallow ground forks and warps it. Raise deep, loose, well-drained mounds, lift the moment the vines die back, and cure the tubers somewhere dry and airy before they store.
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
Holed leaves and pests packed on the vine
Torn, holed leaves flag leaf-feeding beetles, while sticky knots tucked in the leaf axils are mealybugs or soft scale drawing the sap. Turn the leaves and check the stem joints to see which sits there. Hose off or pick the light cases, wet the stubborn ones with insecticidal soap, and keep the plant strong enough to outgrow the bites.
Fix:
- Hand-pick or hose off light infestations
- Treat persistent outbreaks with insecticidal soap or an appropriate insecticide
- Encourage natural predators
Toxicity
Cooked, the pale starchy flesh is safe, everyday food. Raw, it is not. The raw tuber holds bitter saponins and skin-irritating crystals, and wild or bitter species carry the toxic alkaloid dioscorine. Cook every yam, keep raw tubers and trimmings 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 bite of raw tuber or vine can leave a cat drooling and off its food for a while. Only a small amount of cooked yam is worth offering.
Dogs Caution
A dog that gnaws the raw tuber or foliage may end up drooling and sick to the stomach, from the bitter saponins and sharp crystals in the raw flesh.
Horses Caution
The raw tuber and the trailing vines belong out of reach. A grazed mouthful can leave a horse with mild digestive upset.
Rabbits Caution
Raw yam runs far too sharp and saponin-rich for a rabbit's gut. Its everyday greens are the safe feed, and tubers and foliage stay clear of the hutch.
Livestock Caution
Cattle, goats, and sheep may crop the foliage or a stray raw tuber, which can bring a passing stomach upset, sharper with the bitter wild types.
Birds Caution
Backyard poultry can react to raw tuber scraps and peelings, so keep trimmings out of the run and cook anything shared.
People Cooked tuber safe
Cooked yam is a dense, wholesome staple eaten the world over. The tuber is never eaten raw, bitter or wild types are leached and cooked through, and hands get washed after handling cut raw flesh, which stings the 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 spans a whole genus of edible species rather than a shelf of garden cultivars. These are the main ones you will meet.
Dioscorea alata (water / winged / greater yam)
The most widely grown yam anywhere, known at a glance by its squarish, winged stems and its very large tubers, white to purple inside.
Dioscorea rotundata (white Guinea yam)
The leading food staple of West and Central Africa, the region that grows the great majority of the world's yams.
Dioscorea esculenta (lesser yam)
A spiny vine carrying smaller, tender tubers with a sweeter taste than the larger yams.
Dioscorea bulbifera (air potato)
Grown chiefly for the aerial bulbils studding its vines, though many forms are bitter and toxic, and it has turned invasive across the southeastern US.
How to propagate
Yams start from the tubers themselves, not from seed. With clean, healthy planting stock in hand, a new crop begins from the flesh you already have.
Bulbs & cormsModerate
Spring, after frost danger has passed and soil is warm · 3-6 weeks to sprout
Yams rise from setts, chunks of seed tuber or small whole tubers of roughly 100 to 300 grams, pressed into warm spring soil once frost is gone. Pre-sprouting them in warm shade wakes them sooner, and the shoots break in about 3 to 6 weeks.
Bulbs & cormsModerate
Spring · 3-6 weeks to sprout
Some species, D. alata and D. bulbifera among them, also hang aerial bulbils in the leaf axils. Gathered and planted in spring like small tubers, they sprout on the same 3 to 6 week clock.
Myths
A yam and a sweet potato are one and the same.
A true yam is a Dioscorea of the family Dioscoreaceae, while the sweet potato is Ipomoea batatas in the morning glory family, a plant apart. Most produce sold as "yam" in US supermarkets is really sweet potato.
A raw yam is fine to bite into like a carrot.
Raw yam is loaded with bitter saponins and skin-irritating crystals, and wild or bitter species carry the toxic alkaloid dioscorine. Every yam has to be cooked first, and some wild types need soaking and leaching before they are safe at all.