How to Grow Horse Mushroom (Agaricus arvensis)
How to Grow Horse Mushroom (Agaricus arvensis)
Horse mushroom (Agaricus arvensis) is grown by inoculating sterilized grain with liquid culture to build grain spawn, then incorporating that spawn into a composted straw-and-manure substrate loaded into trays and topped with a casing layer to trigger fruiting. This species is significantly less domesticated than button mushrooms and behaves poorly on standard non-composted media — the compost-based substrate is not optional; replacing it with sawdust or raw straw will stall colonization entirely.
Horse Mushroom (Agaricus arvensis): Indoor Tray Method
Horse Mushroom Equipment — Indoor Tray on Composted Substrate
| Item | Specification / Notes |
|---|---|
| Pressure cooker or autoclave | Capable of holding 15 PSI |
| Mushroom grow bags with filter patch | Polypropylene, 0.2-micron filter; size appropriate for 1 lb dry grain |
| Horse mushroom liquid culture syringe | Agaricus arvensis liquid culture — see inline link in Step 1 |
| Grain (wheat berries or rye) | 1 lb dry per spawn bag |
| Composted wheat straw | Phase II pasteurized, ammonia-off; 5 lbs per standard tray |
| Horse manure or poultry manure | Composted and aged; 2 lbs per standard tray |
| Gypsum | Agricultural grade; ¼ lb per standard tray |
| Peat-based casing material | 1 to 1.5 inches depth per tray |
| Lime (hydrated) | For pH adjustment of casing to 7.5–7.8 |
| Growing tray | Flat-sided, minimum 4 inches deep; one per substrate batch |
| Thermometer (probe or infrared) | For monitoring compost and air temperatures |
| Spray bottle or ultrasonic humidifier | For maintaining 90–95% relative humidity during fruiting |
| Hygrometer | For tracking relative humidity |
| Still-air box or laminar flow hood | For inoculation work |
| Isopropyl alcohol (70%) | Surface sterilization |
- 1 lb dry wheat berries or rye berries
- Water for soaking and simmering
- 1 tsp calcium carbonate (food-grade)
- 1 mushroom grow bag with 0.2-micron filter patch
- Pressure cooker rated to 15 PSI
Scale-up: for 3 bags use 3 lbs grain; for 5 bags use 5 lbs grain.
Rinse the grain, then soak in cold water for 12 hours. Drain, transfer to a pot, cover with fresh water, and bring to a simmer. Simmer for 15–20 minutes until kernels are fully hydrated and swollen but intact — do not allow skins to split. Drain thoroughly, spread on a clean surface or tray, and allow to air-dry until the surface of each kernel feels dry to the touch with no visible moisture — the inside should remain moist. Toss with the calcium carbonate, load into the grow bag, fold the top twice, and secure with a zip tie or tape. Sterilize at 15 PSI for 90–120 minutes. Allow the pressure cooker to drop to zero naturally, then remove bags and allow to cool completely to room temperature — at least 8 hours — before inoculating. Out-Grow sells Agaricus arvensis horse mushroom (Agaricus arvensis) liquid culture ready to inject: Horse Mushroom Agaricus arvensis.
→ Ready for Step 2 when grain bags are fully cooled and the exterior of the bag feels room temperature to the touch.Start with this culture — Agaricus arvensis
For one standard tray (approximately 7 lbs finished wet mushroom substrate):
- 5 lbs wheat straw (dry weight), pre-composted or sourced pasteurized
- 2 lbs composted horse or poultry manure
- ¼ lb agricultural gypsum
- Water as needed to reach field capacity
- One growing tray, minimum 4 inches deep
Scale-up: for 3 trays multiply all ingredients by 3; for 5 trays multiply by 5.
Combine the straw, composted manure, and gypsum in a large container and mix thoroughly. Add water gradually while mixing, checking moisture after each addition by squeezing a handful tightly — the mushroom substrate is at correct field capacity when one or two drops express from the mass, not a stream. Transfer the mushroom substrate to an oven-safe pan or heat-tolerant container. Pasteurize at 140–145°F for 2–4 hours; maintain this temperature range throughout — do not exceed 160°F. After pasteurization, lower the temperature and hold at 120–128°F until the substrate no longer smells of ammonia, which indicates conditioning is complete. Allow the pasteurized mushroom substrate to cool to below 75°F before spawning. Out-Grow also carries a ready-made 50/50 Horse Manure and Straw mushroom substrate if you want to skip this preparation step.
→ Ready for Step 3 when the mushroom substrate smells earthy (not of ammonia) and has cooled to below 75°F throughout.- Cooled, sterilized grain bag from Step 1
- Agaricus arvensis horse mushroom liquid culture syringe — 3–5 cc per 1 lb bag
- Alcohol-flamed needle and swabbed injection port
- Still-air box or laminar flow hood
Inside a still-air box or under a laminar flow hood, wipe the injection port of the grain bag with 70% isopropyl alcohol. Inject 3–5 cc of horse mushroom (Agaricus arvensis) liquid culture into the bag through the port. Seal the hole with micropore tape if the bag does not have a self-healing port. Set aside at 74–77°F in darkness. Once the grain bag shows visible white mycelial growth throughout — typically taking longer than standard mushroom species — squeeze and knead the bag fully until all grain separates completely and no clumps remain. Working in a clean environment, open the bag and distribute the colonized grain evenly across the surface of the cooled mushroom substrate in the tray before mixing it in. Mix until no isolated pockets of grain remain separate from the substrate and coverage is uniform. Do not inoculate warm mushroom substrate — let it cool fully before adding spawn. Use a spawn rate of approximately 1–2% fresh spawn by weight of wet substrate.
→ Ready for Step 4 when grain spawn is thoroughly mixed into the composted mushroom substrate with no dry pockets visible at the surface.- Spawned tray from Step 3
- Loose cover (damp newsprint or breathable plastic sheeting) to retain humidity
- Thermometer to monitor substrate temperature
Cover the tray loosely to retain moisture while still allowing minimal gas exchange. Maintain the substrate temperature between 74–77°F throughout the spawn run — do not allow substrate temperature to exceed 80°F, as this can damage or kill mycelium. Keep the space dark or use very low light. Mist lightly if the surface begins to dry out, aiming for roughly 90% relative humidity around the tray. Horse mushroom (Agaricus arvensis) is a slower colonizer than most cultivated mushroom species; expect the spawn run to take longer than typical Agaricus mushroom cultivation timelines. Do not rush this stage or add more spawn — allow the mycelium to run fully at its own pace.
→ Ready for Step 5 when the compost surface is covered uniformly with white mycelial growth and no bare dark patches remain.- Peat moss — enough for a 1–1.5-inch layer across the tray
- Hydrated lime — sufficient to adjust casing pH to 7.5–7.8
- Water to bring casing to correct moisture
Mix peat moss with a small amount of hydrated lime and water until the casing holds moisture without dripping and pH tests between 7.5 and 7.8. Apply the casing evenly across the fully colonized surface of the compost tray at a depth of 1–1.5 inches. Level gently — do not press firmly, which would compact the peat and restrict mycelial growth upward. Mist the surface lightly and cover loosely. Maintain the same 74–77°F temperature and 90% relative humidity during casing colonization.
→ Ready for Step 6 when the casing layer surface shows fine white mycelial threads threading through the peat, typically indicating the casing is colonized and ready for fruiting conditions.- Colonized and cased tray from Step 5
- Environment capable of holding 60–65°F air temperature
- Hygrometer maintaining 90–95% relative humidity
- Fresh air exchange (FAE) — fan or venting to reduce CO₂
Once the casing is colonized, reduce the air temperature to 60–65°F. This temperature drop is the primary fruiting trigger for horse mushroom (Agaricus arvensis) and must be maintained — the compost will remain a few degrees warmer than the ambient air. Maintain 90–95% relative humidity by misting gently and frequently. Increase fresh air exchange (FAE) compared to the colonization phase — elevated CO₂ suppresses pinning, so fan the tray area or vent the space for several minutes at least twice daily. Pins will appear as small, round white buttons emerging through the casing; they are white to cream in color. Do not disturb the casing surface once pins begin to form.
→ Ready for Step 7 when small white pin clusters are visibly breaking through the casing surface.- Clean hands or thin gloves
Harvest horse mushroom (Agaricus arvensis) while the cap is still convex, gills are pink to light brown, and the veil connecting the cap edge to the stem remains intact or has just broken. Do not wait until caps flatten fully, margins lift, or gills darken to chocolate brown — spore deposit increases sharply at that stage. To harvest, grip the mushroom at the base, twist gently and lift upward in one motion to remove cleanly without gouging the casing. Avoid pulling straight up forcefully, which can tear out chunks of casing and reduce subsequent flush yields. Remove any spent stubs left in the casing immediately after each harvest session.
→ Ready for Step 8 once the tray has been fully harvested and all spent stubs removed from the casing.- Harvested tray from Step 7
- Clean water for surface rehydration
After harvesting, mist the casing surface lightly to restore moisture lost during the fruiting period — do not flood or waterlog the tray. Return the tray to fruiting conditions: 60–65°F air temperature, 90–95% relative humidity, and regular fresh air exchange. Allow the tray to rest and recover. Experimental trials with horse mushroom (Agaricus arvensis) have documented up to four flushes from a single tray, though each subsequent flush may produce fewer fruitbodies. Discontinue the tray when the casing shows no new pin formation after an extended recovery period, or when contamination becomes visible in the spent substrate.
→ Tray is spent when no new pins emerge after a full recovery rest period and the compost appears exhausted or patchy.Horse Mushroom (Agaricus arvensis) Troubleshooting
Horse mushroom (Agaricus arvensis) mushroom cultivation runs into problems most often at the colonization stage, because this species is demonstrably finicky about its mushroom substrate in a way that most gourmet mushroom species are not. If your grain spawn shows very slow or no growth after inoculation with horse mushroom (Agaricus arvensis) liquid culture, the most likely causes are an underpowered or struggling mushroom culture — Agaricus arvensis is known to grow poorly in standard nutrient liquid culture without compost extract, which means the liquid culture syringe may produce thin, nearly clear growth rather than the vigorous cloud visible in oyster or shiitake inoculations — and a grain bag temperature that has slipped above 80°F. Keep grain spawn for horse mushroom (Agaricus arvensis) at 74–77°F and expect a slower colonization timeline than for other Agaricus mushroom cultivation projects. If the liquid culture appears stagnant or the grain shows no growth after two weeks, begin a fresh inoculation from a new syringe before proceeding.
Contamination is the primary risk in horse mushroom (Agaricus arvensis) mushroom cultivation because the composted mushroom substrate is a rich medium that competitor molds find equally hospitable. Green patches — ranging from bright to dull green — appearing over the compost or casing are almost certainly Trichoderma, signaling that the Phase II pasteurization of the mushroom substrate did not reach or hold 140–145°F for the full 2–4 hours, or that hygiene during spawning was inadequate. Blue-green or grey powdery patches on the casing indicate Penicillium or Aspergillus, typically entering through improperly pasteurized casing material or dry spots that crack and expose the tray to airborne spores. Slimy, darkened areas on the casing surface with an off smell point to bacterial blotch, usually caused by standing surface moisture combined with high humidity and poor airflow — water earlier in the day and increase fresh air exchange (FAE) to allow surface drying between mistings. Trays with heavy contamination spread across more than one-third of the surface should be removed and discarded outside the grow space immediately to protect other batches.
Pinning failure is the second most reported problem in Agaricus arvensis mushroom cultivation. If white mycelium reaches the casing surface but no pins form, the most common cause is temperature that has not been reduced sufficiently — horse mushroom (Agaricus arvensis) requires the air temperature to drop to 60–65°F to trigger fruiting, and partial or inconsistent drops will not produce pins reliably. CO₂ buildup from insufficient fresh air exchange (FAE) is the second likely cause, particularly in enclosed grow tents where venting is passive only. Finally, a casing layer that is either too dry — below the one-drop-on-squeeze moisture threshold — or waterlogged will both prevent pinning; aim for evenly moist peat throughout the 1–1.5 inch depth. Note that reliable fruiting of horse mushroom (Agaricus arvensis) under home mushroom cultivation conditions is not yet standardized in the documented literature — growers should treat this as an experimental mushroom cultivation project and approach each tray as an opportunity to refine their process.
Shop 50/50 horse manure and straw mushroom substrate at Out-Grow.
How to Grow Agaricus arvensis
Questions and Answers About Agaricus arvensis Cultivation
Q. Can horse mushroom (Agaricus arvensis) be grown using a liquid culture syringe?
A. Yes, but with important caveats. Agaricus arvensis is known to grow poorly in standard nutrient liquid culture media — growers report little to no visible turbidity in the liquid culture syringe compared to other species, and inoculated grain spawn may colonize slowly or stall. The liquid culture works best when inoculated into sterilized grain that is then transferred into a fully composted mushroom substrate. Do not interpret a thin or slow liquid culture as a failed mushroom culture without giving the grain spawn adequate time at 74–77°F in darkness.
Q. Why does horse mushroom colonization of grain spawn take so long compared to other mushroom cultivation projects?
A. Agaricus arvensis is a less domesticated species than the button mushroom or portobello, with no established commercial mushroom cultivation standard. It grows more slowly on grain spawn than oyster, shiitake, or other gourmet species, and its mycelium appears to benefit from compost-derived nutrients that are absent in plain grain. Mushroom cultivation of horse mushroom (Agaricus arvensis) requires patience — treat grain spawn colonization as a slow spawn run and avoid adjusting temperature or opening bags prematurely, as disruptions during this stage increase contamination risk significantly.
Q. What is the correct mushroom substrate for growing horse mushroom (Agaricus arvensis)?
A. The correct mushroom substrate is a composted straw-and-manure mix — typically wheat straw composted with horse or poultry manure, with gypsum added as a conditioner. This mushroom substrate must go through Phase II pasteurization (140–145°F for 2–4 hours) and ammonia conditioning before spawning. Raw straw, sawdust blocks, and standard wood-based mushroom substrates are not compatible with Agaricus arvensis mushroom cultivation and will result in failed or contaminated trays. Out-Grow carries a pre-made 50/50 horse manure and straw mushroom substrate ready for pasteurization and spawning.
Q. How many flushes can I expect from a horse mushroom tray?
A. Experimental mushroom cultivation trials with Agaricus arvensis have documented up to four flushes from a single tray under controlled conditions. However, because horse mushroom (Agaricus arvensis) cultivation is still experimental and no standardized home mushroom cultivation protocol exists, flush count and yield per flush will vary considerably based on mushroom substrate quality, casing moisture management, and fruiting environment consistency. Maintain fruiting conditions — 60–65°F, 90–95% relative humidity, and regular fresh air exchange — between flushes, and rehydrate the casing surface gently after each harvest.
Q. My horse mushroom mycelium reached the casing but no pins are forming — what went wrong?
A. Pinning failure in Agaricus arvensis mushroom cultivation most commonly results from insufficient temperature reduction, CO₂ buildup, or incorrect casing moisture. The air temperature must drop to 60–65°F — partial drops to 68–70°F are typically not enough to trigger pinning in this species. Increase fresh air exchange (FAE) significantly by fanning or venting the grow space at least twice daily to remove accumulated CO₂. Check the casing layer by squeezing a small handful — it should express one or two drops, not be dry to the touch. If all three conditions are correct and no pins appear after two weeks of fruiting conditions, the tray may have colonized unevenly or the mushroom substrate may have been too depleted at the time of casing.
Q. How do I store fresh horse mushroom after harvest?
A. Fresh Agaricus arvensis should be refrigerated as promptly as possible after harvest. Store in ventilated containers or paper bags at 32–39°F — sealed airtight containers trap moisture and accelerate deterioration. Based on general Agaricus mushroom post-harvest standards, expect a usable shelf life of approximately 5–10 days depending on how mature the fruitbodies were at harvest and how carefully they were handled. Mushrooms harvested before the veil fully breaks will keep longer than over-mature specimens. No drying studies specific to Agaricus arvensis have been published.