Many plants suitable for miniature terrariums are easy to root. The Sedum spathulifolium in the photo grows new shoots from between the leaves on mature rosettes. These rosettes will produce roots when they drop off the plant and hit a suitable piece of ground. To prepare them to root in a miniature terrarium, cut the baby plants free of the mother rosette, leaving 1/2 inch of the adjoining shoot on the baby plant. Insert the shoot into the soil or water crystals in your terrarium, and leave the plant to root, it is that simple.
Rooting special plants - if you want to share sections of a heritage Christmas cactus, or some other plant by making miniature terrariums containing these plants as gifts, look up information on how the plant roots, and follow those instructions within your terrarium, or root a cutting outside the terrarium and plant it once it has roots. Any cutting which likes humid conditions for rooting can be started inside a glass ball terrarium. If the cuttings require sand to irritate the cambium layer beneath the bark in order to form roots, you might be better off rooting them outside the terrarium and inserting them once they are growing. Sand is heavy, so it is easier to use in a pot, rather than inside a terrarium ball. For cuttings which require a rooting hormone to start, use the rooting hormone as directed, and insert the plant cuttings into soil in the base of the miniature ball terrarium.