1. A Spring Shoot Has Been Growing Underground Since the Autumn Before
Short answer: a bamboo shoot spends a long time getting ready underground, then finishes growing very fast above it. Many bamboos, including the moso and madake bamboos common in Japan, spread by underground stems called rhizomes. Each year, buds on the rhizome joints (nodes) grow into new bamboo [1]. A forestry research center in Kumamoto, in southwestern Japan, notes that the rhizomes grow from late May or early June until late October or early November, and that shoot growth has already begun by then. In autumn, people tending a bamboo grove sometimes find quite fat shoots still in the soil [2].
Shoots there start to appear above ground from about early March, and the bamboo stem is finished in about 50 days [2]. So the part we see is the last, fast chapter of a story that began months earlier. The next sections ask how a shoot can be fed in the dark, and why the last chapter is so quick.
2. The Yellow Tip Is a Clue That a Shoot Has Not Seen Sunlight Yet
A freshly dug shoot often has a yellow tip. According to a summary of a page from Japan's Ministry of Agriculture, Forestry and Fisheries, a yellow tip shows that the shoot has not yet come out of the soil. Once the tip is in the light, it turns green and gets more bitter [3]. (The page's explanation of why, photosynthesis, is what I would expect, but I did not read it in the original.)
That raises a puzzle. If a buried shoot cannot use light to make food, what is feeding it? A reasonable guess, based on the sources above, is that it must be using food made earlier. The next section checks that guess.
3. Where a Shoot Gets Its Energy: Food Stored in the Underground Stems
The Kumamoto center says that most of the nutrition a shoot needs, in its short and vigorous growth, comes from nutrients stored in the rhizome [2]. In other words, the shoot is not building itself from nothing. It is spending savings that the bamboo grove put away earlier.
This also helps explain why a grove does not produce the same number of shoots every year. Japan's Forestry Agency says rhizomes in their third and fourth years produce the most shoots, and production drops after the fifth year. It also says that good years and poor years come roughly every other year [1]. The Kumamoto guide gives a slightly different range: young rhizomes, roughly two to five years old, produce the most, and first-year rhizomes produce very few [2]. The two sources agree that younger rhizomes produce more, but the exact ages differ. The sources I read do not say why good and poor years alternate.
4. Why a Bamboo Can Grow So Fast Once It Is Above Ground
Most trees get longer mainly at the tips of their branches and trunk. Bamboo has growth points, places where cells divide, at every node along the stem [4]. Many places can lengthen at the same time, which helps explain the speed. (How much each part contributes was not given in the sources I read.)
The Forestry Agency reports records of 121 cm (about 4 feet) in 24 hours for madake and 119 cm for moso bamboo [1]. These are record-setting days, not ordinary ones, so a bamboo does not grow a meter every day. From the day a shoot emerges, the stem is finished in about 50 days, according to the Kumamoto guide [2]. Another ministry article says that in about half a year a shoot becomes a fine young bamboo [4]. Those time spans differ, probably because the sources count different stages, but the sources I read do not explain the gap. The Forestry Agency also notes that bamboo has no layer that makes it thicker each year the way trees do, so it does not keep getting wider after it has grown [1].
5. What the Rhizomes Are Doing While Shoots Race Upward
The rhizomes do not stop working. The Forestry Agency says a rhizome can lengthen by as much as 5 m (about 16 feet) in a year, depending on soil and weather [1]. Bamboo, the Kumamoto guide says, is growing somewhere on the plant all year round [2]. So the year makes a loop: rhizomes spread and buds swell in summer and autumn, shoots emerge in spring, and the new stems finish growing by early summer. The Kumamoto guide describes the bamboo stem and leaves as complete by late May or early June, just when the rhizomes start growing again [2]. This is a timeline for the Kumamoto grove. Other places and other species may run on a different calendar.
6. How to Look for Signs of This Yourself, Safely
If there is a bamboo grove or a park with bamboo near you, ask an adult to go along and stay on the paths. In spring, look for shoots of different heights: tiny ones next to ones already taller than you. Is the bottom of the tall ones as thick as the top? Do the tips of the shortest ones look pale or green? You can also count the nodes on a mature stem from bottom to top, and compare thin and thick stems. Bamboo cut ends can be sharp, so look and do not cut or dig. For more on how bamboo grows, the Forestry Agency page and the ministry's March 2021 magazine feature are good places to start [1] [4].