An important factor in determining the suitability of a plant to your home and envisioning the care it will need, is to know the origin of the plant. Plants that are native to desert regions will require substantial amount of sunlight and loose, fast-draining soil. Plants from the floor of a rainforest will need some protection from strong sunlight and generous humidity.
There are hundreds of Selaginella species, most originating in tropical regions. Those plants most commonly grown as houseplants, called spike moss, are admired for their decorative, branching, mossy foliage. Spike moss has a constant need for moisture and is most successful when cared for in a terrarium environment. It is difficult to grow successfully without the terrarium's protection from dry air, draughts and heat, though a spot in a humid bathroom may suffice.
Spike mosses are forest dwellers in origin and need protection from strong sunlight. Indoors, provide semi-shade or shade, away from the window. The small leaves will scorch if exposed to bright sunlight.
Water frequently, keeping the soil damp but not wet. The maintenance of moisture and humidity is essential or the plant's small leaves will wither. If you do not have a terrarium, spray mist the plant at least once a day with tepid, soft water and keep the pot in a tray of damp peat or wet pebbles.
Typical, warm home temperatures are fine. Do not let the temperature fall below 55°F.
The terrarium keeps your spike moss bathed in a layer of protective mist, regulating temperature, humidity and exposure to draughts. Your terrarium should be transparent (glass ideally) with no more than a small opening to the surrounding air.
You can house your spike moss in the same terrarium with other moisture and shade loving plants. Use a good quality potting mix that includes peat and perlite and no moisture retaining agents or fertilizer supplements. Keep your spike moss in its own pot of soil or plant the entire terrarium with the soil mix.
Ensure that your terrarium is well cleaned and disinfected before planting. After cleaning, allow the terrarium to air out so that no chemical products affect your plant(s). A layer of stones at the bottom of the soil mix is common in a terrarium, as is the inclusion of filtering activated charcoal which helps control fungi in the closed environment. Up to an inch of this mixture can be placed in the bottom of the terrarium before adding the soil. The soil depth should relate to the size of your plant's root ball.
You will need to gently rinse the accumulated fertilizer salts from you plant's container every year or two, or replace the soil in the terrarium with a fresh mixture.
Very little supplemental nutrition is required. Feed only twice a year with a half-strength, standard, houseplant fertilizer. Do not feed a newly planted spike moss or the roots will be shocked and damaged.
Do not use cold water on your plants or they will wither. Spike moss likes stability, of temperature, light, humidity, air movement and moisture. Brown leaves usually indicate a shock to the plant from one of these sources. Remove dead stems as they occur.
You can prune gently or pinch back growing tips occasionally to promote bushiness.