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An Overview of Mushroom Farming at Mountain Meadow Mushrooms


Cultivation of the Agaricus (white or crimini) mushroom consists of six steps: Phase I composting, Phase II composting (Pasteurization), spawning, casing, pinning, and cropping.  These steps are described in their naturally occurring sequence.  Of course, there are other related activities necessary to support these basic steps.  Overall, it takes approximately 9 weeks to complete an entire growing cycle from filling the room to the final clean up after harvesting has ended.  The composting process on the wharf takes about 3 weeks prior to filling the room.



            Substrate is the growing medium that provides the nutrients needed for mushrooms to grow.  Mushroom composting is designed to change the nutrients e.g. ammonia bearing compounds found in the initial ingredients such as cotton seed meal into compounds that are available to and selective for mushrooms.  Composting produces a food source most suited for the growth of the mushroom to the exclusion of competing fungi and bacteria.  Since the mushroom mycelium does not have a chlorophyll system it must have a growing media that supplies the sugars and carbohydrates it needs as well as the correct moisture, oxygen, and nitrogen throughout the process or the nutrients will be inadequate to support the future mushroom crop.  Mushroom farmers use various recipes when formulating mushroom compost based on the availability and cost of ingredients in their area and the individual characteristics of their farms.  Generally speaking, however, mushroom farmers in the United States use straw based stable bedding containing horse manure.  A variety of other ingredients are added in order to develop the nutrition and structure of the developing compost.   The preparation of mushroom growing media recycles vast quantities of agricultural nutrients, especially stable bedding from horse farms and race tracks and/or poultry manure.  By utilizing these nutrient-rich materials productively, the mushroom farming community performs a service to the community in terms of minimizing waste disposal.  At Mountain Meadow we only utilize stable straw, cotton seed hulls, cotton seed meal, along with canola fines and gypsum to make our growing substrate.

            The preparation of mushroom compost occurs in two steps referred as Phase I and Phase II composting.  Phase I compost preparation occurs outdoors on a cement slab, referred to as a wharf.  A specially designed compost turning machine is used to mix and water the ingredients, and a front-end loader is necessary to move the ingredients on the wharf.  Mountain Meadow has two such loaders costing about $200,000+ each.

            Phase I composting begins on all mushroom farms with a preliminary or “pre-wet” step.  Piles of the straw are wetted down by pumping reclaimed wharf water on them.  This wetting step initiates the heating process, a result of the growth and reproduction of microorganisms naturally present in the bedding straw.  This serves to soften the hay or straw and makes it more water absorbent.  These piles are mixed several times to produce uniform starting compost.  The pre-wet stage may last from 5 to 15 days depending on the season and condition of the straw. At this point the manure in the straw has been consumed by many strains of bacteria that were naturally present in the bedding straw.

            Following the pre-wet stage, the materials are arranged in a long pile over which are spread nitrogen supplements and gypsum, and the pile or rick is wetted again and thoroughly mixed with a turning machine.  Aerobic composting continues after the pile is wetted and formed.

            The compost rick must be carefully created and managed.  Most compost ricks are roughly 5 to 6 feet wide, 5 to 6 feet high, and as long as necessary.  The rick must hold its shape, but be loose enough to allow for aerobic conditions throughout.  Turning and watering are done at approximately 2-day intervals based on internal temperature profiles.  We strive to reach temperatures of 170o to 180o F in the ricks.  Turning provides the opportunity to water and mix the ingredients, as well as to relocate the compost from the cooler exterior to the warmer interior and vice versa.  The aeration accomplished by turning is short-lived, so pile construction, structure, and contents are critical in promoting aerobic degradation.  The number of turnings and the time between turnings depend on the condition of the starting material, the season and the time necessary for the compost to heat up.

            Mountain Meadow is a technological leader in aeration of compost ricks.  This experimental system was installed approximately 5 years ago and has eliminated all anaerobic odors in the rick.  We are currently designing a similar system for our pre-wet piles.

            Water addition is critical.  Too much water will exclude oxygen by occupying the pore space and may lead to unnecessary loss of nutrients due to leaching.  Too little water can limit the growth of bacteria and fungi.  As a general rule, most of the water is added when the pile is formed and at the time of first turning, and thereafter water is added only to adjust the moisture content.  On the last turning of Phase I composting, water may be applied generously to carry sufficient water into Phase II.  Water, nutritive value, microbial activity and temperature are like links in a chain.  When one condition is limiting, the whole chain is often affected.

            Phase I composting lasts from 16 to 28 days, depending on the nature of the material at the start, its characteristics at each turn, and the season.  At the end of Phase I, the compost should have: a) a brown color; b) short, pliable straws; c) a moisture content from 68 to 74 percent; and d) a slight odor of ammonia.  When these conditions have been reached, Phase I composting is completed.



            Phase II continues the conversion of nutrients into a selective food supply for the mushroom.  First, pasteurization with live steam is performed to kill any insects, nematodes, competing fungi, or other pests that may be present in the compost.  Second, any remaining ammonia levels which formed during Phase I composting are eliminated.  Ammonia can damage mushroom spawn growth.  Phase II composting can be viewed as a controlled, temperature dependent, ecological process using air and steam to maintain the compost in a temperature range best suited for thermophilic organisms to grow and reproduce.  The growth of these thermophilic (heat-living) organisms depends on the availability of carbohydrates in a usable form and the presence of nitrogen, some of which is in the form of inbound ammonia.

            Mountain Meadow uses a single zone bed of shelf growing system where the compost is placed directly in the stationary beds, which are in a room used for all steps of crop culture.  This growing method is both energy efficient and minimizes the use of water in the growing process.  As a result we use less than half the water in our growing rooms compared with “tray” farms (90% of all farms in California are tray farms).  Hence we do not use as much clean up water or moisture replacement water due to evaporation.



            Spawning is the mushroom culture equivalent of planting seeds for a field crop.  However, mushrooms are “planted” using fungal mycelia rather than seeds.  Fungal mycelium propagated vegetatively is known as spawn (Latin expandere= to spread out).  Spawn making requires special “clean room” laboratory facilities to propagate mycelia, so the mushroom mycelia do not get mixed with the mycelia of other fungi.  Spawn making starts by sterilizing a mixture of cereal grains plus water and chalk; rye, wheat, millet, and other small grains are used.  Once the sterilized grain has bits of mycelia added to it, it is incubated to promote its full colonization.  Mushroom farmers purchase spawn from commercial laboratories around the U.S. and Europe.  The “seed” for a typical room at Mountain Meadow costs about $1,200.00.

            At our farm, spawn is thoroughly mixed into the compost using a special hydraulically powered spawning machine.  Once the spawn has been mixed throughout the compost, the compost temperature and the relative humidity in the growing room are managed to optimize mycelium growth.  The spawn grows out in all directions from a spawn grain.  The time needed for spawn to fully colonize the compost depends on the amount of spawn added and its distribution, the compost moisture, temperature, and the nature or quality of the compost.  A complete spawn run usually requires 10 to 14 days.



            Casing is a top-dressing applied to the spawn-run compost and is necessary for mushrooms to grow from the mycelia, which have grown throughout the compost.  Sugar beet lime mixed with peat moss and limestone are used as a casing material.  It is very critical that nematodes carried by dust not get into the casing material as that can cause major crop losses.  For this reason, dust control on our farm is very important.  Casing is not used to supply nutrients; rather, it acts as a water reservoir and provides a place where rhizomorphs form.  Rhizomorphs look like thick strings and form when the very fine mycelia grow together.  Casing must be able to hold moisture since moisture is essential for the development of a firm mushroom (a mushroom is 97% water).  During the period following casing, some water in a very fine spray must be applied intermittently to maintain the moisture level at its maximum water holding capacity before the mushroom pins form.



            Mushroom fruiting bodies or pins are small outgrowths from the rhizomorphs, which form, in the casing layer.  These fruiting bodies continue to grow larger through a button stage, and ultimately enlarge into a mushroom.  Harvestable mushrooms appear 16 to 21 days after casing.  Pinning affects both the potential yield and quality of a crop.  The management of water content of the casing, relative humidity, and carbon dioxide content of the air are all essential in determining crop yield and quality.  We must measure these and other factors like temperature and air flow 4 to 5 times in each room over a 24 hour period.



            The terms flush and break are names given to the repeated 3 to 6 day harvest periods during the cropping cycle.  They are followed by a few days when no mushrooms are available to harvest.  Once mature mushrooms are picked, an inhibitor to mushroom development is removed, and the next flush moves toward maturity.  The regrowth repeats itself in a 7 to 10 day cycle, and harvesting can be repeated as long as mushrooms continue to mature, though the yield decreases with each picking cycle.  Most mushroom farmers harvest for 21 to 35 days, but harvest can continue for as long as 150 days.  Temperature, water management, and ventilation continue to be critical parameters throughout the growing period.  At Mountain Meadow our harvest period is 21 days because of our integrated pest control program we eliminate or control phorid and sciarid flies.

            One deciding factor related to the length of harvesting time is the necessity to maintain low levels of disease pathogens and insect pests so as to minimize the use of pesticides.  Mushroom pests can cause total crop failure. These pathogens and insects are controlled by cultural practices coupled sometimes with the use of government approved pesticides.  Mountain Meadow focuses its efforts on physically excluding these insects from the growing rooms.  Shorter harvesting periods reduce the time for pests to become established and proliferate in the crop or growing rooms.  After the last flush of mushrooms has been picked, the growing room is closed off and a clean up performed.  The room is pasteurized again with steam at 150oF  for 18 – 24 hours.  This final step is designed to destroy any pests, which may be present in the crop or the growing room, thus minimizing the likelihood of infesting the next crop or other rooms.  This also results in a used compost free of weeds and other pests that can be used by gardeners and landscapers.



As discussed earlier generating a substrate for the growth of the mushroom crop is a critical first step in the mushroom culture for commercially growing mushrooms.  A poorly produced compost will invariably mean disappointing yields, losses and jobs.  So, good composting is the secret of growing good mushrooms in commercial quantities.

Because some materials used in this substrate are high in nutrients, Mountain Meadow takes care to keep these nutrients separated from the natural water resources because of cost and the environment.  Mountain Meadow stresses the use of “best management practices” to protect the environment and minimize the farm’s impact on neighbors.  Both industry and county inspectors have found that Mountain Meadow has a well-run operation for preparing substrate, an efficient growing process, orderliness of operation along with good housekeeping and waste management.  We have pledged ourselves to continue our lead in the industry in this regard.