How to grow yam

Pick deep, loose, fertile soil that drains well - a sandy loam or loam you can push a stick into easily. Yam roots reach down about 1 m, so avoid thin soil sitting over rock or a hard pan.

Choose fresh or rested (fallow) land if you can, and put yam first in your rotation, before other crops take the richness. Yam is a heavy feeder and gives its best on new ground.

Avoid low, wet spots where water sits after rain - yam tubers rot in waterlogged ground. If the land is flat, plan to plant on high mounds or ridges to lift the tuber above the wet.

Clear the bush and weeds early, in the 2 months before planting, so the ground is ready and the cut trash has time to rot down.

Take your planting pieces only from healthy plants and sound tubers - no soft spots, no dark dry cracks under the skin, no shrivelling. A yam grown from a sick tuber stays sick.

A 'sett' is a cut piece of yam tuber used for planting. Cut setts of about 100-300 g each - from a large egg up to your fist. To stretch scarce seed, cut 'minisetts' of 25-50 g - about a matchbox each - with a bit of skin on every piece.

Let the cut faces dry in a shady, airy place for a day or two until they skin over - a wet, raw cut face invites rot.

Treat the setts against dry-rot worms (nematodes - tiny soil worms that cause dry, cracked rot) and anthracnose (a fungus that blackens leaves and vine tips). Choose one: roll the cut faces in wood ash or lime; or dip them in a fungicide (follow the label); or dip them a few minutes in hot - not boiling - water at about 50-53 C. With no thermometer, mix 1 part boiling water to 2 parts cold - steaming, not bubbling.

Use virus-free material to keep yam mosaic virus (YMV) out: never cut setts from any plant whose leaves showed a yellow-and-green mottled pattern last season.

Sort as you go: throw out any piece that is soft, smelly, or shows dark dry rot under the skin. Better to plant fewer good setts than to carry disease into the field.

If too many of your setts are rotten, mottled, or worm-damaged to plant a full field, don't risk them - buy clean, certified seed-yam or minisetts from a trusted source instead.

Build mounds or ridges 30-45 cm high - up to knee height - spaced 1 m apart every way - one stride. That gives about 10,000 mounds per hectare, one plant each.

Break and loosen the soil well as you heap it - the softer and deeper the mound, the bigger and cleaner the tuber.

Work a generous handful or two of well-rotted manure or compost into each mound as you build it, mixed right through the soil. Use only rotted manure - fresh manure burns the sett and brings weed seeds.

On flat or wetter land, make the mounds taller so the tuber sits high and dry; on slopes, run ridges across the slope to hold the rain and soil.

Plant your treated setts into the ready mounds, one sett per mound.

Set each sett about 10-15 cm deep - three finger-joints - on top of the mound, cut-face down or sideways, and cover it firmly with loose soil.

Keep to 1 m between mounds every way - one stride. Don't crowd them; a yam vine needs room and light to fill its tuber.

Plant when the soil is moist but not waterlogged - into a mound that will stay damp, not into dust and not into a puddle.

Firm the soil over the sett so it sits snug in moist earth, and keep the mound shape intact.

From about a week after planting until the vines are well up (roughly the first 6 weeks), walk the mounds once a week.

Check which mounds have sprouted and mark the ones that haven't - yam is slow and uneven, so give late ones time, but keep note.

Termites: look for mud tunnels and soil sheeting climbing the mound or the stake, and setts eaten hollow. Scratch open a non-sprouting mound or two - a tunnelled, hollow sett means termites are working.

Look for rotten or foul-smelling setts (wet rot, or the dry, cracked rot of nematode worms) in mounds that fail to sprout.

Where termites are active, break up any nests nearby and keep the mounds clear of dry trash, and replant failed mounds with a fresh treated sett early, while there is still season to catch up.

Through the first 6 weeks after planting, keep the mound moist but never waterlogged - damp like a wrung-out cloth, not muddy.

After heavy rain, rebuild any mounds that have slumped or washed down, and re-cover any sett left exposed.

In a dry spell, a light mulch of grass or leaves over the mound holds the moisture in and keeps the soil cool - but keep mulch clear of any active termite ground.

Don't let water pool around the base - open a small channel to drain low spots so the sett doesn't rot.

As soon as the vines appear (about 6-10 weeks after planting), set a stake by every mound - a strong pole 2-3 m tall, well taller than you. Push it in firmly, 20 cm (a hand-span) from the sett so you don't spear the tuber.

Wind the young vine gently onto the stake in the direction it naturally twines, and tie it loosely if it won't hold on its own.

Walk the field again a few days later and re-train or re-stake any vines that have fallen or missed their pole.

Where poles are scarce, lead several vines up one tall stake as a tripod, or train them onto a live tree or trellis - the whole point is to get the leaves up into the light.

Weed 3-4 times through the critical window - weeks 8 to 16 after planting - going over the field every 2-3 weeks.

Keep the mound and the base of each vine clean; pull or hoe weeds while they are small and easy.

Hoe shallowly near the mound - the tuber is swelling just under the surface, and deep hoeing cuts or bruises it.

Lay the pulled weeds between the mounds as a mulch to hold moisture and slow the next flush (keep them off the vine base).

Top-dress about 8-12 weeks after planting, once the vines are climbing and the field is freshly weeded (feed the yam, not the weeds).

Use a compound fertiliser supplying both nitrogen (N) and potash (K) - your recommendation gives the type and amount for your field.

Spread it in a ring around each plant, about 20 cm (a hand-span) out from the stem - never against the vine base or over the tuber.

Apply onto moist soil (after weeding or a light rain) and cover it lightly or let rain wash it in, so it doesn't blow off or gas away.

From about 6 weeks to 4 months after planting, walk the field once a week and look closely at the leaves, vines and mound bases.

Anthracnose (a fungus, also called dieback): look for dark brown-to-black spots on the leaves, and blackened, dying vine tips and shoots - worst on water yam in wet weather. Cut out and carry off badly hit vines.

Yam mosaic virus (YMV): look for a yellow-and-green mottled or streaky pattern on the leaves, with crinkling and stunting. There is no cure - uproot and remove badly mottled plants, and never save setts from them.

Yam beetle: look for round bore-holes and tunnels in exposed tubers and at the mound base; the brown beetles feed at night and come in from nearby fallow or swampy ground. Firm, clean mounds and clearing crop trash keep them down.

Note where you see each problem and remove diseased vines and plants promptly - clean-up is your main tool here, as this plan uses no spray.

From about 4 to 8 months after planting, check the field every 2 weeks, looking over the whole canopy.

Anthracnose: watch for spreading dark leaf spots and blackened, dying vine tips, especially in wet spells and on water yam. Cut out and carry off badly hit vines to slow it.

Keep watching too for mosaic-mottled leaves (YMV - the yellow-and-green patterning) and pull out any badly affected plants.

Keep the canopy up on its stakes and in the light - a green, healthy leaf is what fills the tuber.

From about 4 to 8 months, walk the field every 3-4 weeks and pull any weeds off the mounds and vine bases.

Weed by hand, or hoe very shallowly - the tuber is swelling just below and is easily cut.

Leave the pulled weeds between the mounds as mulch to hold moisture through the dry-season bulking.

If you can water, keep the soil steadily moist through bulking - roughly months 4 to 8 - watering every 7-10 days in dry spells.

Water deeply enough to wet the tuber zone (down a hand-span and more), then let the top just start to dry before the next watering - yam still hates standing water.

Never let the crop wilt during bulking - a thirsty vine stops filling the tuber, and you can't get that growth back.

Ease off only near the very end, as the leaves begin to yellow and the tuber matures for storage.

From about 8 months after planting, watch for the plant to finish on its own.

The tuber is mature when the vines and leaves turn yellow and die back, and the skin on the tuber has set firm and no longer rubs off when you scratch a little soil away from the top.

Scrape gently to feel the shoulder of a tuber - a firm, tight skin means ready; a thin skin that scuffs off means it's still filling.

If the vines are still green and the skin still rubs off, it isn't ready - wait and check again in a couple of weeks. An unfilled, thin-skinned tuber will not store.

Harvest at leaf-yellowing, in the dry season (about 7-11 months after planting). Choose a dry day - damp tubers store worse.

Dig a wide ring around the mound first, well out from the tuber, and loosen the soil down the side - never chop straight down onto the tuber.

Work slowly by hand, easing the whole tuber out sideways and lifting it whole and unbruised. Support its weight so it doesn't snap.

To 'milk' early white-yam types: at about 5-6 months, cut the first tuber off below its head and set the head (with the vine) back in the mound to grow a second tuber for a later harvest.

Lay the dug tubers gently in the shade - never toss or drop them - and keep them out of the sun while you finish.

Right after harvest, cure the tubers for 2-4 days before they go into store.

Hold them warm and humid - about 32-40 C and very damp air - in a shaded, airy spot: heap them under a shelter and cover the heap loosely with grass or leaves to trap the warmth and moisture.

This lets the cuts and grazes skin over and the outer skin harden, so rot cannot get in.

Handle gently even now - a fresh bruise during curing is a fresh doorway for rot.

Cure the tubers first (see the curing step), then move only sound, unbruised tubers into store - set aside any cut or soft ones to eat first.

Store in a traditional yam barn: a shaded, well-ventilated frame where you rack or tie the tubers so air moves all around them - out of the sun, rain and reach of rodents.

The tuber's natural resting sleep (dormancy) lasts roughly 2-7 months and carries it through the dry season with no special treatment.

Check the barn regularly. Once the sleep breaks and tubers start to sprout, rub off the new sprouts by hand every week - sprouting drains the food and shrivels the tuber.

Pull out and use any tuber that turns soft, mouldy, foul-smelling, or shows dry, cracked rot under the skin before it spreads to its neighbours.