Reintroducing beneficial organisms
In most cases you probably don’t want your substrate to be totally sterile. Bacteria, insects, and other organisms play an important role in keeping the ecosystem healthy in nature, and this applies in horticulture and terrariums as well. It’s possible to reintroduce beneficial organisms by mixing in a bit of substrate from an existing grow bin or container, and by adding microfauna such as isopods and springtails. If you add substrate from an existing setup, be sure the source material doesn’t have any pests or disease.
Optimizing your use of sterile Sphagnum
A pro tip is to fill the plant pots with ½ non-sterile Sphagnum at the bottom, and only use sterile Sphagnum for the top ½. In general, the lack of light and the distance to the surface are enough to suppress any Sphagnum spores on the bottom from germinating. In this way you don’t have to sterilize as much, which saves you time and energy.
Storing your sterile moss
For extended storage and to reduce mold, I like to refrigerate the sterilized Sphagnum. I let it warm to room-temperature again before using it for plants.