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Mountain yam does not form a round tuber near the surface. It sends a single pale tuber straight down, three to four feet (90 to 120 cm) in a good year, tapering to a point and brittle enough to break under its own weight if you lift it wrong. Everything about how you plant follows from that. You are not preparing a bed, you are preparing a column of loose material for the tuber to travel through, and the shape of that column is fixed on planting day.
There are three workable answers and they are not interchangeable. You can dig a deep trench and refill it with loosened soil. You can lay a length of pipe or corrugated drainage tube at a shallow angle so the tuber grows along it and lifts out whole. Or you can plant into a tall container, accepting a shorter tuber in exchange for never digging at all. Choose one now, build it now, and plant into it. Half-measures produce forked, stubby tubers that break during harvest.
| Method | Depth or size needed | Harvest effort | Typical tuber |
|---|---|---|---|
| Deep trench | Loosened to 3 ft (90 cm) | Hard: a long dig per plant | Full length, straight if soil is even |
| Angled pipe or drain tube | 4 ft (120 cm) tube at 20 to 30 degrees | Easy: open the tube and lift | Long, gently curved, unbroken |
| Tall container | At least 24 in (60 cm) deep | Easy: tip the container out | Shorter and thicker, sometimes clubbed |
Preparing a trench that the tuber can actually travel
If you go the trench route, dig two feet (60 cm) wide and three feet (90 cm) deep, and keep the topsoil and subsoil in separate piles. Break up the floor of the trench with a fork, then refill with the topsoil first and the subsoil on top, mixed with plenty of coarse compost or leaf mold. What you are trying to remove is any hard layer, any stone, and any pocket of raw clay, because a tuber that meets an obstruction forks around it and both forks end up thin.
Do not add fresh manure or a heavy dose of high-nitrogen fertilizer to the lower part of the trench. Undecomposed material sitting in the tuber's path causes surface pitting and side branching. Mature compost worked evenly through the whole profile is the right approach. Firm the refilled trench gently and water it once so it settles before planting rather than settling around the growing tuber.
Setting a pipe or drainage tube
The pipe method is the reason gardeners with heavy or stony ground can still grow a straight yam. A four-foot (120 cm) length of wide corrugated drainage tube, split lengthwise, or a length of large-bore plastic pipe, is laid in a shallow trench at a twenty to thirty degree angle from horizontal. Fill it with a light, uniform mix of sand and sifted compost, set the planting piece at the high end, and the tuber grows down inside the tube instead of into the ground.
- Dig a sloping channel Take out a channel about a foot (30 cm) deep at the top end and running down to two feet (60 cm) at the far end, long enough for the whole tube to lie in it.
- Lay and fill the tube Split the tube lengthwise or use an open half-round. Fill it level with a mix of roughly equal parts coarse sand and sifted compost. Keep the mix uniform: any lump makes a kink in the tuber.
- Set the piece at the high end Place the planting piece at the raised end, cut side down, covered by three inches (8 cm) of the same mix. The shoot rises from here, so this is where your support goes.
- Backfill and mark both ends Cover the tube with four to six inches (10 to 15 cm) of soil and push a cane in at each end. At harvest you will be digging blind, and the markers save you from cutting the tube in half with a spade.
Planting pieces, spacing and soil warmth
Mountain yam is planted from a cut section of last year's tuber, from a whole small tuber, or from the aerial tubers that form at the leaf joints. A cut section should weigh two to three ounces (60 to 85 g) and carry part of the crown or upper shoulder, which is where the growth bud sits; pieces cut from the pointed lower end sprout weakly or not at all. Leave cut faces to dry and callus in a shaded airy place for two or three days before planting, or dip them in wood ash, otherwise they rot in cool damp ground.
Plant when the soil at six inches (15 cm) has reached 59F (15C) and is climbing, which is around the time of your last frost or a little after. Set pieces three inches (8 cm) deep and allow twelve to sixteen inches (30 to 40 cm) between plants along the row, with rows at least two feet (60 cm) apart so the supports do not shade each other. Emergence is slow and uneven: three to five weeks is normal, and a piece that has not shown at four weeks is usually still alive underground.
Wear gloves from the first cut. The sap of every part of this plant carries needle-shaped crystals that lodge in skin and cause an intense, persistent itch, and it is worst on wet hands. Cutting planting pieces is when most gardeners get caught, because the job feels harmless.
Never plant a cut piece from the thin lower end of the tuber and expect it to grow. Use the crown end, the shoulders, whole small tubers, or aerial tubers. This single choice explains most reports of yams that never came up.