How to Grow Button Mushrooms (Agaricus bisporus)

• 1 lb dry whole wheat or rye grain
• Water for soaking
• 1 sterilizable grain bag with 0.2 micron filter patch and injection port
• Pressure cooker
• White button mushroom (Agaricus bisporus) liquid culture — 1–2 cc per pound of hydrated grain

Scale-up: 3 lbs grain → 3 bags | 5 lbs grain → 5 bags

Rinse the grain, then submerge it in cold water and soak for 12–18 hours. Drain, then transfer to a pot and simmer on medium heat for 15–20 minutes until kernels are fully hydrated but not split open. Drain and spread the grain across a clean surface or sheet pan; let it air-dry until each kernel feels dry to the touch on the outside with no surface moisture — moist inside, dry outside. Wet-surfaced grain pressurizes poorly and creates anaerobic pockets that invite bacteria. Load the surface-dry grain into grain bags, filling each bag no more than two-thirds full. Fold and seal the bag top with a zip tie or heat seal, leaving the injection port accessible. Sterilize at 15 PSI for 90–120 minutes. Let bags cool completely to room temperature — at least 65 °F or below — before proceeding. Out-Grow sells White Button Mushroom (Agaricus bisporus) liquid culture ready to inject.

Flame-sterilize your needle, let it cool 10 seconds, then inject 1–2 cc of liquid culture per 1 lb bag through the self-healing injection port. Agitate the bag gently to distribute the inoculum. Place in a warm spot at 72–77 °F out of direct light.

→ Ready for Step 2 when grain is uniformly white and densely colonized with no green, black, or pink patches visible — typically 12–14 days.

• 3 lbs wheat straw (chopped or whole)
• 1 lb aged, composted poultry or horse manure (not fresh — free ammonia will kill mycelium)
• 3 tbsp gypsum
• Water — enough to bring mix to field capacity (a firmly squeezed handful leaves the palm barely damp, no dripping)
• Large stock pot or heat-safe container with lid

Scale-up: 3 trays → multiply each quantity by 3 | 5 trays → multiply by 5

Combine straw, manure, and gypsum in a large container. Add water gradually and mix thoroughly until the substrate reaches field capacity. Pack the mix into a large pot or heat-safe tub and cover. Pasteurize at 140–145 °F for 2–4 hours, holding temperature steady with an oven or water bath. After pasteurization, allow the substrate to cool uncovered in a clean space until it drops below 70 °F and any sharp ammonia smell has dissipated fully — residual ammonia above safe levels causes spawn failure. Out-Grow also carries manure-based mushroom substrate bags that arrive pre-pasteurized and ready to inoculate if you want to skip this step.

→ Ready for Step 3 when substrate smells earthy and composted with no sharp ammonia, temperature is below 70 °F throughout.

• Fully colonized grain spawn from Step 1 (1 lb colonized grain per single tray batch)
• Pasteurized compost substrate from Step 2
• Grow tray or plastic tub (12" × 18", 4–6" deep)
• Gloves and clean work surface

Before opening the grain bag, squeeze and knead the bag firmly until colonized grain breaks apart into individual kernels. Do not describe the size of pieces — work until no solid clumps remain inside the sealed bag. Never open the bag over substrate that is still warm. Spread the colonized grain evenly across the surface of the compost substrate before mixing in — do not dump it all in one area. Work the grain into the compost until no isolated grain pockets remain visible and spawn is distributed evenly throughout. The spawn rate for this single tray is approximately 3–4% by wet weight of the compost. Fill the grow tray with the inoculated compost to a depth of 3–4 inches, pressing gently to remove large air voids without compacting.

→ Ready for Step 4 when tray is loaded, spawn is evenly distributed throughout the depth, and the surface looks uniform with no concentrated grain patches.

• Colonized tray from Step 3
• Loose plastic sheeting or humidity dome to retain moisture
• Incubation space held at 72–77 °F

Cover the tray loosely with plastic sheeting or a dome to maintain humidity around 90% without sealing air out completely. Place the tray in a dark or low-light space at 72–77 °F. Check the tray every 2–3 days to confirm the compost surface is not drying out; mist lightly if needed. Do not flood or over-wet. Avoid hot spots above 86–90 °F — Agaricus bisporus mycelium dies at those temperatures and the colony will not recover. Keep the area clean and minimize air disturbance to reduce contamination risk.

→ Ready for Step 5 when the compost surface is uniformly white, fully knit with button mushroom (Agaricus bisporus) mycelium, and no bare brown patches remain — typically 12–14 days at 75 °F.

• 2 cups peat moss
• ½ cup agricultural limestone or hydrated lime
• Water — enough to bring the casing mix to 80–85% moisture (a squeezed handful glistens and barely drips)

Scale-up: multiply casing quantities proportionally to tray footprint

Mix peat and limestone together in a bowl. Add water gradually and work through the mix until it reaches the correct moisture — a firmly squeezed handful should feel heavy and glisten at the surface without water streaming out freely. Spread the casing mix evenly over the colonized compost in a layer ½–¾ inch thick. Do not press down hard; the casing layer must remain loose enough for mycelium threads to grow up into it. Once casing is applied, drop the room temperature from 75 °F down to 61–64 °F. Maintain relative humidity at 85–90%. Increase fresh air exchange by opening vents or lids slightly — elevated CO₂ suppresses pinning in Agaricus bisporus. Light is not required; commercial button mushroom cropping rooms operate at very low light.

→ Ready for Step 6 when fine white mycelium threads from button mushrooms (Agaricus bisporus) are visible rising 1–⅛ inch up into the casing layer surface — typically 7–10 days after casing application.

• Cropping room or grow tent capable of holding 61–66 °F
• Humidifier or misting bottle to maintain 85–90% RH
• Adequate fresh air exchange

Hold room temperature at 61–66 °F throughout the fruiting phase. Maintain RH at 85–90% by regular misting of the casing surface or running a humidifier — avoid pooling water directly on pins or caps, which invites bacterial blotch. Fan the tray briefly once or twice daily to exchange CO₂ for fresh air; Agaricus bisporus is particularly sensitive to high CO₂ during pinning and will produce long, thin stems with small caps if ventilation is inadequate. Mist irrigation should target the casing surface, not overhead directly onto developing mushrooms.

→ Ready for Step 7 when white, spherical pins ranging from 1–⅛ inch appear clustered across the casing surface — typically 7–10 days after the temperature drop.

• Clean hands or gloves
• Extra casing mix to fill harvest holes

Harvest button mushrooms (Agaricus bisporus) when caps are fully rounded and convex with the veil still intact — do not wait for the veil to tear or gills to become visible. Grip each mushroom at the base and twist gently with a slight upward pull to remove it and its stump in one motion. Remove the entire stump; leaving it in the casing creates a rot point that invites disease. Fill each harvest hole with reserved casing mix to maintain surface integrity. Harvest quickly when caps are at the right stage — open-gilled, flat-capped mushrooms decline rapidly in quality. A single flush typically runs 7–9 days from first visible pins to completion.

→ First flush complete when all buttons in a cluster have been picked or caps have flattened and veils begun to tear.

• Irrigation water for casing surface
• Continued temperature control at 61–66 °F

After the first flush, do not dunk or submerge the tray — button mushrooms (Agaricus bisporus) are re-watered through irrigation of the casing surface. Mist the casing thoroughly to restore moisture lost during cropping. Hold environmental conditions at 61–66 °F and 85–90% RH and continue fresh air exchange. The second flush usually peaks 7–10 days after the first; it typically produces the heaviest crop of the three flushes. A third flush follows at the same interval. Compost is spent when flushes drop to negligible yield with small, poor-quality mushrooms despite maintained conditions and adequate casing moisture.

→ Substrate is spent when the third flush yields fewer and smaller button mushrooms (Agaricus bisporus) despite stable conditions — typically after 5–7 weeks of total cropping.

Follow Method 1, Step 1 exactly — grain preparation, sterilization, LC inoculation, and colonization are identical. Out-Grow also carries sterilized grain bags ready to inoculate if you want to skip grain preparation: sterilized grain bags collection.

→ Ready for Step 2 when grain is uniformly white throughout — typically 12–14 days.

• 1 lb colonized grain spawn
• 1 × 5 lb manure-based pre-composted mushroom substrate bag from Out-Grow
• Grow tray or plastic tub

Break grain spawn down completely inside the sealed bag before opening — squeeze and knead until individual kernels separate. Open the pre-composted substrate bag and empty the contents into the grow tray to a depth of 3–4 inches. Spread colonized grain evenly across the entire substrate surface before folding it in. Mix thoroughly until no concentrated grain pockets remain. Out-Grow's manure-based mushroom substrate bags arrive pasteurized and pH-adjusted for Agaricus bisporus and require no additional conditioning.

→ Ready for Step 3 when tray is loaded and spawn is evenly distributed throughout.

Follow Method 1, Steps 4–8 exactly — colonization at 72–77 °F, casing layer application, temperature drop to 61–64 °F, fruiting environment maintenance, harvest technique, and flush recovery are identical. Pre-composted substrate bags eliminate the composting and pasteurization steps but require the same environmental parameters throughout colonization and cropping.

→ Ready for casing when compost surface is uniformly white — typically 12–14 days at 75 °F.