Mushroom Growth: How Do They Form?

Mushrooms are the fruiting bodies of fungi and are grown in a highly controlled environment. They are grown indoors on a year-round cycle and are usually cultivated in labs and farms. The process of growing mushrooms is scientific and technical, and the right ingredients must be blended to create a quality product. The growth of mushrooms depends on the availability of usable carbohydrates and nitrogen, some of which is in the form of ammonia. Mushrooms are grown in different places, both inside and outside, and the optimum growing temperature is a minimum of 15°C.

The role of spores, mycelium and spawn

Mushrooms have a unique and fascinating production process. Fungi, along with algae, plants, protozoa, and bacteria, produce spores—small, usually microscopic, single-celled structures. The spores are released from the mushroom fruit body and dispersed through air, water, or by hitching a ride on an animal. When a spore lands in the right environment, it germinates and sends out a small filamentous thread called a hypha. The hyphae quickly begin branching, forming a network known as mycelium.

Mycelium, the hidden part of the fungus, is a network of thin fungal strands that explore the soil or any other substrate where fungi are growing and secrete digestive enzymes onto their food source. Mycelium plays a crucial role in fungal reproduction. The mycelia of two compatible fungi can fuse together, allowing the cells of each fungus to combine and their DNA to mix. After fusing, the cells end up in new spores held inside (like in truffles) or exposed outside (like in mushrooms) the fungus' reproductive structures.

Mycelium can also be propagated vegetatively from germinated spores, allowing spawn makers to multiply the culture for spawn production. Commercial mushroom farmers purchase spawn from companies specializing in its manufacture. Spawn is the commercial term for mycelium colonizing a prepared substrate, such as straw, wood chips, or sawdust, and forms the backbone of all large-scale mushroom growing operations.

Mushroom spawn, purchased from commercial labs, is mixed with the substrate. The mixture is transferred to several hundred beds or trays. Casing, made of peat moss, is spread over the mushroom bed, serving as a reservoir to hold in moisture. During the pinning stage, “pins” of mushrooms push up through the casing. Mushrooms are harvested by hand throughout a 16-35 day cycle.

Environmental factors

Mushrooms are the fruiting bodies of fungi, and their growth is influenced by various environmental factors. They can adapt to growing in different places, both indoors and outdoors. However, their growth is significantly influenced by temperature, humidity, and moisture levels in their surroundings.

Temperature plays a crucial role in mushroom formation. In nature, changes in temperature can trigger the mycelium to start fruiting and developing into mushrooms. In controlled indoor environments, temperature is carefully regulated to optimize mushroom growth. The ideal temperature for growing mushrooms is around 70 degrees Fahrenheit, and they thrive in high humidity conditions, typically above 90%.

Humidity and moisture are also critical factors in mushroom formation. Casing, made of materials like peat moss, plays a vital role in maintaining the necessary humidity and moisture levels. It acts as a reservoir, holding water and preventing the compost from drying out. Providing adequate moisture to the casing early on enhances yield potential.

Additionally, mushrooms require specific substrates or growing media to form. These substrates can be synthetic compost made from wheat or rye straw, hay, crushed corn cobs, cottonseed meal, cocoa shells, or gypsum. Alternatively, manure-based compost can be used, derived from stable bedding or poultry litter. The substrate provides the necessary nutritional balance of organic material for mushroom growth.

Environmental conditions also influence the growth rate of mushrooms. Mushrooms have a rapid growth rate, doubling in size within 24 hours. Once they are visible, they can be harvested in as little as four days or left to grow larger. The entire growth cycle, from spawning to harvest, typically ranges from 16 to 35 days, depending on the mushroom variety.

The role of casing

Casing is a crucial component in mushroom farming, serving as a top dressing applied to the spawn-run compost on which mushrooms eventually form. It is typically made of peat moss, a substrate that provides a nutritional balance of organic material necessary for mushroom growth. This substrate, or growing mix, is distinct from soil, which is the typical growing medium for field crops.

Another important function of casing is providing structural support for developing mushrooms. As mushrooms grow, they need a stable foundation to anchor themselves and facilitate proper development. The casing layer offers this support, helping mushrooms grow upright and maintain their shape. This is especially crucial for larger mushroom varieties.

Furthermore, casing plays a protective role by resisting structural breakdown following repeated watering. Mushroom farming involves frequent irrigation, and the casing layer helps maintain the structural integrity of the growing medium, preventing it from eroding or breaking down. This ensures that the mushrooms have a stable environment to grow and prevents the compost from washing away.

Lastly, casing creates an optimal environment for the formation of rhizomorphs, which are thick strings that form when fine mycelium fuses together. Mushroom initials, primordia, or pins develop on these rhizomorphs, so a sufficient casing layer ensures the successful formation of mushrooms. Overall, the casing plays a critical role in providing the necessary water, support, and structural integrity for mushrooms to thrive.

The growth process

Initiation

The process of mushroom growth usually begins in a laboratory, where sterile conditions are maintained. The spores, or natural seeds of the mushroom, are inoculated into sterile cereal grains like millet, rye, or wheat and incubated until they develop into a viable product. These grains become "spawn," which can then be sown.

Preparation

At the farm, the grower prepares the basic growing medium for mushroom production, known as the substrate. The substrate provides a nutritional balance of organic material to serve as food for the mushrooms. Two types of starting material are generally used for mushroom substrate: synthetic compost (consisting of wheat or rye straw, hay, crushed corn cobs, cottonseed meal, cocoa shells, and gypsum) or manure-based compost (made from stable bedding or poultry litter).

Mixing and Spreading

The mushroom spawn is then mixed with the substrate. The mixture is transferred to beds or trays. A top dressing, known as casing, is spread over the mushroom bed. The casing is usually made of peat moss and acts as a reservoir to hold in moisture, which is essential for mushroom development.

Growth

The spawn-run compost provides a conducive environment for the mushrooms to form and grow. During this stage, the mycelium (thin, thread-like cells) grows, gathering nutrients and water from its surroundings. When it encounters another compatible mycelium, it fuses and forms a secondary mycelium, which continues to grow until it receives signals like heavy rainfall or temperature changes to start fruiting. The mycelium then condenses into the earliest stage of mushrooms, known as primordia or pins.

Fruiting and Harvesting

With the right conditions, the primordia develop into fully formed mushrooms, complete with structures to produce spores and continue the life cycle. Mushrooms are typically harvested by hand throughout a cycle that can range from 16 to 35 days, depending on the variety.

Mushrooms have a rapid growth rate, doubling in size within 24 hours. They can be picked within four days of appearing or left to grow into larger sizes. The ideal growing conditions for mushrooms are around 70 degrees Fahrenheit and greater than 90% humidity.

Harvesting and storage

Harvesting mushrooms at the right time is crucial to maximising their potency and quality, as well as ensuring that subsequent fruiting or flushes are successful. Generally, mushrooms are ready to be harvested when the partial veil, the membrane that connects the cap to the stem, begins to break down. This is an indicator that the mushroom has reached maturity but has not yet released spores, which can compromise subsequent cleaning and handling, as well as future flushes. In a mossy forest floor habitat, it is best to gently push the mushroom at the base of the stalk and pull it up from the forest floor. Other mushrooms, such as the oyster mushroom and the cauliflower fungus, occur on the trunks and stumps of trees and must be cut off with a knife.

When harvesting, it is important to keep your collecting basket or bag free from any dirt. You can use baskets, cloth bags, or even recycled plastic bags for harvesting, as long as you immediately place the mushrooms in a packing basket (a mesh-sided container or wicker basket) when you return to your vehicle. If you store mushrooms in a plastic bag, they will sweat moisture and the decaying process will be accelerated.

To clean wild mushrooms, thorough washing may be necessary to remove any earth and sand. To store fresh mushrooms, wipe each mushroom clean and trim the stem bottom off. Only cultivated mushrooms should be eaten raw. For tough stems like shiitake, you can trim the whole stem and use it as a flavour base for stock.

Fresh mushrooms will usually stay fresh for seven to ten days. However, if you dry them, they can be stored for months. To dry mushrooms, place them on a rack or mesh in a single layer, without stacking them, making sure there is enough space between them for air circulation. You can also put them on kitchen paper. If they are very large specimens, the best option is to cut them into sheets and let them dry.

Once dried, store the mushrooms in airtight containers with desiccants, in a cool, dry, dark place. Refrigeration is recommended for long-term storage, and exposure to moisture should be avoided. Pickling with salt, vinegar, and sugar is another relatively safe method of preservation, although it is a complicated process.

Frequently asked questions