Vegetable · Dioscoreaceae
Yam
Dioscorea
With heat, a tall trellis, and a little patience, the yam pays you back in big, storable tubers.
Full sun
Every 5–7 days
68–86°F · 20–30°C
240–330 days
Overview
If you have never grown a yam before, you are in good company with cooks across Africa, Asia, and the Caribbean who have leaned on this twining tropical vine for thousands of years. It is grown for the starchy tubers that swell underground, and it belongs to the genus Dioscorea, a plant quite separate from the sweet potato that keeps borrowing its name. Most of the world's yams are raised in West and Central Africa, a good deal of it the white Guinea yam, so you are joining a very old and forgiving tradition.
These are generous, good-sized plants, so do not be shy about giving them room. The vines climb 7 feet and will reach higher on a strong support, which is why a tall, sturdy trellis goes in right at planting. Underground the tubers can grow surprisingly big, and a winged yam now and then tops 100 pounds, though the yams most people grow for the kitchen stay much smaller and easy to handle.
What a yam really asks of you is warmth and patience. It runs as a warm-season crop over a long stretch of 8 to 11 months, so it settles in happily where frost stays away, or with a head start indoors when your season is short. Once it is cooked the tuber becomes a comforting, wholesome staple, but do remember it should never be eaten raw.
Care
The short version: give your yam full sun, warm ground, and a tall trellis set in at planting, and it will look after itself. Keep that deep, loose, well-drained soil evenly moist all season, feed it monthly with plenty of potassium, and gently ease back on the water as the vines yellow toward harvest.
Light
Your yam will be happiest in full sun, which powers both the long climb and the tubers filling out below. It will put up with a little partial shade if that is what you have, though the growth will be softer and the harvest smaller, so give it the brightest spot you can.
Water
Aim to keep the soil evenly moist all through the long growing season while the vines and tubers are filling out, which usually means a drink every 5 to 7 days. As the foliage begins to yellow toward harvest you can ease right off, and try not to let the ground sit waterlogged, since soggy soil is what rots the tubers.
Soil
Give the roots a deep, loose, well-drained sandy loam with plenty of organic matter, so the tubers can stretch out without forking. A slightly acid pH of 5.5 to 6.5 keeps them content, and if your ground is on the heavy side, gentle mounds or ridges will help the water drain away.
Temperature
Remember these are tropical plants at heart, most comfortable between 68°F and 86°F (20°C to 30°C). They have no patience for frost at all, so wait until the soil has genuinely warmed through before the setts go in.
Humidity
Yams enjoy humid air, somewhere around 60% to 90%, which suits their tropical background. Do pair that dampness with airy spacing and tidy trellising, because crowded, wet foliage is exactly what anthracnose loves.
Feeding
Feed once a month through the growing season with a balanced fertilizer high in potassium, since that potassium is what builds strong, healthy tubers. Little and often over the long season does far more good than one big heavy dose.
Pruning
Happily, there is very little pruning to do here. Train the twining vines up a tall, sturdy trellis or stakes set in at planting, then simply pick off any dead or diseased leaves and keep the vines up off the ground.
Size & spacing
On their support the vines can climb anywhere from 7 to 30 feet (2 to 9 m), while each plant spreads roughly 2 to 4 feet (60 to 120 cm) across the base. Give them that space with a clear conscience, because good airflow leaves disease far less room to settle in.
When to plant
Yams keep their own gentle schedule, built around heat and a long calendar. If your summers are on the short side, give the setts a head start indoors a few weeks early, then move them outside only once the frost has gone and the soil feels warm. About 8 to 11 months on, when the vines yellow and fade, your tubers will be 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
Most hiccups with yams turn up in one of two places: on the leaves during muggy weather, or in the tubers when you dig them up. The good news is that catching leaf troubles early and getting the soil right before you plant heads off most of it. Find your plant's symptom below, then act while it still does some good, and know that most setbacks are recoverable.
Dark, blighted patches creeping over the leaves
Do not panic if you spot this: it is yam anthracnose, the most serious leaf disease of water yam, and it takes hold when warm, wet weather meets a crowded, closed-in canopy. The dark spots tend to start low and work their way up, and they can move into the stems if left alone. Pick off the worst-hit leaves, open the vines up with wider spacing and neat trellising, and lean on tolerant cultivars and clean planting material to keep the upper hand.
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 soft tubers when you dig them up
If the tubers come up misshapen or soft, the cause is usually wet or compacted ground, and sometimes root-knot nematodes nibbling the developing roots. Heavy, soggy soil rots the flesh, while stony or shallow ground makes it fork and twist. Grow in deep, loose, well-drained mounds, lift the crop as soon as the vines die back, and let the tubers cure somewhere dry and airy before you store them.
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 leaves and little pests gathering on the vine
Ragged, holed leaves usually point to leaf-feeding beetles, while sticky little clusters tucked into the leaf joints are mealybugs or soft scale sipping the sap. A quick look at the leaf undersides and stem joints will tell you which visitor you have. Hose off or hand-pick the light cases, reach for insecticidal soap on the stubborn ones, and keep the plants strong and healthy so they shrug the damage off.
Fix:
- Hand-pick or hose off light infestations
- Treat persistent outbreaks with insecticidal soap or an appropriate insecticide
- Encourage natural predators
Toxicity
There is nothing to worry about with cooked yam, which is a safe, everyday food. The raw tuber is the part to respect: raw flesh holds bitter saponins and skin-irritating crystals, and wild or bitter species carry the toxic alkaloid dioscorine. Cook every yam before it reaches the plate, keep raw tubers and trimmings out of reach of pets and children, and give your hands a wash after handling cut raw flesh.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Caution
A nibble of raw tuber or vine can leave a cat drooling and off its food for a while, so it is best kept away. Only a little cooked yam is worth offering.
Dogs Caution
If a dog chews the raw tuber or leaves, expect some mild drooling and a queasy stomach, since the raw flesh carries bitter saponins and irritant crystals.
Horses Caution
Keep the raw tuber and the vines out of reach of horses, as a mouthful can leave one with mild digestive upset.
Rabbits Caution
Raw yam is far too sharp and saponin-rich for a rabbit's gentle gut, so keep to its usual greens and leave the tubers and foliage well outside the hutch.
Livestock Caution
Cattle, goats, and sheep might nibble the foliage or a stray raw tuber, which can bring on a passing stomach upset, more so 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 you decide to share.
People Cooked tuber safe
Cooked yam is a wholesome staple enjoyed the world over. Never eat the tuber raw, leach and cook bitter or wild types thoroughly, and wash your hands after handling cut raw flesh, which can irritate 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 is really a whole genus of edible species rather than a tidy row of garden cultivars, so the names below are species you will meet rather than named varieties. These are the main ones worth knowing as you get started.
Dioscorea alata (water / winged / greater yam)
This is the most widely grown yam anywhere, easy to know by its squarish, winged stems and its very large tubers, which can be anything from white to purple inside.
Dioscorea rotundata (white Guinea yam)
The staple that feeds much of West and Central Africa, the very region that raises the great majority of the world's yams.
Dioscorea esculenta (lesser yam)
A spiny little vine that carries smaller, tender tubers with a sweeter flavor than the bigger yams.
Dioscorea bulbifera (air potato)
Grown mostly for the aerial bulbils dotting its vines, though many forms are bitter and toxic, and it has turned invasive across the southeastern US.
How to propagate
Here is the reassuring part: yams grow from the tubers themselves rather than from fiddly seed. Once you have some clean, healthy planting stock, starting a fresh crop is really quite straightforward.
Bulbs & cormsModerate
Spring, after frost danger has passed and soil is warm · 3-6 weeks to sprout
Yams start from setts, which are simply pieces of seed tuber or small whole tubers of around 100 to 300 grams, tucked into warm soil in spring once the frost has passed. If you pre-sprout them somewhere warm and shaded first they get moving sooner, and either way they take about 3 to 6 weeks to sprout.
Bulbs & cormsModerate
Spring · 3-6 weeks to sprout
A few species such as D. alata and D. bulbifera also form little aerial bulbils in the leaf joints. Gather these and pop them in during spring just like small tubers, and they will sprout on that same 3 to 6 week timeline.
Myths
Yams and sweet potatoes are just two names for one vegetable.
It is an easy mix-up, but a true yam is a Dioscorea in the family Dioscoreaceae and has no relation to the sweet potato, which is Ipomoea batatas over in the morning glory family. In fact, most produce sold as "yam" in US supermarkets is really sweet potato, so the label is doing you no favors.
A yam tuber is fine to eat raw, straight out of the ground like a carrot.
This one is worth taking to heart. Raw yam carries bitter saponins and skin-irritating crystals, and wild or bitter species hold the toxic alkaloid dioscorine, so yams must always be cooked first. Some wild types even need soaking and leaching before they are safe to eat at all.