Considering terrarium plants are typically held in an enclosed space, select species that thrive in humidity. Some top picks include:
Too much sun can fry plants in a terrarium, but too little light leads to mush. An east- or west- facing window provides the best lighting. Rotate the terrarium regularly to ensure even sunlight exposure.
Ventilate a terrarium occasionally. Every two weeks or so, open the terrarium for a few hours, then close it. When placed in the right location, a terrarium is self-watering—condensation from the moisture in the soil collects on the glass and dribbles down, keeping the soil moist in a continuous cycle. If no condensation collects on the glass, give it a light watering using an eyedropper.
Remove spent flowers and leaves. Decomposing plant parts are unsightly and can cause fungal and bacterial infections. Keep glass clean by wiping off debris, inside and out.
To keep your terrarium under control, give plants periodic trims. When ferns and tropical mosses begin to gain girth and crowd their comrades, remove the excess and start another terrarium.
At the first hint of fungus growth, get rid of it by covering it with a tissue and removing it completely so the spores don't spread. As long as the fungus is not visible on the other plants in your terrarium, they should be fine, but keep a close eye on them.