Spawn is distributed on the compost and then thoroughly mixed into the compost. For years this was done by hand, broadcasting the spawn over the surface of the compost and ruffling it in with a small rake-like tool. In recent years, however, for the bed system, spawn is mixed into the compost by a special spawning machine which mixes the compost and spawn with tines or small finger-like devices. In a tray or batch system, spawn is mixed into the compost as it moves along a conveyer belt or while falling from a conveyor into a tray. The spawning rate is expressed as a unit or quart per so many square feet of bed surface; 1 unit per 10 ft is desirable. The rate is sometimes expressed on the basis of spawn weight versus compost weight; a 2 percent spawning rate is desirable.
Once the spawn has been mixed throughout the compost and the compost worked so the surface is level, the compost temperature is maintained at 75°F and the relative humidity is kept high to minimize drying of the compost surface or the spawn. Under these conditions the spawn will grow – producing a thread-like network of mycelium throughout the compost. The mycelium grows in all directions from a spawn grain, and eventually the mycelium from the different spawn grains fuse together, making a spawned bed of compost one biological entity. The spawn appears as a white to blue-white mass throughout the compost after fusion has occurred. As the spawn grows it generates heat, and if the compost temperature increases to above 80° to 85°F, depending on the cultivar, the heat may kill or damage the mycelium and eliminate the possibility of maximum crop productivity and/or mushroom quality. At temperatures below 74°F, spawn growth is slowed and the time interval between spawning and harvesting is extended.
The time needed for spawn to colonize the compost depends on the spawning rate and its distribution, the compost moisture and temperature, and the nature or quality of the compost. A complete spawn run usually requires 14 to 21 days. Once the compost is fully grown with spawn, the next step in production is at hand.
Mushroom compost must be inoculated with mushroom spawn (Latin expandere = to spread out) if one expects mushrooms to grow. The mushroom itself is the fruit of a plant as tomatoes are of tomato plants. Within the tomato one finds seeds, and these are used to start the next season’s crop. Microscopic spores form within a mushroom cap, but their small size precludes handling them like seeds. As the tomato comes from a plant with roots, stems, and leaves, the mushroom arises from thin, thread-like cells called mycelium. Fungus mycelium is the white, thread-like plant often seen on rotting wood or moldy bread. Mycelium can be propagated vegetatively, like separating daffodil bulbs and getting more daffodil plants. Specialized facilities are required to propagate mycelium, so the mushroom mycelium does not get mixed with the mycelium of other fungi. Mycelium propagated vegetatively is known as spawn, and commercial mushroom farmers purchase spawn from any of about a dozen spawn companies.