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Grain spawn in jars

Sterilised grain in jars, colonised as the spawn for a bigger substrate.

day 0a week oncolonised
Plate 4 of 16Grain spawn in jars, drawn for All the Tek.

What it is

Grain is a high-nutrient medium that mycelium expands through quickly. It is usually the step after agar, and growers also add it to straw in large amounts to boost yields1. Because it is so rich, it has to be sterilised with pressurised steam and inoculated in lab conditions, and it contaminates easily. Cornell’s guide says it is not a beginner substrate1.

The UF/IFAS method soaks seeds for about twelve hours, sterilises them in their containers at 15 psi for 30 minutes, inoculates them with pieces of a clean agar culture and incubates them at about 23 °C (73 °F) in the dark until fully colonised2.

Jars are the small-batch version. They are easy to see into, reusable, and fit in a home pressure cooker.

What it suits

Every species. Grain spawn feeds straw, sawdust, buckets and beds alike.

Equipment

  • Pressure cooker that holds 15 psi
  • Wide-mouth jars with filtered lids
  • Whole grain (rye, wheat, millet, sorghum and similar)
  • Clean agar culture or liquid culture
  • Still air box or flow hood

Time to first harvest

10–20 days to colonise. UF/IFAS gives about 10 to 20 days for grain spawn to fully colonise2.

Contamination and other risks

  • Bacterial and mould contamination; grain is very easy to contaminate1.
  • Fermentation from soaking the grain too long2.
  • Clumped grain that colonises unevenly if it is not shaken after cooking2.

Steps, in overview

  1. Soak. Soak the grain for about twelve hours; longer soaking causes fermentation2.
  2. Drain and fill. Drain well and fill jars, leaving room to shake.
  3. Sterilise. 15 psi for 30 minutes in the UF/IFAS method2. Many growers run longer for full jars.
  4. Shake and cool. Shake soon after removing the jars so the grain does not clump2, then let them cool fully.
  5. Inoculate. Add agar pieces or liquid culture inside the still air box2.
  6. Incubate. Keep at about 23 °C in the dark until the jars are white all the way through2.

Common mistakes

  • Wet grain. Free water in the bottom of the jar invites bacteria.
  • Opening the cooker while it still holds pressure2.
  • Making grain spawn before you have tried bought spawn. UF/IFAS says buying spawn is the fastest and simplest start3.

Safety

Pressure cookers: vent before pressurising, never force-cool, and wait about 10 minutes after the pressure reads zero before opening the lid away from your face4.

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Sources

  1. Methods of Commercial Mushroom Cultivation in the Northeastern United States. Willie Crosby (Fungi Ally) and Steve Gabriel, Cornell Small Farms Program, 2019 (Northeast SARE). Extension publication, checked 26 October 2026.
  2. Isolation of Mother Cultures and Preparation of Spawn for Oyster Mushroom Cultivation. Liao, Hsu, Hameed and Cotter, UF/IFAS Extension (EDIS SS663). Extension publication, checked 26 October 2026.
  3. D.I.Y. FunGuide: Grow Your Own Oyster Mushrooms at Home. Hameed, Cotter, Liao and Hsu, UF/IFAS Extension (EDIS SS662). Extension publication, checked 26 October 2026.
  4. Using Pressure Canners. National Center for Home Food Preservation (University of Georgia). Extension publication, checked 26 October 2026.