Terrariums can be open at the top, or fully enclosed.
Certain plants, like cacti and other succulents, do well in an open terrarium. Others practically take care of themselves for months, even years, in enclosed terrariums. Whichever you choose, terrariums are a fun and easy way to enjoy indoor plants. Your own green world can be any size. For single small plants you can use an old mayonnaise jar, a larger gallon-size pickle or candy jar, or even fish bowls and aquariums.