Mushroom substrate is the growing medium your mycelium colonizes and feeds on before producing fruiting bodies. At Foxfire Mushrooms in Longmont, Colorado, we carry two primary substrates — CVG (coco coir, vermiculite, and gypsum) and our Oak Soy Hull hardwood blend — and the right choice depends almost entirely on what species you’re growing.
The short answer: CVG for oyster varieties (Blue, Pink, Yellow, White), Oak Soy Hull for Lion’s Mane, Shiitake, King Oyster, and Reishi. Read on for the full breakdown, including how to prepare each, sterilization requirements, spawn-to-substrate ratios, and common mistakes to avoid.
If you’re still figuring out what you want to grow, check out our beginners guide to growing mushrooms before diving into substrate selection.
Mushroom substrate is the organic material that mushroom mycelium colonizes, digests, and feeds on — it’s the soil equivalent in mushroom cultivation. Unlike plants rooting in dirt, mycelium secretes enzymes that break down the substrate’s organic compounds, extracting carbon, nitrogen, and minerals it needs to grow. When the mycelium has fully colonized the substrate and the environmental conditions are right, it transitions to producing fruiting bodies: the mushrooms you harvest.
Choosing the wrong substrate for your species is one of the most common reasons home growers struggle. A substrate that’s too nutritious for a low-demand species invites contamination. One that’s too sparse starves a high-demand species into producing weak or absent flushes. Getting this right is foundational.
We’ve deliberately simplified our substrate lineup to two formulas — one for each major category of gourmet mushroom cultivation:
Both are available pre-made and ready to use from Foxfire — our CVG in 10lb bulk bags, our Oak Soy Hull in pre-sterilized 9.5lb bags.
CVG stands for Coco Coir, Vermiculite, and Gypsum. It’s one of the most widely used mushroom substrates in home cultivation because it’s forgiving, inexpensive, widely available, and doesn’t require a pressure cooker to prepare.
Each component has a specific role:
Standard ratio:
Or by volume: roughly 5 parts coco coir : 5 parts vermiculite : 1 part gypsum.
This test is the single most important skill in substrate preparation. Substrate that’s too wet invites anaerobic contamination; too dry and mycelium colonizes poorly and flushes weakly.
CVG’s low nutritional content means competing organisms — molds, bacteria — have little to feed on. This allows you to pasteurize rather than fully sterilize:
Do not inoculate hot substrate — you’ll kill your spawn.
CVG is best for:
CVG is not recommended for Lion’s Mane, Shiitake, King Oyster, or Reishi. These species evolved to digest complex wood compounds and need supplemented hardwood substrate to fruit well.
Our Oak Soy Hull substrate is a supplemented hardwood blend: hardwood sawdust as the primary carbon and nutrition source, with soy hulls providing additional nitrogen and nutrition for more productive flushes.
Our OSH bags are available in pre-sterilized 9.5lb bags, ready to inoculate immediately after opening. No pressure cooker required when you buy ready-made.
The added nutrition in supplemented hardwood is a double-edged sword. The same nitrogen and carbohydrates that feed your mycelium are attractive to contaminating organisms — particularly Trichoderma (green mold), Bacillus (wet rot), and various other bacteria and molds. At the inoculation-ready moisture level, an unsterilized OSH bag is a warm, nutritious environment that contamination can colonize in hours.
Pasteurization — sufficient for CVG — does not reach the temperatures needed to kill heat-resistant bacterial spores (Bacillus in particular). OSH substrate requires pressure sterilization at 15 PSI for 2.5–3 hours in a pressure cooker or autoclave. This is why many home growers opt to purchase pre-sterilized bags: the equipment investment and time involved in DIY sterilization is significant.
See our mushroom contamination prevention guide for more on identifying and preventing contamination at every stage of the grow.
OSH is the go-to substrate for:
Sterilizing supplemented hardwood substrate yourself means: acquiring a large pressure cooker (23-quart minimum for meaningful batch sizes), loading it with bags, running a 3-hour sterilization cycle, cooling the bags for 8–12 hours, then transferring to a clean environment for inoculation — all with the risk that any breach in the bag during handling introduces contamination. Our pre-sterilized 9.5lb OSH bags eliminate every one of those steps. They pair directly with our colonized grain spawn or liquid culture syringes for a streamlined grow from day one.
| Feature | CVG Substrate | Oak Soy Hull Substrate |
| Best for | Oyster varieties, Pioppino | Lion’s Mane, Shiitake, Reishi, King Oyster |
| Nutritional level | Low | High |
| Pasteurize or sterilize? | Pasteurize only (160–180°F) | Must sterilize (15 PSI, 2.5–3 hrs) |
| Contamination risk | Low | Moderate–High if not sterilized |
| Colonization speed | Fast for oysters | Moderate; varies by species |
| pH | 6.0–7.0 | 5.5–6.5 |
| Yield potential | Good for oysters | Higher for wood-loving species |
| Available pre-made? | Yes — Foxfire 10lb CVG bags | Yes — Foxfire 9.5lb pre-sterilized OSH bags |
| Best grow bag pairing | 3T or 4T Unicorn bags | 4T Unicorn bags (more volume) |
Use this quick reference to match your target species to the right substrate:
| Species | CVG | Oak Soy Hull |
| Blue Oyster | Best choice | Not needed |
| Pink Oyster | Best choice | Not needed |
| Yellow Oyster | Best choice | Not needed |
| Lion’s Mane | Will not perform well | Best choice |
| Shiitake | Will not perform well | Best choice |
| King Oyster (Chestnut) | Marginal | Best choice |
| Reishi | Not recommended | Best choice |
| Pioppino | Good option | Also works |
When in doubt: oyster mushrooms = CVG. Wood-lovers = Oak Soy Hull.
Regardless of which substrate you use, the general rule for bulk grows is 10–20% spawn by weight relative to substrate. In practice:
Higher spawn rates — closer to 20% — colonize faster and reduce the contamination window, which is especially important with nutritious OSH substrate. Lower spawn rates — 10% — are fine for CVG given its low contamination risk.
For detailed inoculation steps including how to mix spawn into bulk substrate, see our how to inoculate grain spawn guide.
For guidance on choosing between colonized grain spawn and liquid culture for your initial inoculation, see our colonized vs. sterilized grain spawn comparison.
Mistake 1: Wrong moisture level
The field capacity test exists for a reason. Substrate that’s too wet leads to anaerobic conditions, slow colonization, wet rot, and bacterial contamination. Too dry means mycelium can’t fully colonize. Squeeze-test every batch.
Mistake 2: Pasteurizing OSH instead of sterilizing it
CVG can be pasteurized. OSH cannot — not if you want reliable results. The nutrition in supplemented hardwood keeps bacterial spores alive through pasteurization temperatures. A pressure cooker is not optional for DIY OSH. If you don’t have one, buy Foxfire’s pre-sterilized bags.
Mistake 3: Using low-quality or contaminated coco coir
Not all coco coir brands are created equal. Some arrive with visible mold or have been stored poorly. Buy from reputable sources. Our pre-mixed CVG bags are sourced and quality-checked before they ship.
Mistake 4: Inoculating before the substrate has cooled
Sterilized or pasteurized substrate is hot. Inoculating too soon — before temps drop below 80°F — will kill your spawn. Wait until you can hold the bag comfortably.
Mistake 5: Wrong bag size for the substrate
CVG is typically lighter in density and works well in 3T Unicorn bags. OSH blocks are denser and larger — 4T Unicorn bags give you better volume and breathable filter area for wood-loving species. See our unicorn mushroom bags guide for filter patch sizing and bag selection detail.
At Foxfire Mushrooms in Longmont, Colorado, we’ve made it simple: buy the substrate that matches your species, and it arrives ready to use.
All Foxfire substrates and grow supplies ship from Longmont, Colorado, with our 30-day money-back guarantee. Questions about what you need for your specific species? Our team is here to help.
Yes — sterilizing CVG won’t harm it, and some growers prefer it for extra peace of mind. It’s just not necessary. CVG’s low nutritional content makes it an inhospitable environment for most contaminants after pasteurization. If you’re already running a pressure cooker for OSH, you can throw a CVG bag in the same run without issue.
Oversaturated substrate creates anaerobic (oxygen-deprived) pockets that mycelium can’t colonize. Worse, bacteria thrive in these wet zones. You’ll likely see wet rot — a slimy, discolored, foul-smelling patch — rather than healthy white mycelium. If you catch it early (before inoculation), add dry vermiculite (for CVG) or dry hardwood sawdust (for OSH) and re-test. If it’s already inoculated and wet rot develops, the bag is likely lost.
CVG can sometimes support a second or third flush of oyster mushrooms if the mycelium is still viable — just maintain humidity and give the block time to recover. OSH blocks for Lion’s Mane and Shiitake can also produce multiple flushes. However, each subsequent flush yields less than the last, and the risk of contamination increases as the block ages. After 2–3 flushes, spent substrate makes excellent garden compost or worm bin addition.
CVG and Blue or Pink Oysters. It’s the most forgiving combination in home cultivation — no pressure cooker needed, fast colonization, fast fruiting, and oysters are visually rewarding and hard to mess up. Once you’re comfortable with the process, you can move to OSH and wood-loving species.
You can absolutely make your own. CVG is straightforward — the ingredients are widely available at garden centers and online. DIY OSH is more involved because you need a pressure cooker and the discipline of sterile technique throughout. Foxfire’s pre-made bags exist for growers who want to skip that overhead — especially useful for OSH where equipment requirements are significant.
A standard 3T Unicorn bag holds roughly 2–3lbs of hydrated substrate; a 4T Unicorn bag holds roughly 4–5lbs. Our 10lb CVG bag typically fills 3–5 standard bags depending on final hydration and how densely you pack. Our 9.5lb pre-sterilized OSH bags are single-use: one bag per block.
Share this article
Grab a beginner-friendly kit and apply what you just read —
or take the quiz and we’ll match you to the right
starting point.
