What is mycelium?

Mycelium is made of hyphae, and mushrooms grow from mycelium. A germinating spore sends out long, thin hyphae; a single hypha is too small to see without a microscope, but groups of them show as a mass of threadlike growth, the mycelium. When the mycelium has developed enough, it can produce mushrooms (University of Arizona Extension).

Spawn is "intentionally cultivated mycelium" of a known species, grown clean in a lab, and most growers buy it from a supplier (Cornell Small Farms).

Mycelium colonizes a substrate by growing out from each piece of spawn until the pieces meet. Cornell calls the start "leap off", typically about four days after inoculation. Each colony expands as a sphere, the colonies fuse "until the entire substrate becomes one mycelial organism", and a fully colonized block is ready for fruiting (Cornell Small Farms).

Cornell's figures for incubation:

• Temperature: ideally about 70°F (21°C) and not above 80°F (27°C).
• Heat: growing mycelium makes its own heat. The core of a 5- or 10-pound bag can run 10–15°F (about 6–8°C) warmer than the room. In an 80°F room, a core of 95°F (35°C) is getting "warm enough for the mycelium to cook itself and introduce contamination".
• Spacing: about a hand's width between bags.
• Time: oyster on sawdust takes 10–20 days, shiitake on sawdust 42–84 days.

Healthy mycelium on grain or bulk substrate is white and spreads evenly. Grain spawn is ready when every kernel is covered and the grain is "uniformly white" (GroCycle), and UF/IFAS waits until a straw bag turns white all over before fruiting oyster (UF/IFAS). See colonization for how to judge it.

Not every species stays white. Shiitake blocks turn brown, or "bark over", before they fruit (Field & Forest).

Mycelium grows into the substrate and usually stays white, while most molds turn a color as they make spores. Trichoderma starts as white growth that can pass for mycelium, then turns green, and cobweb mold sits in grey, wispy tufts above the surface. When unsure, isolate the jar or block, photograph it and look again in 24 hours, as our contamination ID guide explains.

Hypha organizes photos by grow, with side-by-side comparisons, so you can watch mycelium spread through each block and spot a change from one day to the next.