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Mountain yam is ready when the vine has yellowed and died back completely, which happens in late autumn after the first cold nights. Until the leaves go, the tuber is still gaining weight, and a plant lifted while still green gives you a thin tuber with soft skin that will not keep. There is no benefit in being early.
Once the top has died down, the tuber sits perfectly well in the ground and its skin firms up further. In climates where the soil does not freeze hard, you can leave plants in place and lift them as you need them through winter, which stores them better than any shed. Where the ground freezes deeply, lift everything before the freeze reaches tuber depth.
Cut the dead vine off at about six inches (15 cm) above ground before you start digging, and leave that stub standing. It is your only marker for exactly where the tuber begins, and you will want it.
Digging a trench-grown tuber
This is the slow way and there is no shortcut. The rule is that you never dig toward the tuber, you dig beside it and then work sideways into the wall of the hole. A spade driven down next to the stub goes straight through the tuber almost every time.
- Open a pit to one side Start twelve inches (30 cm) away from the stub and dig a pit as deep as you can manage, at least two feet (60 cm) and ideally three. Throw the soil well clear so it does not slide back.
- Work sideways with your hands From inside the pit, pull soil away from the wall toward you until the pale tuber shows. Use fingers and a trowel handle, not a blade, once you can see it.
- Follow it down before you pull Expose the full length, right to the pointed tip, before applying any lifting force. A tuber freed for two feet and still gripped at three will snap at the boundary.
- Lift with two hands under the weight Support the tuber along its length rather than pulling at the crown. Lay it flat straight away instead of standing it upright.
Allow forty minutes for a single mature trench-grown tuber the first time you do this. Gardeners who rush the last six inches account for almost every broken yam.
Lifting from a pipe or a container
This is where the planting decision repays itself. With an angled pipe, scrape the covering soil off the tube, lift the whole tube out or open the split, and the tuber is lying there in the loose sand mix, whole and clean. It takes minutes rather than the better part of an hour, and breakages are rare. Rinse the mix off gently rather than scraping it.
A container is tipped rather than dug. Lay the container on its side on a tarpaulin, ease the whole mass out, and part it by hand. Do not upend the container and shake it: the tuber falls point-first and breaks on landing. Container tubers are shorter and thicker than trench ones and often carry a club at the tip, which is normal and does not affect quality.
| Growing method | Time per plant | Risk of breaking | Cleaning needed |
|---|---|---|---|
| Deep trench | 30 to 45 minutes | High | Considerable, soil packs into skin |
| Angled pipe | 5 to 10 minutes | Low | Minimal, sand rinses off |
| Tall container | 10 minutes | Low if laid on its side | Minimal |
The aerial tubers are a second harvest
From late summer, small brown tubers the size of a pea to a marble form at the leaf joints all along the vine. They ripen through autumn and detach at the lightest touch when ready, which is the whole difficulty: shake the vine and you lose them into the mulch. Run a hand up each stem with a tray or a sheet held underneath and collect what falls.
They are a crop and they are also your planting material for next year. Store them in barely damp sand or peat in a cool frost-free place over winter and plant them in spring exactly as you would a cut piece. They take two seasons to make a full-sized tuber, so a bed started from aerial tubers is a two-year project, but the material costs nothing and carries no cut surface to rot.
After lifting: curing, storing and what to note
Do not wash a tuber you intend to store. Let the surface soil dry, then brush it off, and leave the tuber in a shaded airy place for a few days so any grazes on the skin dry over. Store at 50 to 59F (10 to 15C) in slightly damp sand, in the dark, with the tubers laid flat and not touching. A broken tuber will not keep, so use those first.
Before you finish for the day, write down the depth you actually reached, the length and weight of each tuber, whether it was straight or forked, and which planting method it came from. Note also where each plant stood, because next season's rotation depends on remembering where the yams were. Those figures tell you within one season whether the extra effort of a deeper trench is worth it on your own ground, which no general guidance can settle for you.