Gadenin日本語Get the appHow tall, and how soon
The vine is slender, twining, and fast once warm weather arrives. Expect six to eight feet (180 to 240 cm) of growth in a single season, occasionally more on rich ground, and expect almost all of it in the eight weeks after midsummer. A plant left to sprawl on the ground produces a fraction of the tuber, because the leaf area that feeds the tuber gets buried, shaded and eaten.
Put the support in before the shoot breaks the surface. Driving a stake next to an emerged plant risks spearing the developing tuber, which is directly below and already several inches long by the time you see leaves. If you are using a pipe, the support goes at the raised end where the planting piece sits, angled away from the tube's run.
Frames that hold up in wind
The load is not heavy but it is high and it catches wind like a sail once the vine is in full leaf, so anything you build must resist toppling rather than resist weight. A single cane pushed a few inches into soft ground will go over in the first summer storm and take the vine with it. Sink supports at least twelve inches (30 cm) and tie the tops together.
| Frame | Best for | What to watch |
|---|---|---|
| A-frame of paired canes with a ridge cane | Rows of four or more plants | Tie every crossing point; untied A-frames rack sideways |
| Tripod of three 8 ft (240 cm) canes | Two or three plants in a group or a container | Vines crowd at the apex and shade each other |
| Netting stretched between two posts | Long rows along a fence line | Netting must be taut; slack netting sags into a hammock |
| Single stout stake with twine to a top wire | Container plants near a wall | The stake must be driven, not pushed, or wind lifts it |
Training the shoot in the first month
Mountain yam twines anticlockwise, seen from above, and it needs something under about half an inch (12 mm) thick to grip. On a fat post it will slide back down. Give it string, thin cane or netting for the first three feet and it will manage the rest by itself.
Once a vine has wrapped, do not try to unwind it. The stems are brittle at the nodes and a snapped shoot in early summer costs you most of the season's leaf area. If a shoot is going the wrong way, cut a new string and lead it, rather than pulling the vine off what it has already climbed.
- Wind the first shoot by hand When the shoot is eight inches (20 cm) long, loop it gently anticlockwise around the string. One turn is enough; it takes over from there.
- Keep one or two shoots, remove the rest A planting piece often sends up three or four shoots. Pinch all but the strongest one or two at ground level. More shoots means more thin tubers rather than one good one.
- Spread the vines across the frame Lead each vine to its own string or panel of net. Vines bundled together shade one another and the lower leaves yellow and drop by late summer.
- Stop the vine at the top When the growing tip reaches the top of the frame, pinch it out. This pushes growth into side shoots and, later in the season, into aerial tubers at the leaf joints.
What the frame does for the aerial tubers
Small brown aerial tubers form at the leaf joints from late summer onward. They are a crop in their own right and they are also the easiest planting material you will ever handle, so it pays to make them easy to collect. A vine held clear of the ground on a frame drops its aerial tubers where you can find them; a sprawling vine drops them into the mulch and you lose them, or you find them next spring as a thicket of unwanted seedlings.
If you intend to save them, slip a strip of fine netting or a shallow tray under the busiest section of the frame in late summer. They detach at the lightest touch when ripe, and you will not be there at the moment they let go.
Do not build the frame from wire or from anything thicker than half an inch (12 mm). The vine cannot get a grip on a broad smooth surface and will simply spiral up and slip down repeatedly, wasting weeks of growth.