Mountain Yam Care: Water, Feeding, Mulch and Shoot Thinning

Water steadily, never in bursts

The tuber lays down its bulk from midsummer to autumn, and it does that in a narrow column of soil that dries out from the top. What damages a mountain yam is not drought alone but alternation: a fortnight dry then a soaking. The tuber responds to that pattern by cracking lengthwise or by forming a constriction, and both faults show up only at harvest when it is far too late.

Aim for soil that is damp four inches (10 cm) down at all times through summer. In practice that is a deep watering every four to five days in hot dry weather, applied slowly enough to soak in rather than run off. Container plants need far more attention than that: a 24 inch (60 cm) container in full summer sun can need water daily, and once the mix in a deep container has dried right through, rewetting it evenly is very difficult.

Ease off as the leaves begin to yellow in autumn. The tuber has finished sizing by then, and wet soil around a mature tuber invites rot at the shoulders where the stem joins.

Feeding: less nitrogen than you think

Heavy nitrogen produces a spectacular vine and a disappointing tuber. Work mature compost through the planting trench or container mix before planting and, in most gardens, that is the whole base dressing. On genuinely poor ground add a balanced feed at planting, then a single side dressing weighted toward potassium once the vine is climbing well.

Mulch and leaving the root zone alone

Put down three inches (8 cm) of straw, leaf mold or grass clippings once the soil has warmed and the vine is a foot high. The mulch evens out soil moisture, which is exactly the problem you are trying to solve, and keeps the shallow feeding roots near the crown from cooking in hot sun.

Do not hoe around a mountain yam. The feeding roots run out sideways in the top few inches and the growing tuber is directly beneath the crown. A hoe blade that nicks the tuber shoulder in midsummer opens a wound that rots slowly for the rest of the season. Pull weeds by hand through the mulch, or smother them and accept a few.

1. Thin to one or two shoots early Do this while shoots are under a foot (30 cm) tall. Pinch or cut at ground level rather than pulling, which disturbs the planting piece.
2. Mulch after the soil has warmed Mulching cold ground in spring keeps it cold and slows emergence. Wait until the vine is climbing, then lay it thick and keep it clear of the stem itself.
3. Check ties and frame after every storm A leaning frame drags vines across the ground and snaps them at the nodes. Straighten and retie the same day.
4. Remove nothing but damaged leaves Every healthy leaf is feeding the tuber. Resist tidying the vine; strip only leaves that are yellow, torn or diseased.

Wear gloves for any job that involves cutting or breaking the plant, including pinching shoots and tidying leaves. The sap irritates skin badly and the itch can last several hours after a job that took two minutes.

What to write down this season

Three notes make next year much easier. Record the date each plant emerged, because emergence spread over five weeks tells you which planting pieces were sound and which were cut from the wrong end. Record which method each plant was given, trench, pipe or container, alongside what came out at harvest, since the comparison on your own soil is worth more than any general advice. And mark the exact position of every plant with a stake or on a sketch, because the tuber sits directly below the crown and by autumn the vine has died back and the ground gives you nothing to aim at.

Add the date you first watered deeply and how many days you went between waterings in the hottest fortnight. Cracked or constricted tubers at harvest almost always trace back to a gap you can find in those notes.