Mycology Tool

Grain Spawn Calculator

Dial in your grain spawn recipe: dry grain weight, water volume at field capacity, pressure-cooker time adjusted for your altitude, and inoculation dose per jar or bag. Pick your species in Pro mode to get a colonization time estimate too.

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Calculator

Build your spawn recipe

Pro mode tips

Altitude correction kicks in above 2000 ft (+5% time per additional 1000 ft). 10 PSI cookers run 1.5× longer than 15 PSI. Hydration override is for growers who've dialed in a specific moisture sweet spot for their setup — otherwise the grain default is correct.

Grain character

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Select a grain to see its profile.

Common grain spawn mistakes

  • Too-wet grain from overboiling — if water puddles at the jar bottom after sterilization, hydration was too high. Bacteria colonize that puddle before mycelium can reach it. Pre-drain and surface-dry grain before loading jars.
  • Inoculating hot jars — PC-cooked jars stay above 100°F for 8+ hours. Inoculating a warm jar kills your LC or denatures spores. Wait until jars are at room temp, minimum 8 hours post-cycle.
  • Shaking too early — the urge to shake at day 4 is strong. Don't. Wait until jars show 20–30% colonization (small circular white patches), then shake to distribute. Early shaking ruptures undercolonized mycelium and invites contamination.
  • Skipping the dry-down after hydration — surface moisture on grain gives bacteria a head start. After simmering or soaking, drain and air-dry grain on towels for 30–60 minutes. Kernels should feel dry to the touch but yield when squeezed.

Shopping list

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dry grain total

Water to add

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PC time (per load)

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Dose per container

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Colonization

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Setup procedure

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    How to use

    From calculator to colonized spawn in 4 steps

    1

    Pick your grain and container

    Rye berries are the default for a reason — small, dense, hydrate evenly, colonize fast. WBS (wild bird seed) is the cheap alternative that works almost as well for beginners. Oats shine for lion's mane and oyster. Millet colonizes fastest and is the best grain for future grain-to-grain transfers. Container size drives everything downstream: quart jars hold ~500g dry grain, half-gallons hold ~1kg, 5 lb bags hold ~2.2kg.

    2

    Hydrate to field capacity

    This is the step most beginners get wrong. The calculator tells you exactly how much water — not "1 cup per jar," but field-capacity water for your grain's absorption rate. Rye at 43% wants less water than oats at 50%. Simmer rye/WBS/millet for 15–25 min, drain. Soak oats and wheat overnight instead — they absorb too unevenly under heat. Air-dry the drained grain on towels for 30–60 minutes before loading jars. Surface moisture is the #1 bacterial vector.

    3

    Pressure cook

    Load jars or bags about half-full with hydrated grain. For jars: hammer a nail through the lid for an injection port (cover with micropore tape). For bags: make sure the filter patch is unobstructed. Pressure cook at 15 PSI — standard times are 90 min for quarts, 120 min for half-gallons, 180 min for gallons and 5 lb bags. If you're above 2000 ft elevation, the calculator adds time automatically because atmospheric pressure drops with altitude. 10 PSI canners need 1.5× as long — upgrade if you can.

    4

    Inoculate and incubate

    Cool jars fully to room temperature — minimum 8 hours, overnight is better. Inoculating a warm jar kills your culture. Dose per container depends on method: 1–3 mL liquid culture per quart, 1–2 mL spore syringe per quart, or ~100g grain-to-grain per quart. Inject through the self-healing port or bag injection sticker. Incubate at 72–78°F in the dark. First check for contamination at day 5, then every 3 days until full colonization.

    Choosing an inoculation method

    Liquid culture (LC) is active mycelium suspended in nutrient broth — colonization starts within 48 hours, consistent genetics, lowest contamination risk. More expensive per dose but faster to grain spawn. Spore syringes are dormant spores — cheaper, but add 5–10 days to colonization because spores have to germinate first, and each grow is a genetic lottery. Grain-to-grain transfer uses already-colonized jars to inoculate new ones — fastest method of all (hours to visible growth) but requires existing master jars. Beginners should start with LC.

    The contamination check schedule

    Check jars visually on days 5, 8, 11, and every 3 days thereafter. Look for green (trichoderma), pink (bacterial), orange/yellow (cobweb mold), or slimy/wet patches. Any of these = toss the jar and thoroughly sanitize your incubation area before the next batch. Healthy mycelium is pure white and fluffy — no off-colors, no zones, no liquid. Don't shake any jar until it shows 20–30% colonization (scattered white patches), otherwise you'll rupture the mycelial front and give contaminants a path in.

    The science

    Why grain spawn math matters

    Grain spawn is the workhorse of indoor mushroom cultivation — a colonized grain base that rapidly inoculates larger volumes of bulk substrate. Every number in this calculator — hydration percentage, sterilization time, inoculation dose — traces to a specific biological or physical constraint. The Spawn guide in the Mycology Learning Center covers the full lifecycle; this section explains why the numbers are what they are. Getting hydration wrong by 5% or sterilization wrong by 20 minutes is the difference between clean colonization and a contaminated batch.

    43%

    Rye field capacity

    Rye's optimal moisture — high enough for mycelium to metabolize kernels, low enough to prevent bacterial puddling. Below 40%: stalled colonization. Above 47%: contamination.

    250°F

    PC sterilization temp

    15 PSI steam reaches 250°F internal — hot enough to kill bacterial endospores in 90 min. 212°F boiling water can't; endospores survive atmospheric boiling indefinitely.

    2 mL

    LC dose per quart

    Enough to seed dozens of colonization points without flooding the grain. More isn't faster — 5 mL gives the same timeline as 2 mL but wastes culture.

    Why hydration percentage is a razor's edge

    Fungi metabolize substrate through extracellular enzymes released from hyphal tips. Those enzymes need liquid water to function — dry grain (below 35% moisture) is effectively inert to mycelium. But the same liquid water is an open invitation to bacteria. Bacterial growth rate vastly exceeds fungal growth rate in liquid environments — a drop of free water in a grain jar hosts a bacterial colony that doubles every 20 minutes, while the fastest fungal mycelium doubles every 4–8 hours. The hydration target for each grain is the point where grain kernels are saturated but no free water pools anywhere.

    Field capacity — the physics term behind hydration percentage — is the maximum water a porous substrate can hold against gravity. The formula is straightforward: water mass = dry mass × (target fraction / (1 − target fraction)). For 500g rye at 43%, that's 500 × (0.43/0.57) = 377g water, which gives a final grain mass of 877g at exactly 43% moisture. Every grain type has a different field capacity because kernel density and starch composition vary. Rye at 43% is not the same condition as oats at 43% — oats at 43% would be undersaturated.

    Pressure cooker physics

    At sea level, water boils at 212°F (100°C). Steam sterilization requires higher temperatures to kill bacterial endospores — dormant, heat-resistant structures that survive atmospheric boiling indefinitely. A pressure cooker raises the boiling point by trapping steam: 15 PSI above atmospheric produces 250°F interior steam, which kills endospores in 15–30 minutes for small volumes. Grain spawn times (90 min for quarts, 180 min for gallons) include heat-penetration time — the center of the jar has to reach 250°F long enough to sterilize, not just the outer kernels.

    Altitude changes this equation because atmospheric pressure drops with elevation. At 5000 ft, atmospheric pressure is ~83% of sea level. When your gauge reads 15 PSI, you're actually at 15 + 12.1 = 27.1 PSI absolute — slightly lower than the 29.7 PSI absolute you'd hit at sea level, meaning your interior temperature tops out a few degrees lower. The calculator compensates with +5% time per 1000 ft above 2000 ft, conservative enough to handle any commercial PC at any altitude.

    Grain comparison table

    Grain Hydration target Prep method Best for species Character
    Rye berries 43% Simmer 20 min Cubensis, reishi, king trumpet, lion's mane Gold standard. Small dense kernels, even hydration, clean colonization.
    WBS (bird seed) 45% Simmer 15 min Cubensis, oyster Cheap and effective. Millet/milo blend. Quality varies by brand.
    Whole oats 50% Soak 24 hours Lion's mane, oyster Higher protein than rye. Uneven absorption — must soak, not simmer.
    Millet 48% Simmer 15 min Oyster, cubensis Fastest colonization. Small kernels = high surface area = rapid mycelial spread.
    Wheat berries 45% Soak 24 hours Cubensis, lion's mane Works but clumps. Thorough drying critical before sterilization.

    Inoculation method tradeoffs

    The three inoculation methods represent different tradeoffs between speed, cost, and contamination risk. Liquid culture is pre-grown mycelium in nutrient broth — colonization begins immediately upon injection because you're introducing active hyphal fragments. Fastest method from inoculation to fully colonized grain, but requires buying or producing LC (~$15–25 per 10 mL syringe). Spore syringes contain dormant spores that must germinate before colonization can begin, adding 5–10 days to the timeline. Cheaper (~$15 per 10 mL) but each grow is a genetic lottery — spores from a single print can produce highly variable fruits. Grain-to-grain transfer takes a fully colonized master jar, breaks it into chunks, and distributes them across new sterilized grain. Fastest colonization of all three because the new grain is being seeded by mature mycelium at dozens of points simultaneously, but requires a successful master jar to expand from.

    Colonization time and temperature

    7–10 days

    Oyster

    Fastest cultivated species. Aggressive colonization on most grains. Can outrun many contaminants.

    10–14 days

    Cubensis

    Reliable at 75°F on rye. Drops below 70°F add ~20%, below 65°F add ~40%.

    12–16 days

    Lion's mane / King trumpet

    Wood-lovers on grain. Slower than dung-lovers but still reliable. Lion's mane on oats ≈ cubensis on rye.

    14–21 days

    Reishi

    Slowest of the common cultivated species. Dense, almost leathery colonization. Patience required.

    Why 75°F is the default

    Mesophilic fungi — the category containing every species in this calculator — have optimal metabolic rates between 70–80°F. Too cold, enzymatic activity slows and colonization drags out (giving contaminants more time to establish). Too hot (above 82°F), growth accelerates briefly but bacterial contamination risk rises sharply because bacteria thrive at warmer temps than fungi. 75°F is the sweet spot — fast enough for rapid colonization, cool enough to suppress most bacterial competitors. Commercial cultivators run banks of grain spawn at 72–76°F year-round for exactly this reason.

    Sources and references

    Hydration targets and sterilization times draw from Paul Stamets' Growing Gourmet and Medicinal Mushrooms, Shroomery community consensus for cubensis cultivation, North Spore's commercial spawn documentation, and Out-Grow's species-specific guides. Field capacity math is standard soil/substrate physics. Pressure-cooker temperature-pressure relationships follow the steam tables in any thermodynamics reference. For first-time grain spawners, start with the Mycology Beginner Overview if this was your entry point into mushroom cultivation.

    FAQ

    Common grain spawn questions

    What's the right hydration percentage for rye?

    43% is the sweet spot. Below 40% your mycelium will colonize slowly or stall entirely — extracellular enzymes need liquid water to metabolize kernels. Above 47% you'll see free water puddling at the jar bottom during cooling, which is where contamination starts. The field-capacity formula (water mass = dry mass × (0.43 / 0.57)) gets you to 43% exactly when you add the calculated water to dry grain and drain nothing. If you simmer grain and drain afterward, aim for the grain to be saturated but not dripping when spread on towels.

    Rye vs WBS — which should I use?

    Rye is objectively better — more even hydration, cleaner colonization, slightly faster. WBS (wild bird seed, usually a millet/milo mix) is a distant second for reliability but wins on cost: you can buy a 50 lb bag of bird seed for $20–30 at any feed store, whereas organic rye berries run $2–4 per pound. For a first grow, either works. For serial cultivation where consistency matters, go rye. Avoid WBS mixes that include sunflower seeds, cracked corn, or peanut fragments — those contaminate easily and throw off hydration targets.

    Do I really need a pressure cooker? Can't I just boil?

    For grain spawn, yes — you need a pressure cooker. Atmospheric boiling water tops out at 212°F (100°C), which isn't hot enough to kill bacterial endospores. Endospores survive repeated boiling cycles indefinitely. A pressure cooker at 15 PSI produces 250°F steam, which kills endospores in 15–30 minutes depending on substrate mass. Without a PC, every grain jar you make will host endospores that activate during incubation and contaminate the grow. For bulk substrate (straw, manure, coco), pasteurization at 160–180°F for 90 minutes is sufficient because the bulk substrate chemistry disfavors bacterial growth. Grain is nutrient-rich and pH-neutral — no shortcuts, use a PC.

    How long can I store colonized grain spawn?

    Fully colonized grain spawn keeps 4–6 weeks refrigerated (35–40°F), or 2–3 weeks at room temperature before mycelium starts to consolidate and release metabolic byproducts. For longer storage, you can vacuum-seal colonized jars and refrigerate for up to 8 weeks, though vigor drops noticeably past 4 weeks. Don't freeze — freezing ruptures hyphae and kills the culture. Best practice: time your grain spawn prep to the substrate it'll inoculate. Grain sitting in a fridge for a month is grain with reduced colonization vigor.

    What's the difference between liquid culture, spore syringe, and grain-to-grain?

    Liquid culture (LC) is pre-grown active mycelium suspended in nutrient broth. Inject it and colonization starts within 48 hours. Most consistent results, lowest contamination risk, moderate cost. Spore syringe contains dormant spores in sterile water. Spores must germinate first (5–10 days added to timeline), and each grow produces genetic variation because spores are sexually produced. Cheapest inoculation method but highest contamination risk because spores come contact-packed with environmental bacteria. Grain-to-grain (G2G) breaks a fully colonized master jar into chunks and distributes them across new sterilized grain. Fastest method (hours to visible growth) because you're transferring established mycelium, not starting from scratch. Requires a clean master jar and sterile technique — one contaminated master ruins ten transfers. Beginners should start with LC.

    Why is my grain turning green?

    Green growth in a grain jar is almost always Trichoderma — a fast-growing fungal contaminant that colonizes undersaturated or bacterially-weakened grain. Toss the jar. Don't try to save it, don't open it near your other jars, don't treat it with anything. Common root causes: understerilized grain (PC time too short or altitude uncompensated), free water in jar (hydration too high), contaminated inoculant (old or unsterile LC/spores), or dirty workspace during inoculation. For your next batch: verify PC time matches altitude, drain grain more thoroughly before jarring, and flame-sterilize your injection needle between jars.

    Can I use a Presto canner at 10 PSI?

    Technically yes, but plan for 1.5× longer cycles. 10 PSI produces ~240°F steam vs 15 PSI at 250°F — still above the endospore kill threshold, but heat penetration takes substantially longer. A quart jar that sterilizes in 90 minutes at 15 PSI needs about 135 minutes at 10 PSI. Half-gallon jars need 3 hours. If you're shopping for a pressure cooker and will be doing grain spawn regularly, buy a 15 PSI model (or one with a 15 PSI weight setting like the Presto 23qt). The time savings add up fast when you're doing multiple loads.

    When and how should I shake my jars?

    Wait until jars show 20–30% colonization — visible as scattered white patches of mycelium spread across the grain, typically 5–8 days after inoculation (LC) or 10–14 days (spore syringe). Shaking earlier is counterproductive: you fragment undercolonized mycelium and expose unprotected grain to any contamination lurking in the jar. Shake by holding the jar upright and vigorously tapping the side against your palm 8–12 times, rotating the jar a quarter turn between each set. The goal is to break up the colonized chunks and distribute them through the grain. After shaking, you should see white flecks spread throughout — full colonization typically follows within 4–7 days. Shake once, not repeatedly; repeated agitation stresses mycelium.