This tutorial shows you how to construct a closed terrarium using a maidenhair fern as the plant. Because closed terrariums hold a lot of moisture, you must carefully research which plants to use. Some plants require less moisture and would quickly develop fungal problems in a closed terrarium. While others enjoy this humid environment, and can thrive. The maidenhair fern (adiantum raddianum) is a great plant to use in closed terrariums because is enjoys a humid environment.