Growing mushrooms from mycelium involves inoculating a nutrient-rich substrate with living fungal tissue, letting it colonize in warm darkness, then shifting conditions to trigger fruiting. The full process from inoculation to your first harvest typically takes anywhere from three weeks to three months, depending on the species. Here’s how to do it at each stage.
Understanding Your Starting Material
When you buy mycelium to grow mushrooms, it usually comes in one of two forms: grain spawn or liquid culture. Grain spawn is mycelium that has already colonized sterilized grain (often whole oats, rye, or wheat berries). Each colonized grain acts as a separate inoculation point when you mix it into your growing substrate, so grain spawn colonizes quickly and is the most beginner-friendly option.
Liquid culture is living mycelium suspended in a sterile nutrient solution. It’s typically a single genetic individual, meaning you’ll get consistent results. Liquid culture is often used to inoculate grain first (creating your own grain spawn) rather than going directly into a bulk substrate. If you’re just starting out, buying ready-made grain spawn skips an entire step and reduces your contamination risk significantly.
Both of these are different from spore syringes, which contain dormant reproductive cells that still need to germinate and mate before they can form fruiting mycelium. Spores are like unfertilized seeds. They add weeks to your timeline and introduce genetic variability, so they’re better suited to experienced growers or people who want to experiment.
Choosing and Preparing Your Substrate
The substrate is whatever material your mycelium eats as it grows. Different species prefer different substrates. Oyster mushrooms do well on straw or supplemented hardwood sawdust. Shiitake thrive on hardwood sawdust blocks. Button and portobello mushrooms need composted manure. For most home growers starting with oysters, straw or hardwood fuel pellets are the easiest to work with.
The substrate needs to be hydrated and either pasteurized or sterilized before use to kill competing organisms. Pasteurization means heating the material to 140°F (60°C) and holding it there for at least four hours. For straw, many growers submerge it in hot water in a large pot or cooler. Sawdust-based substrates, especially those supplemented with bran or soy hulls, typically need full sterilization in a pressure cooker because the added nutrients attract aggressive molds. Hardwood fuel pellets are convenient because they arrive nearly sterile and expand to the right moisture level (about 60% water by weight) when you add measured amounts of boiling water.
Mixing Spawn Into Substrate
Cleanliness matters most at this step. Wash your hands, wear gloves, and wipe your tools and work surface with isopropyl alcohol. The goal is to distribute the grain spawn as evenly as possible throughout the substrate so the mycelium colonizes from many points at once, outrunning any contaminants.
Break up your grain spawn first by firmly tapping and squeezing the bag to separate individual grains. Then cut a corner of the spawn bag to create a pouring spout. Open your substrate bag or container and pour the spawn in, mixing thoroughly with clean hands or a sterilized utensil. A common spawn-to-substrate ratio is roughly 10% to 20% spawn by weight. More spawn means faster colonization and less chance of contamination, but it also costs more.
Once mixed, pack the inoculated substrate into your growing container. This could be a filter-patch grow bag, a 5-gallon bucket with holes drilled in the sides, or a plastic storage bin (monotub). Seal or close the container to maintain humidity while still allowing minimal gas exchange through filters or micropore tape over small holes.
Incubation: Letting Mycelium Colonize
Place your inoculated container in a dark, warm spot. The ideal temperature during incubation is between 68°F and 75°F (20°C to 24°C). Keep humidity moderate, around 70%. You don’t want it bone dry, but excess moisture encourages bacteria and mold. A closet, shelf, or cabinet works fine for most setups.
During this phase, the mycelium spreads through the substrate, breaking it down and building a dense white network. Don’t open, shake, or disturb the container. The timeline varies by species:
- Oyster mushrooms on straw: 14 to 28 days
- Oyster mushrooms on sawdust: 10 to 20 days
- Most specialty species on sawdust: about 21 days
- Shiitake on sawdust: 42 to 84 days
You’ll know colonization is complete when the entire surface of the substrate is covered in thick white mycelium with no visible patches of bare substrate remaining.
Spotting Contamination Early
Not all white growth is healthy mycelium. Cobweb mold is a common look-alike that grows wispy and gray-white, spreading faster and more loosely than true mycelium. It causes soft rot in mushrooms and should be removed or isolated immediately if spotted.
The most notorious contaminant is Trichoderma, or green mold. It starts as an aggressive white mycelium that can initially look like healthy growth, but within days it sporulates into unmistakable bright blue-green patches. Once you see green, that container is lost. Don’t try to save it. Seal it and remove it from your growing area to avoid spreading spores.
Bacterial contamination shows up differently. On grain, it looks dull gray and slimy, with a sour or rotten smell. In a substrate block or monotub, you might notice yellowing patches. That yellow color is actually the mycelium secreting defensive enzymes in reaction to bacteria. Small isolated spots of bacteria can sometimes be overcome by healthy mycelium, but widespread contamination means starting over.
Triggering Fruiting
Once colonization is complete, you need to shift conditions to tell the mycelium it’s time to produce mushrooms. The primary trigger for most species is fresh air. During colonization, CO2 builds up inside the sealed container. Introducing fresh air (lowering CO2) signals the mycelium to form pins, the tiny bumps that become mushrooms.
Open or cut your grow bag, remove the lid of your monotub, or move the colonized block into a fruiting chamber where it gets regular air exchange. For a simple setup, a large plastic tub with holes covered in micropore tape and fanned a few times a day works well. The exact air exchange rate depends on your species, but the principle is the same: stale air out, fresh air in.
Humidity needs to increase during fruiting, up to 85% to 95%. Mist the inside walls of your fruiting chamber (not directly onto pins) with a spray bottle a few times daily, or use a humidifier on a timer. Indirect light from a window or a simple LED on a 12-hours-on, 12-hours-off cycle helps pins orient their growth upward, though light is not what triggers pinning in the first place.
Shiitake blocks respond especially well to “shocking.” Soaking a fully colonized shiitake block in cold water for 5 to 12 hours, or refrigerating it for 12 to 24 hours, stimulates a strong flush of pins. This cold-shock technique is specific to shiitake and isn’t typically used with other species.
Harvesting at the Right Time
Mushrooms grow quickly once they pin. You’ll often go from tiny bumps to harvestable mushrooms in 5 to 10 days, depending on the species and temperature. Harvest daily once mushrooms start maturing, especially if your fruiting area is above 60°F.
The ideal harvest window depends on the species. For button-type mushrooms, pick them when the caps are well-rounded and the thin membrane (partial veil) connecting the cap to the stem is still intact. Once that veil tears and the cap opens flat, the mushroom is past peak quality and beginning to release spores, which can coat your fruiting area in a fine powder.
For oyster mushrooms, harvest when the cap edges are still slightly curled downward or just beginning to flatten. If the edges curl upward, you’ve waited too long. Twist and pull clusters gently from the substrate, or cut them at the base with a clean knife.
Getting a Second (and Third) Flush
After your first harvest, the substrate still contains nutrients for additional rounds of mushrooms, called flushes. The key is rehydration, since the first flush pulls a lot of water from the block.
The most reliable method is dunking. Submerge the entire block in clean, non-chlorinated water at room temperature (65°F to 70°F) for 4 to 6 hours. Don’t exceed 12 hours, as prolonged soaking starves the mycelium of oxygen and invites bacteria. If the block floats, weigh it down with a clean plate or jar. After soaking, drain it for 30 to 60 minutes on a rack or at an angle, then return it to your fruiting chamber.
If dunking isn’t practical (the block is too fragile or too large), misting works as a gentler alternative. Spray the surface of the block 2 to 3 times daily for 2 to 3 days, focusing on any dry, dull, or cracked areas. Cover the block lightly with a humidity tent or plastic dome during this rehydration period to hold moisture in.
Most substrates produce two to three flushes before they’re spent, with each flush yielding somewhat less than the one before. After the final flush, the spent substrate makes excellent compost or garden mulch.