Regardless of the location, species, and method, there are seven main steps to the process of growing mushrooms. Keep in mind that while the steps below are generally true for all methods and species, there is quite a bit of detail depending on the species of mushrooms and growing medium you choose. Here we overview the key points to each step in the process. From this, you can see which strategies emerge for your enterprise that align with your mission and goals.
- Strain selection
- Substrate preparation
- Inoculation
- Spawn run
- Initiation
- Fruiting
- Harvest
Fact Sheet Overview
Strain Selection
Strain selection is one of the commonly overlooked but extremely important factors to successfully growing mushrooms. The strain is similar to a plant breed. Individual species of mushrooms like pleurotus ostreatus (commonly known as Oyster mushrooms) will have thousands of different strains or expressions of the genetic make- up. This is really easy to illustrate using humans. All humans are in the species Homo sapien but each individual human or “strain” of Homo sapien has a particular set of genetic information that influences all sorts of factors. Factors like size, strength, color, health, longevity, maybe even personality are influenced by the particular strain.
With mushrooms very important factors are influenced by the strain, including:
- how fast the mycelium grows
- color of the fruiting body
- shelf life of the fruiting body
- fruiting temperature
- size and quantity of fruiting bodies (yield)
The simple act of selecting the right strain can easily triple yields! Fungi Ally has studied this with cordyceps and shiitake mushrooms, both species showing large variables in the correlation between strain and yield.
In the cordyceps trials, when controlling for strain, yield varied from 3.5 grams per jar to 11.8 grams per jar. The shiitake trials were done on two different substrates: logs and supplemented sawdust blocks. Both had large variables in yield depending on the strain selected. For logs, average yield varied from .5 – 2.3 pounds per log, with strain LE-46 performing the best. On sawdust, yields varied from .3 – 1.4 pounds per block with strain 3782 performing the best and strain 46 doing the worst. This illustrates how some strains are developed to consume certain substrates. This data also shows that proper strain selection can be the difference between a profitable farm and going out of business.
Any spawn or ready-to-fruit block provider will provide high quality strains selected for maximum performance. If you are producing strains in house, its best to trial 4-5 strains side by side to see which one does best. Every couple of months it is good to experiment with a new strain to see if improvements can be made. Growers are constantly trading strains to look for the best combination of strain and substrate. Once the strain is selected, spawn can be ordered or produced.
“Spawn” is intentionally cultivated mycelium from a known species that has been isolated, generally in a sterile lab facility, to ensure it is free of outside contaminants. For most growers, spawn is purchased from a supplier, much like a vegetable grower buying seed from a company. Spawn comes in three common forms: grain, sawdust, and plug.
Substrate Preparation
All organic material is broken down by fungi and bacteria. The job of a cultivator is to get the mycelium of the desired edible mushroom established before anything else can take hold. For the highest chance of success, the material or “substrate” needs to be prepared. The word substrate refers to any material that is a food source for mushroom mycelium. The way the substrate is prepared will be guided by the type of substrate and the equipment that is available. Preparation includes ensuring moisture content is optimal and the substrate is clean of contaminants, mixing substrates if desired, and sometimes placing the substrate into a bag. Common substrates include logs, stumps, woodchips, straw, sawdust, coffee grounds, grain hulls, and other carbon-rich materials. Some species are very particular about the type of substrate used, while others are flexible.
There are several options for substrate preparation depending on the grower’s goals, available resources, and infrastructure on the farm or growing site. Each technique carries a set of pros and cons. In general, lower tech methods focus on using low nitrogen, high carbon materials like oyster mushrooms on straw or shiitake on logs. The higher tech methods generally use higher nutrient materials to increase speed of colonization and yield. High tech methods utilize sterilization methods and employ a mix of sawdust or hardwood fuel pellets plus a high nitrogen supplement (grain or seed hulls, coffee grounds etc).
Below is a review of some common substrates and how to prepare them:
Logs, wood chips, and stumps
On these wood-based substrates, it takes some time for native fungi to get established. It is best to inoculate these substrates within a month of cutting. If temperatures are below freezing, fungi are not active so these materials can be stored until temperatures are consistently in the 40s – 50s at night. There is a common misconception that these substrate materials need to sit for two weeks until inoculation, but this is not true. Trees deal with fungal infections primarily by walling off the infection, not through antifungal compounds. Once inoculated, the mycelium takes upwards of 10 days to jump off the spawn into the substrate. At the same time, the wood is losing any ability to wall off an infection. There is no harm in inoculating stumps, logs, or wood chips immediately after harvesting. As long as the tree is healthy, no further preparation is needed. When using logs, if there are cracks large enough for a dime could be placed into, soak the log for 24 hours before inoculation.
Straw
Straw is a very common substrate for home and beginning commercial growers. It does not produce the best yield and can be difficult to work with as it needs to be chopped for optimal yield. However, there are pre-treated and chopped bags of straw available at local stores and online which can be used successfully. For most farms it is likely more in line with their values to source local substrates and support the local economy. Oyster mushrooms grow very well on straw and are the best species to grow on straw. The straw for oyster cultivation can be treated through fermentation, pasteurization, or a lime soak (covered in detail later in this booklet).
Coffee Grounds
Coffee grounds are an interesting substrate to use because they are effectively sterilized when they are used to produce coffee. Two important steps for successfully growing on coffee grounds are: 1) to inoculate the same day as brewing the coffee with them (or resteralize), and 2) to add sawdust to the spent coffee grounds. The nutrients in spent coffee grounds are much more readily available than in wood so ambient bacteria and fungi will start growing within two or three days. Because of this, the grounds should either be inoculated the same day they are used to make coffee or frozen until they can be inoculated. When inoculation does happen, grounds should be mixed with 50% sawdust to increase aeration. The sawdust, which has low nutritional value does not need to be treated.
Supplemental sawdust/wood pellets
This is what most specialty mushroom growers use for a fruiting substrate. Mixing sawdust or wood pellets with wheat bran or soybean hulls helps to achieve a high nutrient and cost effective food source for the mycelium. This requires steam sterilization and a lab for inoculation but produces the highest yields.
Grain
Grain is used as a high nutrient medium for rapid mycelial expansion. It is typically the step after mycelial growth on agar medium. Grain is too expensive and too high in nutrients for bulk substrate fruiting, but can be a good supplement to the sawdust or wood pellet base. It works well to boost yields by inoculating straw with large amounts of grain spawn. Grain also needs to be treated using pressurized steam and inoculated in lab conditions. It is very easy to get contamination on grain, so this is not a beginner substrate.
Agar
There are many different nutrient formulas to make petri plates that are extremely high in nutrients and easy for mycelium to grow on. This is extremely easy to contaminate and is only used in small quantities. For example, an agar substrate is used when storing a mushroom strain or tissue cloning from a mushroom. These must be sterilized and used in a lab-like condition. Sometimes you can get away with doing cloning work in makeshift lab areas, but it is not always successful.
The different substrates are “cleaned” or prepared by several different methods. The table below illustrates common treatment methods and for what substrates they are used.
| Method | Substrate | Materials Needed |
|---|---|---|
| Fresh | Logs, wood chips, straw at cold temperatures, hot compost | Source for substrate |
| Cold Ferment | Straw | Bucket, water, cinder block |
| Hot Pasteurization | Straw, compost, wood pellets, coffee grounds | Heat source, metal container, cinder block |
| Lime | Straw, wood pellets | Lime, water barrel |
| Atmospheric Steam | Supplemental sawdust, straw | Steam source and metal vessel |
| Pressurized Steam | Agar, grain, supplemented sawdust | Pressure canner pot, autoclave |
Each of the treatment methods below has its own set of positive and negative attributes. Growers may want to consider trialing several methods to see what works best for them.
| Method | Pro | Con |
|---|---|---|
| Fresh (logs, stumps, wood chips, straw) | – no equipment or additional treatment needed – easy to obtain and do – kid friendly | – long incubation time – fruiting yields compared to weight of the substrate are low |
| Cold Ferment (straw) | – no energy needed to heat water – very flexible | – STINKY! – time from soaking to inoculation varies with seasonal temps – success is variable |
| Hot Pasteurization (straw) | – reliable process for consistent yields on straw | – energy (propane, gas, electric) needed to heat water – lower yields compared to sterilization |
| Lime (straw) | – no energy needed to heat water – moderately reliable – if easy to obtain, lime is cheap ($10-15 per 50 lb bag) | – disposal of high pH water – can be challenging to acquire specific lime needed depending on location – lower yields compared to sterilization |
| Atmospheric steam (sawdust, wood pellets, grain) | – high-yield substrates – fast spawn run – effective for a variety of mushroom species – low cost for steam treatment | – energy intensive – requires specialized equipment hook-up – requires lab for inoculations – higher contamination than pressurized |
| Pressurized steam (sawdust, wood pellets, grain, agar) | – thorough sterilization – high-yield substrates – fast incubation time – effective for a variety of species | – energy intensive – expensive and infrastructure intensive – lab required |
Inoculation
This step of the process involves bringing the spawn into contact with the substrate to initiate its growth and development. Depending on the substrate, inoculation may entail drilling holes into a log, cutting wedges into a stump, or mixing spawn into individual bags. Outdoor methods can generally be done with little concern for introducing contaminants (mostly molds), while most of the indoor methods require the substrate to be inoculated in a sterile space to avoid contaminating the substrate. There are three factors in this step to consider:
- Where to inoculate
- Inoculation rates
- Spawn distribution
Where the process of inoculation happens depends on the nutrient level of the substrate. Low nitrogen, high carbon materials like logs, woodchips, and straw are fine to inoculate outside in the open air. During this form of inoculation it is helpful to be clean, using a clean table and clean hands but sterility is not a concern. Sterility becomes much more critical when using high nitrogen materials like agar, grain, and supplemented sawdust. These are easier for ambient microorganisms to grow on, so require the use of a lab-like setting for inoculations. On a small scale, it is possible to create low budget equipment to mimic a lab setting, but the success rate is lower. A commercial lab would include a separate space for lab use, cleaning and maintaining that space, using HEPA (high efficiency particulate air) filters, and equipment for steam treatment. If inoculations are done in a lab, being mindful of the seven vectors of contamination and implementing proper lab technique is critical.
Inoculation rates are a balance between economics and speed of spawn run. The cultivator is trying to get the desired edible mushroom mycelium established before anything else. The higher the inoculation rate or amount of spawn added to the substrate, the faster the mycelium will grow through the substrate. The substrate likely will not produce a larger amount of mushrooms, so a high inoculation rate means more money spent on spawn per pound of mushrooms. For a new cultivator, using a high inoculation rate can really boost the success rate. As the grower dials in the process of cultivation, they can start to lower the inoculation rate and observe if there is a difference in colonization time.
At Fungi Ally, this sort of trial was conducted on supplemented sawdust. Five pound bags of shiitake spawn were used to inoculate 20 five-pound bags of bulk substrate. These bags colonized in about 8 weeks. Slowly the amount of bags inoculated by the five pound bag of spawn was increased. At 30 five-pound bags of bulk substrate, colonization time was also 8 weeks and contamination rates did not go up. At -40 five-pounds bags, time until colonization was still 8 weeks, and little to no contamination increase was observed. At 50 five-pound bags both colonization time and contamination increased. The sweet spot was using one five-pound bag of spawn for every 200 pounds (40 bags) of substrate. The amount of money spent on spawn was decreased by 50% per pound of mushrooms. Saving .25 cents per pound of mushrooms adds up to a lot when growing 300 pounds per week – almost $4,000 per year!
Spawn distribution also impacts the speed of colonization. Two methods of distribution commonly used are “top spawning” and “through spawning”. Top-spawning is the process of adding the spawn on top of the substrate and letting it grow down. This process is good when inoculating full containers like mason jars that can not be shaken, or for low stakes cultivation. Through-spawning refers to shaking the substrate after inoculation to distribute the spawn throughout the substrate. This shortens the length of spawn run but adds a small amount of labor. In low tech methods like wood chip beds and straw tubes, through-spawning is achieved by layering the substrate and spawn like lasagna. Alternating between the spawn and substrate allows for faster colonization than putting all the spawn on top. In supplemented sawdust this typically means shaking the substrate after inoculation to distribute the spawn evenly.
The impact of top vs. through-spawning was inadvertently tested at Fungi Ally. One day, the crew was feeling like shaving some time off of the inoculation process. Instead of shaking after inoculating they left the spawn on top for 50% of the bags. Four shelves containing 40 bags each, were filled in the incubation room. The top two shelves were top-spawned, and the bottom two shelves were through-spawned. After 14 days the bottom two through-spawned shelves were fully colonized while the top spawned bags were closer to half colonized. By day 24, the top shelves were also fully colonized. In total it took ten extra days for the same amount of spawn to colonize a five pound bag of substrate. It saved a small amount of labor but was not worth the extra time in incubation. Not only did the top-spawned bags take up more space for a longer amount of time, but the extra time also left more opportunity for contaminants to take hold.
