Edible Mushroom Cultivation

R. A. Davis and B. J. Aegerter, Department of Plant Pathology, University of California, Davis
(A copy of the handout from SOMA meeting 11/17/00)

Just as the popularity of mushrooms in the American diet continues to increase, so has interest in growing gourmet mushrooms for a hobby or small business. Today, many stores carry a variety of wild mushrooms, specialty mushrooms, and several forms of the common button mushroom. The following is a brief description of the materials and methods used for producing crops of some popular cultivated mushrooms.

What is a mushroom?

A mushroom is the 'fruit' of certain fungi, analogous to the apple on a tree. Most of the fungus goes unseen as it colonizes and absorbs nutrients from wood, fallen leaves, organic matter in soil, etc. As a group, fungi can grow on almost any carbon source (a substrate). A fungus is composed of tubular, branched filaments known as hyphae (a mass of hyphae is called a mycelium). Many fungi, including some that form mushrooms, are saprophytes, obtaining their food by colonizing dead organic matter. If a mycelium thrives, it will eventually have enough energy to reproduce. When the combination of temperature, relative humidity, carbon dioxide levels, etc., are just right, the fungus will develop a highly organized structure called a mushroom. The mushroom releases millions of spores, which function like the seeds of plants. A number of these saprophytic fungi are cultivated for their edible mushrooms.

Some fungi are parasites, living on plants, animals, or other fungi. Some are parasitic and pathogenic, while many are parasitic and beneficial. A mutual beneficial partnership between a fungus that lives in the roots of a plant (which provides sugars to the fungus) and the plant (which receives minerals from the fungus), is called a mycorrhiza. Many fungi that form mushrooms exist in mycorrhizal relationships with trees, and this is one of the reasons why forests are often generous to mushroom hunters. Some wild mushrooms, including the popular Porcini, Matsutake, and Chanterelles, are mycorrhizal mushrooms and as such cannot be cultivated (unless the tree is also cultivated!). These mushrooms are sometimes available in stores, but they are all collected in a forest.

portobello mushrooms

Portobello mushrooms

What are specialty mushrooms?

The most popular mushroom consumed in the United States today is the white button mushroom. Because it dominates the market, many call anything other than the button mushroom a specialty mushroom. Following that logic, the Portobello is the most popular specialty mushroom, even though it is simply a brown variety of the button mushroom that is allowed to mature. In the United States, other popular specialty mushrooms are the Shiitake, Oyster, and Enoki. Several other cultivated mushrooms are grown, but they are not widely available.

What substrates are used to grow mushrooms?

Although the common button mushroom is grown on straw-based compost, most specialty mushrooms are grown on sawdust supplemented with other nutrients. Many types of wood are suitable, but alder and oak are the most popular. Cedar and redwood are resistant to colonization by most fungi, including the cultivated mushrooms, and must be avoided. Pines are also unsuitable since most cultivated fungi are inhibited by the resins in the wood. The coarseness of wood sawdust should be about that from a chain saw. Larger chips can be included (use 2 parts of sawdust to 1 part wood chips). Sawdust should be sterilized at 250°F at 15 PSI (pounds per square inch) for 2 to 4 hours, depending on volume. This is accomplished in an autoclave, a retort, or pressure cooker. A common recipe for supplemented sawdust is: 76% sawdust, 12% millet, and 12% bran, and 65% moisture. Note that recipes are always based on dry weights of the ingredients because moisture contents of the ingredients vary. The standard industry container for growing specialty mushrooms in sawdust is a heat-resistant plastic bag fitted with a filter patch, which allows gas exchange but excludes contaminating microorganisms.

How are mushrooms grown?

There are many ways to grow mushrooms, but production always occurs in three general steps—spawn run, pinning, and fruiting. Spawn run is the complete colonization of a suitable substrate following inoculation of the substrate, called spawning. The second step, pinning, is the stage of growth when pinheads are initiated. Pinheads are knots of mycelium that eventually develop into mushrooms. All species of mushrooms require a set of environmental conditions for pinning that are different from the conditions for optimum mycelial growth. Most, if not all, cultivated mushrooms fruit at lower temperatures than the optimum for substrate colonization. The last step, fruiting, is the development of the pins into mature mushrooms.

Step I. Spawn Run: After the substrate is prepared and sterilized in the plastic bag or other suitable container, spawn is aseptically added. Spawn is typically grown on sterilized grain (usually rye or millet) and is either produced by the cultivator or purchased from a mushroom supply house. Purity of the spawn is absolutely critical. In most specialty mushrooms, spawn is added at a rate of 2.5% or more of the dry weight of the substrate. After the bag is hermetically sealed, the spawn is evenly distributed throughout the sawdust by shaking the bag. Depending on the species of mushroom, the substrate is usually fully captured by the mycelium within 2 to 6 weeks. Spawn run temperatures should be 70-75°F.

Step II. Pinning: To initiate mushroom formation, temperatures are dropped for 2 days to 2 weeks, CO2, levels are lowered by introducing fresh air, and light is provided (if you can read by it, there is enough light). Often, the bags are simply opened and moved to the growing room.

Step III. Fruiting: The colonized bag of substrate is placed in a growing room maintained at cool temperatures (63°F is a good average for most mushrooms) and high relative humidity (85-95%). Although the common button mushroom does not require light, all of the other mushrooms described here need some light for proper development. For most specialty mushrooms, the bag may be removed from the sawdust block after spawn run is complete. It is often desirable, however, to remove only the top half of the bag so the sides of the bag reduce air movement across the top of the block, thus maintaining high humidity. In the production of Oyster and Lion's Mane mushrooms, holes are cut in the plastic bag and the mushrooms are allowed to grow through the holes. In Shiitake production, the plastic bag is usually completely removed since this mushroom develops a tough skin on the surface of the sawdust substrate.

Is special equipment necessary to grow mushrooms?

Although spawning and spawn run can be accomplished in any clean room at nonrial room temperature with fresh air, sterile techniques are absolutely necessary for successful cultivation of most mushrooms. Such methods include: a clean lab bench (swab with 50% Lysol, 10% bleach, or 70% ethyl alcohol); washed hands (wash with soap and spray with alcohol); clean clothes; and clean air. While HEPA filters (High Efficiency Particulate Air Filters) are not absolutely necessary in your work space (0.3 micron particles in the air are removed with 99.99% efficiency, i.e., only one out of every 10,000 particles of that size will pass the filter), most cultivators employ one. Note that typical clean desert air contains 200,000 particles per cubic foot and in urban areas this value is raised tenfold.

The growing room requires special conditions. In addition to cooling requirements (60 to 63°F), the humidity must be maintained at about 95% (or even higher in some situations) with just enough air movement to avoid pockets of stagnant air. Excessive air movement, even with high relative humidity, will dry the surface of a substrate and can damage delicate pinheads. Creating a very humid environment for mushroom production is perhaps the limiting factor in home cultivation. Creative steps may be necessary to provide high humidity. Aquariums, plastic tents, ice chests with cellophane windows, etc., are potential humid chambers used for growing. One way to humidify a growing chamber is to bubble a stream of air (perhaps using an aquarium air pump and porous stone) through water slightly warmer (perhaps using an aquarium heater) than the air you want to humidify. Various humidifiers are also used by the home cultivator.

black poplar mushrooms

Black poplar mushrooms

Black Poplar Mushroom (Agrocybe aegerita)

This mushroom is relatively easy to grow but yields are generally modest. Because the pins are very susceptible to desiccation, a layer of moist peat moss and crushed oyster shells (9 parts peat moss: 1 part crushed oyster shell) on top of the block may improve yields.

common button mushrooms

Common button mushrooms

Common Button Mushroom (Agaricus bisporus)

The white button mushroom and its various forms (Crimini, Portobello, and Portobellini) are grown on compost. Straw, either alone or with horse manure (a little horse manure in straw, usually from racetrack bedding) is the starting material of compost and provides structural and chemical properties. The straw is supplemented with nitrogen in the form of various agricultural wastes such as cottonseed meal and chicken manure. During composting, certain physical qualities (permeability to air and water holding ability) and chemical properties (nutrient availability to the mushroom and the exhaustion of nutrients for competitors) are developed as organic matter is consumed and heat is released by generations of microorganisms. The heat needed for these reactions requires a pile of straw of considerable size. The labor and/or machinery required for handling the compost pile generally prevents the small grower from producing good quality compost. Special rooms with controlled temperature, humidity, and fresh air are also necessary to produce the appropriate conditions to pasteurize and condition the finished compost. Unless compost is available, the production of the button mushroom is fairly technical and generally beyond the ability of the hobbyist or small grower.

Golden needle mushrooms

Golden needle mushrooms

Enoki or Golden Needle Mushroom (Flammulina velutipes)

This mushroom is grown on the standard sawdust recipe and can be grown in a variety of containers, including the filter bag. Commercially, Enoki is grown in jars. After pinning is initiated, the mushrooms are grown in the dark at cold temperatures with collars around the top of the jars to elevate CO2, levels, resulting in long stemmed, almost capless, blanched, crisp mushrooms. Enoki is relatively fast growing.

Maitake mushrooms

Maitake mushrooms

Maitake or Hen-of-the-Woods (Grifola frondosa)

In the wild, this mushroom is known as Hen-of-the-Woods. It fruits at or near the bases of trees or stumps. Although Maitake can be grown on the standard sawdust recipe, yields may be increased with the following recipe: 72% sawdust, 20% cornmeal, 7.5% rice bran, 0.2% CaC03, and 67% moisture.

Nameko mushrooms

Nameko mushrooms

Nameko or Golden Needle Mushroom (Pholiota nameko)

Nameko is one of the most popular mushrooms in Japan. It is grown on the standard sawdust recipe in filter bags. To initiate pinning, the bag must be exposed to very high humidity or a light mist.

Lion's mane mushrooms

Lion's mane mushrooms

Lion's Mane (Hericium erinaceus)

This gourmet mushroom, which tastes something like lobster, is grown on the standard sawdust recipe. Unlike most mushrooms, this fungus does not possess gills. Instead, spores are produced on teeth that hang down like delicate icicles.

oyster mushrooms

Oyster mushrooms

Pink Oyster (Pleurotus species)

The Oyster mushroom is the easiest specialty mushroom to grow. The various Oyster mushrooms can be grown on a variety of sources of cellulose, including coffee grounds, newspaper, and corn husks, to name a few. The standard sawdust recipe is also a suitable substrate. Perhaps the most convenient substrate for the hobbyist is chopped wheat straw (or the straw of other small grains) pasteurized by submerging the straw for I hour in water heated to 160°F. Plastic bags stuffed with the drained straw and inoculated with spawn are often used. In commercial farms, long bags of straw are hung from ceilings. Holes are punched at intervals around the bag for air exchange and openings for the mushrooms to emerge.

Spawning can be conducted without special filters since Pleurotus species are very fast growing and will out compete most organisms in the straw.

Reishi mushrooms

Reishi mushrooms

Reishi or "Mushroom of Immortality" (Ganoderma lucidum)

This mushroom is too tough to be edible but is used in teas for its reputed medicinal effects. It is grown on the standard sawdust recipe and can be grown in a variety of containers, including the filter bag.

Shiitake mushrooms

Shiitake mushrooms

Shiitake or Black Oak Mushroom (Lentinula edodes)

Shiitake is relatively easy to grow and is one of the most popular specialty mushrooms. Today, U.S. growers annually produce over 5 million pounds of Shiitake. Like most specialty mushrooms, Shiitake is grown on sawdust in plastic bags (in environments more humid than California, it is also grown outdoors on natural logs). Various strains of Shiitake differ in the time required for spawn run. Some require a spawn run of 45 days while others require 100 days. The plastic bag is then removed and the block is placed in a fruiting room (about 63°F with 75-90% relative humidity). After harvest, the block may be stimulated to fruit again by soaking the block in water until the block is almost back to its original weight.

How can one identify a poisonous mushroom?

Do not eat any mushroom unless you are absolutely sure of its identity. Folk tales on testing a mushroom for toxins are, without exception, useless. Poisonous mushrooms are known because of someone's unfortunate experience. Eating wild mushrooms can be dangerous!

Production Times

Typical Production Schedules for Specialty Mushrooms Grown in 5-pound Bags of Supplemented Sawdust
Mushroom Spawn run Primordia initiation Fruiting (63° F and high humidity) Total time to harvest
Black Poplar 3-4 weeks 1-2 weeks at 63° F 1 week 5-6 weeks
Enoki 17-21 days 1-2 weeks at 63° F 1 week 6 weeks
Lion's Mane 3-4 weeks During spawn run 1-2 weeks 5-6 weeks
Maitake 35 days or less During spawn ran; primordia darken in 9 days; cut holes over largest, fold over bag Clusters take shape in 11 days; harvest 8-15 days later 63-70 days
Nameko 3 weeks After 1-2weeks in fruiting room, remove block from bag and place in loosely closed bag at 54° F until priniordia form 1-2 weeks 2+ months
Oysters 2 weeks 2-3 days at 63° F 3-7 days 3-4 weeks
Reishi 3-5 weeks During spawn run 7-9 weeks 3+ months
Shiitake 45-100+days
  • (strain dependent)
  • 2-3 days at 40° F 2 weeks 2-4 months

    Reference: Stamets, Paul. 1993. Growing Gourmet and Medicinal Mushrooms. Ten Speed Press, Berkeley, CA, 554 p.

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