How to Grow The Prince (Agaricus augustus)

• 1 lb dry whole oat groats
• Water for soaking and simmering
• Large pot for simmering
• Strainer or colander
• Quart mason jars with injection ports and 0.2-micron filter lids, OR Out-Grow sterilized Oats Mushroom Spawn Bag (skip this step if using pre-sterilized bags)
• Pressure cooker rated for 250°F / 15 PSI

Measure 1 lb of whole oat groats into a large bowl and cover with cold water by several inches. Soak for 12–18 hours at room temperature — this fully hydrates the kernels and reduces sterilization time. After soaking, drain the oats into a strainer and transfer to a pot of fresh water. Simmer at 200–212°F for 10–20 minutes, stirring occasionally, until the kernels are fully swollen and soft through the center but not burst open or mushy. Drain immediately and spread on a clean towel for 20–30 minutes until the surface is visibly dry — wet surface grain dramatically increases contamination risk. Load the surface-dry oats loosely into quart mason jars, filling to the shoulder. Seal with injection-port lids and process in a pressure cooker at 250°F (15 PSI) for 90–120 minutes. Allow pressure to drop naturally before opening. Let jars cool fully to room temperature — at least 8 hours — before liquid culture inoculation. If using Out-Grow pre-sterilized oat spawn bags, the sterilization is done for you; skip directly to Step 2.

• Agaricus augustus liquid culture syringe
• Sterilized grain jars from Step 1 (or Out-Grow grain bags)
• 70% isopropyl alcohol and paper towels
• Still-air box or flow hood
• Butane lighter or alcohol lamp

Work inside a still-air box or in front of a flow hood. Wipe all surfaces and the outside of your liquid culture syringe with 70% isopropyl alcohol. Flame-sterilize the needle until red-hot, let it cool for 10 seconds, then wipe with an alcohol-dampened paper towel. For each quart jar, inject 2–4 mL of Agaricus augustus liquid culture through the self-healing injection port, targeting two or three spots distributed around the top of the grain. For Out-Grow bags with a 0.2-micron filter patch and self-healing injection port, inject 5–10 mL per bag directly through the port — no sealing required. Gently shake each jar or bag to distribute the liquid culture evenly through the grain. Store inoculated jars and bags at 70–75°F in a location without direct light.

• 3 lbs dry wheat straw, chopped to 3–4 inch lengths
• 2 lbs composted horse or cow manure (bagged, fully composted)
• 8 oz fine hardwood sawdust or wood chips
• Water to cover mushroom substrate
• Large stock pot or turkey fryer with thermometer
• Large plastic bags or covered bins for pasteurization
• Alternatively: Out-Grow 50/50 Horse Manure and Straw Mix Mushroom Substrate or Manure-Based Mushroom Growing Substrate 5 Pound Bag (skip pasteurization if using pre-pasteurized bags)

Combine chopped wheat straw, composted horse or cow manure, and hardwood sawdust in a large stock pot or turkey fryer. Cover with water and heat to 140–160°F. Hold that temperature for 60–120 minutes, stirring occasionally, to pasteurize the mushroom substrate. Do not exceed 160°F — higher temperatures push pasteurization toward sterilization and can destroy beneficial micro-organisms that support Agaricus mycelium. After pasteurizing, drain off excess water and test moisture: a firm squeeze of a handful of mushroom substrate should yield a few drops but not a stream. Spread the mushroom substrate on clean plastic sheeting or into a covered container to cool. Let it cool fully to room temperature — below 75°F — before spawning. If using Out-Grow pre-pasteurized manure mushroom substrate bags, they arrive ready to use; no heating is needed.

• Fully colonized grain jars or bags from Step 2
• Pasteurized bulk mushroom substrate from Step 3
• Clean grow tray or plastic tote (at least 6 inches deep)
• 70% isopropyl alcohol for wiping hands and surfaces

Wipe down your tray and all surfaces with 70% isopropyl alcohol. Break the colonized grain spawn into small clumps directly from the jar or bag. Distribute grain spawn evenly over the pasteurized bulk mushroom substrate at a rate of 3–5% by weight — for 5 lbs of mushroom substrate, use roughly 2.5–4 oz of grain spawn. Mix thoroughly by hand or with a sanitized spoon until grain spawn is evenly distributed throughout the tray. Level the surface with a gloved hand or a clean board to a uniform depth of 3–4 inches. Cover the tray loosely with plastic wrap or a humidity tent to retain moisture and keep debris out. Do not seal the tray airtight — some gas exchange is needed during the spawn run.

• 1 cup coarse vermiculite
• 1 cup unfertilized sphagnum peat moss
• 1–2 tbsp agricultural lime (hydrated)
• Water to hydrate casing to field capacity
• Mixing bowl

Combine peat moss and coarse vermiculite in equal parts by volume in a mixing bowl. Add 1–2 tbsp of agricultural lime to raise the pH of the casing layer to approximately 7.0–7.5 — this discourages competitor molds that prefer acidic conditions. Mix in water gradually until the casing reaches field capacity: a firm squeeze should produce only a few drops. Pasteurize the casing layer by placing it in an oven-safe container at 180°F for 30 minutes if desired, then cool fully. Once cooled, spread the casing mix evenly over the fully colonized mushroom substrate in a layer 0.5–0.8 inches deep. Do not pack the casing down — it should remain loose and porous to allow gas exchange and pin formation. Mist the casing surface lightly with water.

• Fruiting environment held at 55–62°F (a basement, garage, or spare refrigerator at the upper setting works)
• Hygrometer to monitor relative humidity
• Spray bottle filled with clean water for misting
• Fan or vents for increased fresh air exchange

Move the tray to a cooler environment held at 55–62°F — a drop of 10–18°F from spawn-run temperature is required to trigger pinning in Agaricus augustus. Increase fresh air exchange by fanning the tray or opening vents twice daily to bring CO₂ levels down to approximately 1,000–1,500 ppm; elevated CO₂ will prevent pins from forming. Maintain relative humidity at 90–95% by misting the walls of the fruiting chamber, not the casing surface directly. Mist the casing lightly if the surface begins to look dry or cracked. Diffuse light is fine; bright direct light is not needed. Expect pins to appear 7–14 days after the temperature shift, though The Prince mushroom (Agaricus augustus) may be slower or less reliable than other species — if no pins appear after 3 weeks at correct conditions, the mushroom cultivation attempt may have stalled, which is normal for this experimental species.

• Clean, sharp knife or mushroom knife
• Cutting board
• Collection tray or basket

Harvest The Prince mushroom (Agaricus augustus) when the cap is still convex to nearly flat and the veil connecting the cap edge to the stem is just beginning to stretch but has not yet torn. At this stage, gills will be pink to light brown — once the veil fully breaks and gills turn dark chocolate-brown, quality declines rapidly. Use a sharp knife to cut each The Prince mushroom (Agaricus augustus) cleanly at the base of the stem rather than twisting or pulling. Twisting dislodges chunks of casing and compost, damaging the surface and reducing the number of pins that form in the next flush. After harvesting, gently remove any remaining stem stubs from the casing surface and mist lightly. Allow the tray to rest at fruiting temperature for 7–10 days between flushes, maintaining 85–90% RH. Rehydrate by surface-misting the casing — do not submerge or dunk manure-based mushroom substrate, as this creates anaerobic pockets.