1. The Small Round Taro in a Shop Are the Plant's Children and Grandchildren
Taro (Colocasia esculenta) grows from a corm, a thick underground stem that stores food. Extension staff at North Carolina State University describe it as a plant that can be divided like other perennials with storage organs [4]. In Japan the small-piece type is the one people buy most, and they call it satoimo. The word means "village potato," because it is a potato-like food grown in villages, as opposed to the wild mountain yams that grow in the hills [1].
A farmer buries a "seed taro," a corm kept from the year before. In a trial by Saitama Prefecture, researchers planted corms weighing 70 to 90 g (about 2.5 to 3 ounces), with the buds pointing up and the top of each corm about 7 cm (almost 3 inches) below the surface [3]. The first bud to swell becomes the mother corm. Buds on the mother's side grow into children, and buds beside the children grow into grandchildren. Above ground, another bud becomes a thick stalk with leaves [1].
What separates the three is where they form and how big they get: mothers are usually large, and children and grandchildren are small and round [1]. So a bag of small round taro is mostly second- and third-generation pieces, not the planted corm. That depends on the variety, though. Some Japanese varieties are grown to eat the mother corm, and a Japanese prefectural summary lists one that is harvested for its mother corms in November and December [2].
How fast does this family grow?
2. How Much Heavier Do the Mother and Grandchildren Get Between June and October?
Ehime Prefecture's agricultural research institute weighed one plant's corms from June to October. The plant went into the ground on April 4, 2023. The mother corm weighed about 54 g on June 16, 270 g on July 19, 414 g on August 15, 720 g on September 16 and 806 g on October 12. That is about fifteen times heavier over four months, but only about three times heavier from mid-July on, so the mother corm swelled early [2].
The grandchildren changed even more. On July 19 the plant had 8.3 of them, weighing 37 g together. By October 12 it had 21, weighing 1,203 g together. The number grew about 2.5 times, but the weight grew about 33 times. New grandchildren appeared, and the ones that already existed also got fatter. Divided out piece by piece, that is roughly 4.5 g each in July and 57 g each in October (my own division of the institute's figures) [2].
The institute says that taro planted in March or April grows leaves and stalks strongly until late August, and then the underground pieces swell fast as the leaves lose vigor. It also says each piece first lengthens and only then widens [2]. The material does not explain why swelling speeds up when leaves weaken, so it is best read as something researchers observed, not as an explained rule. These are also figures from one plant in one trial, so other taro will not match them exactly.
Leaves that spread wide need plenty of water. So what does water do to the pieces below?
3. What Happens to the Underground Pieces When a Taro Plant Goes Short of Water in Summer?
Taro is a plant that likes water and does badly in drought. NC State's plant guide says it should not be allowed to dry out [4]. A Saitama research report adds that whether or not a farmer irrigates in the growing season changes the yield a great deal [3].
Ehime researchers tested timing. They grew taro in containers in a greenhouse and stopped watering one group in June, another in July and another in August, each until the third leaf from the top started to yellow by 5 to 10 percent. Then they watered normally again. A plant that was always watered gave 2,240 g of children and grandchildren. The June group gave 1,680 g, about 25 percent less. The July group gave 1,977 g and the August group 2,184 g. The plants' height recovered once water returned, but the yield did not [2]. In other words, a plant can look healthy again and still have lost some of its crop.
Saitama's trial looked at an earlier moment. In a taro field there, watering from mid-May to early June made the mother corm form its third node sooner, so the first children appeared earlier. Earlier children led to earlier grandchildren, which then had longer to swell, and the harvest included more large, high-value pieces [3].
Read these two results carefully. They come from different prefectures, soils and methods, and the Ehime test was a single greenhouse container trial. They support a modest conclusion: a shortage of water early in the season can reduce what is dug up later. They do not prove that June is "the most important month" for every taro field.
Water is only one of the things a farmer manages. Soil is another.
4. Why Do Farmers Heap Soil Over Taro Ridges in Early Summer?
In Ehime Prefecture, growers cover the taro ridges with plastic sheet (called mulch), which keeps the soil moist and warm. In late May to mid-June they pile more soil on top of the sheet. The Japanese name for this is tsuchi-ire, "putting soil in." The institute calls it a required job in this style of farming [2].
In its data for one season, plants with soil piled on grew 6.1 new shoots per plant in mid-August, against 4.6 without. In November the children and grandchildren weighed 1,793 g with soil and 1,480 g without, about 1.2 times more. Odd-shaped corms, such as "bamboo-shoot taro," which grow long instead of round, numbered 1.8 with soil and 4.8 without [2].
The institute gives two reasons: the soil helps new shoots sprout, and it keeps the inside of the ridge from becoming too hot or too dry [2]. Saitama's trial, which did not use plastic mulch, also heaped soil about 15 cm (6 inches) high around the plants in late June [3]. The Ehime figures come from mulched ridges, so they may not apply to old-style hilling without sheeting.
Both stories point the same way: what the plant does underground depends on the water and warmth it gets there.
5. Why Does the Taro Plant Almost Never Flower?
With so much going on underground, you might expect flowers above it. NC State Extension notes that taro rarely flowers and is better known for its huge leaves [4]. The sources I read do not explain why, and they do not say whether the rarity of flowers is related to taro multiplying through its pieces. It is a question still open to anyone who wants to look further.
6. How Can You Compare a Shop's Taro With the Ehime Numbers?
Do not eat taro raw. NC State warns that taro is poisonous until cooked, and a Japanese review adds that peeling it can make hands itchy, which dabbing fingertips with vinegar is said to help [4] [1]. Cook it fully before tasting.
With that said, you can do some detective work. Weigh a few raw taro from a shop one by one on a kitchen scale, without peeling them, and wash your hands afterward. The Ehime plant's grandchildren averaged about 4.5 g in July and about 57 g in October. Which month do yours resemble? The variety, place and harvest date will differ, so they need not match. The goal is to picture where each piece grew in the family underground, and to decide whether it was a mother, a child or a grandchild.
One more oddity: a Japanese review says taro is hardly eaten outside Japan, yet NC State calls the corm a staple food worldwide. They look at different groups of varieties and I could not settle it from these sources, so I leave that question open.