Hello fellow mushroom growers! Over the last week, I've been considering how important casing colonization is in mushroom production, especially growing our favorite friend, Mr. Cubensis, the magic mushroom. I've discovered that a casing really makes or breaks a tray regardless of the supporting substrate. I know of four common results of casings. Let's go from worst to best.

1. In the first case, a casing will not colonize at all. Perhaps moisture content is incorrect or the mix may be too acidic because the cultivator did not add enough lime flour (peat moss is very acidic by nature). Another common reason, is packing down of the casing either deliberately or accidentally through repeated watering; rather, the casing should be sprinkled down and remain light and fluffy. Yet another reason is that the temperature might not be in the incubation range (approximately 85oF). In each case, the cultivator will wait and wait yet the mycelium never makes it through. Given enough time, mold usually appears. The cultivator might be inclined to think he/she needs to sterilize the casing next time or try a different casing mix, but in reality, incorrect conditions set the stage for contamination, not materials.
2. In the second case, the cultivator will lay the casing but immediately pins will form before the casing is colonized. In this case, the cultivator might be very happy to get pins but will soon realize that the first flush is not particularly impressive. Those mushrooms that do form may even be large, but they will be few. Soon after the first flush, the uncolonized casing has a great risk of contamination.

The reason for premature pinning is usually the cultivator inadvertently presented initiators. Perhaps the casing or substrate incubation temperature dropped too low (for example, in the mid-70s or even lower). Maybe too much air or light was given to the substrate prior to casing provoking pins. For whatever reason, the mycelium switched to the fruiting mode prior to full colonization of the casing resulting in a meager fruiting and a precarious position for the casing.  

3. The third case is nearly the opposite of the second. In this case, the mycelium grows densely through and OVER the casing, and, hence, this is called overlay. The casing effectively becomes part of the substrate mass and so looses its ability to store moisture or form pins within its moist microclimate. The mycelium will often grab the casing so tightly it will pull away from the sides of the container. The cultivator may try to mist the casing thinking this will help yet, in extreme overlay, even water cannot penetrate the casing!

When overlay strikes, the cultivator will sometimes get a decent first flush but struggles to keep the humidity high enough so the surface doesn't dry out. Other times the cultivator will see pins only form along the edges because that's where the moisture is highest. But the worst is still to come: subsequent flushes will not be any more impressive that without a casing at all!

The reason for overlay is usually too long of incubation time. I've also read that too wet of a casing mix can contribute to this condition. It's best prevented by switching to the fruiting temperatures immediately once the valleys of the casing show mycelium. Overlay, while very destructive if not treated, can be remedied by mixing up the casing material with a fork and waiting a couple days until it recovers prior to fruiting.

4. The fourth case is what we all want to happen each time. It's casing perfection. Casing perfection is when the cultivator lays the casing with the correct pH, moisture, consistency, and incubation temperature. In a week or two, the mycelium can be seen running through the casing in many of the valleys but not covering the casing as in overlay. Once the valleys show growth, the tray is uncovered, placed in the fruiting chamber, and in about a week (or even less) there are dozens of pins every square inch! The mushrooms explode everywhere, yet, throughout, the casing remains open and receptive to water.