Hilling Taro: How Earthing Up Increases the Crop of Side Corms

What hilling actually does for a taro plant

A taro plant does not produce one corm. It produces a large central corm and a ring of smaller side corms, sometimes called cormels, on short lateral stems around the base. For most home gardeners the side corms are the better eating and they make up a large share of the total weight. The point of hilling is that those laterals only develop and swell where they are surrounded by soil. Any that push out into the air stay hard, green and stunted.

That is why hilling gives such a clear return on a small amount of work. Drawing four to six inches (10 to 15 cm) of soil up around the base at the right moments buries the whole zone where laterals form, and gives them room to expand outward rather than upward. Gardeners who never hill typically get a decent main corm and a handful of tiny useless side corms; the same plants hilled twice give a similar main corm and several times the weight of usable side corms.

When to hill, and how much soil each time

The first hilling comes when the plant has three or four fully open leaves, usually four to six weeks after emergence. At that point draw about three inches (8 cm) of soil in around the base from either side, filling in part of the planting trench. The second comes four to six weeks later, once the plant is well established and the trench has largely disappeared; add another three to four inches (8 to 10 cm), this time forming a genuine mound. A third light hilling in late summer is worth doing wherever you can see the shoulder of a corm or the pale tip of a lateral breaking the surface.

Do not hill everything in one go early in the season. Burying a young plant deeply chills the growing point in cool spring soil and slows it down, and the laterals it will eventually make have not formed yet, so there is nothing to bury. Equally, do not hill after the leaves start yellowing at the end of the season; by then the corms are finishing and disturbing the soil only bruises them.

Doing it without damaging the roots

Taro roots run wide and shallow, and a careless hoe cuts through them just below the surface. The soil you draw up should come from the middle of the path or between the rows, not from within a foot of the plant. Scraping soil in from close to the stems does more damage than the hilling repairs, and it also creates a hollow that fills with water and rots the crown.

Work when the soil is damp but not waterlogged so it holds its shape rather than running back down. Draw the soil in evenly around the whole base and firm it lightly with the back of a rake to make a mound with a slightly dished top; a dish holds water where you want it, whereas a sharp cone sheds it. If you are mulching, pull the mulch aside first, hill, and then put the mulch back on the new surface.

1. Take soil from the path, not the row Draw material in from at least a foot (30 cm) out. Soil scraped from close to the stems takes the feeder roots with it.
2. Work on damp soil Damp soil piles and stays. Dry soil runs back off the mound within a day and wet soil compacts into a crust.
3. Build a dished mound, not a cone Leave the top of the mound slightly hollow so irrigation collects over the corm rather than running off down the sides.
4. Keep soil off the leaf stalks Bury the base of the plant and the ground around it, but do not pile soil up the stalks themselves, which invites soft rot at the crown.
5. Restore the mulch afterwards Pull mulch back before you hill and spread it over the fresh soil after. Bare mounds dry out fastest of all.

If you garden on soil too thin to spare, hill with compost, well-rotted manure or old potting mix instead. The laterals do not care what the material is, only that it is loose, damp and covering them.

Hilling in containers and on wet sites

In a tub you cannot draw soil in from a path, so plan for hilling by planting low. Fill the container only two thirds full at planting and add three inches (8 cm) of compost at each hilling round, working up to within an inch or two of the rim by late summer. It is the same principle used for potatoes in bags, and it gives the laterals the same buried space to swell into.

Where the bed is genuinely wet or flooded, hilling still applies but the material behaves differently. Saturated soil slumps back flat within a week, so build the mound higher than you think necessary and expect to repeat it. On very wet ground some gardeners abandon mounds altogether and simply top-dress the whole bed with two inches (5 cm) of compost at each hilling date, which achieves the same burial across the row and does not wash away as fast.

1. Plant low in the container Fill the tub two thirds and set the corm on that level. The remaining third is the material you will hill with later.
2. Add compost, not garden soil, in tubs Garden soil compacts in a container and drives out air. Loose compost lets the side corms expand and lifts easily at harvest.
3. Top-dress instead of mounding on wet ground Where mounds slump, spread two inches (5 cm) of compost across the whole row at each hilling date. It buries the laterals just as well.

Checking the result, and keeping a record

You cannot see side corms while they grow, so the only feedback comes at harvest. Make the comparison deliberately: leave one or two plants unhilled at the end of a row, mark them with a cane, and weigh their side corms separately when you dig. In a single season that tells you more about what hilling is worth on your soil than any general advice, and it usually settles the question permanently.

Write down the dates of each hilling round, how much soil you drew up, and then at harvest the count and weight of usable side corms per plant. Note also any corms that came up green or cracked, because that is a direct message that the mound was too shallow at that spot. Two seasons of those numbers turn hilling from a chore you do because you read about it into a scheduled job with a known payoff.

1. Leave a control plant unhilled Mark one plant per row with a cane and do not hill it. Weigh its side corms separately at harvest against a hilled neighbor.
2. Record dates and depth Note when you hilled and roughly how many inches you added. Corm results only mean something alongside what you actually did.
3. Count green or cracked corms Every greened shoulder means soil was too shallow there. Where those cluster in the row tells you which end dries or slumps.
4. Weigh side corms separately from the main corm Total weight hides the effect. Side corm weight is what hilling changes, so it is the number worth tracking year to year.