1. Why the Mushroom You Buy at the Store Is Only a Small Part of the Whole Fungus
The white, round, hat-shaped object you buy at the store is not the entire mushroom. Mushrooms are fungi that lack chlorophyll. What we usually call a "mushroom" is actually the "fruiting body," which forms when fungal threads gather together to produce spores [1].
Below the fruiting body lies the "mycelium," a network of fungal threads. This network spreads through soil or fallen leaves, absorbing nutrients and water from there [2]. The common button mushroom belongs to the family Agaricaceae. It is also known by other names, such as the "cultivated mushroom" or "Western matsutake" [3].
If the part you see is just a small fruit, you might wonder what the hidden network of threads is eating underneath.
2. Why Mushrooms Get Their Nutrients from Compost Made of Straw and Manure, Not Soil
Mushrooms are "litter decomposers." They break down fallen leaves and other organic matter to get their nutrients [2]. Unlike crops in a field, they do not use sunlight to create energy. Because they live by breaking down existing materials, they do not need a sunny field to grow.
In traditional farming, the growing bed is made from straw from horse stables and horse manure, which are piled up and fermented. This is called "stable manure" or compost. Spawn (seed) is then added to this bed [3]. In Japan, artificial compost has also been made using rice or wheat straw mixed with chemicals like ammonium sulfate, lime nitrogen, or urea [3].
Making compost happens in two stages. First, the raw materials are mixed, watered, and piled into a mound. Second, the compost is placed in a growing room and heated to sterilize it, reducing harmful bacteria while leaving useful ones. Microorganisms convert ammonia into nutrients the mushroom can use, and sugars are processed during this stage [5]. Wheat straw and hay are often listed as raw materials.
Once this compost is ready, how does the mushroom fungus get into it?
3. How Spawn Is Mixed into Compost While Growers Wait for the Fungal Threads to Spread
The seed used in cultivation is called "spawn." It consists of grain that has been sterilized and grown with fungal threads. This spawn is mixed into the compost to inoculate it. After inoculation, you must wait for the mycelium to spread throughout the entire compost [6].
In 1707, the botanist Tournefort wrote the first scientific description of mushroom cultivation. However, cultivation at that time was unstable. It was not until the 1890s that pure spawn, free from other contaminants, was created at places like the Pasteur Institute, making cultivation stable [4]. Some sources cite the year as 1890 or 1893, so this article uses "the 1890s" [3].
When the mycelium has spread through the compost, it still does not look like a mushroom. This is where the soil comes into play.
4. How Covering the Compost with Soil Triggers Mushroom Buds
Once the mycelium has spread through the compost, a layer of casing soil, peat, or similar material about 3 to 4 cm (1 to 1.5 inches) thick is placed on top. This casing soil acts as a stimulus, causing the formation of fruiting body buds (primordia) [8]. The casing soil also protects the crop from drying out and from pests or diseases.
Before the buds appear, the mycelium forms thick bundles called rhizomorphs. Fresh air must be circulated in the growing room to keep carbon dioxide levels below 0.08% [6].
So, what exactly is the casing soil doing? The detailed mechanism of how casing soil promotes bud formation has not been fully explained [8]. One theory suggests that a water gradient created between the compost, casing, and air is necessary. However, some researchers argue that no single mechanism is the sole cause [7].
This means that the traditional practice of covering compost with soil is not yet fully understood by researchers. After the buds appear, how often can they be harvested?
5. How Mushrooms Are Harvested in Waves About Every Week
Harvesting begins 15 to 21 days after the casing soil is applied. From the start of compost preparation, this takes about 7 to 8 weeks [6]. Japanese summaries also state that mushrooms appear about 3 weeks after casing, in cycles of about one week [8].
Harvesting is repeated in waves. Collection takes 2 to 4 days, followed by a break of 7 to 10 days before the next wave arrives. These waves are called "flushes." The entire cycle, from starting the compost to steam treatment after harvest, takes about 14 weeks [6].
Cultivation takes place in specialized houses (mushroom houses) where temperature and humidity can be controlled. Using underground bunkers or earthen storehouses is also being advanced [3]. The numbers above are guidelines for commercial cultivation in the United States and may differ for Japanese farms.
When and who started this cultivation?
6. How Mushroom Cultivation Began in France in the 17th Century and Came to Japan in the Taisho Era
Cultivation began in France around the 17th century. It is said to have started when mushrooms were noticed growing naturally in old hotbeds used for melon cultivation [8]. Sources vary on whether this was in the mid-17th or late 17th century. By the 18th century, cultivation was also happening in caves.
Mushroom cultivation was introduced to Japan during the Taisho era. In 1922, Morimoto Hikozaburo built a red-brick mushroom house in Fushimi Ward, Kyoto, and started a business [3].
The beginning seems to have been a chance discovery. Since then, preparations have increased step by step: compost, pure spawn, casing soil, and temperature-controlled houses.
When you see mushrooms in the store, what should you look for?
7. How White, Brown, and Portobello Mushrooms Compare, and How to Calculate the Days
White button mushrooms, brown ones, and large, open Portobello mushrooms are all the same species. The differences are said to be due to the stage of maturity or variety [4]. If you see them lined up in a store, try comparing how open the caps are and their sizes.
You can calculate the days. The 7 to 8 weeks before harvest begins equals 49 to 56 days. The total cycle of about 14 weeks, including post-harvest steam treatment, is 14 × 7 = 98 days. These numbers are just conversions from the source data and may not match actual cultivation days.
You can check the structure of the mushroom body on pages from the Forestry Agency and the Ministry of Agriculture, Forestry and Fisheries. Cultivation processes and numbers can be read in the commentary from Penn State University listed in the sources.