While winter weather restricts outdoor activities in the garden there are many possibilities for green fingered addicts to occupy themselves inside the house. One of these is to build a terrarium which is essentially a closed glass container. The micro climate inside the closed container ensures that once installed the plants take care of themselves. This article from the Penn State Extension website tells you how to set about making one yourself.

In the depths of winter, most gardeners believe their efforts are restricted to armchair activities ? reading books, reviewing catalogs, and planning for the spring garden. Not so!
A closed terrarium allows you to continue gardening on a very small scale. You?ll be able to shop for plants, get your hands in the dirt and create a beautiful green vignette that will enhance any room in your home or workplace.
The first step is to choose a container. It must be clear glass or plastic with a tight-fitting lid or cover. Colored glass will not let enough light in for the plants to flourish. You may use a container with a narrow opening, but it will be more difficult for you to plant and tend. While there are specifically designed terrarium containers, other options include large glass cookie or candy jars, old fish bowls, fish tanks, half-gallon canning jars, old apothecary jars, and lidded brandy sniffers. Choose a container that will blend with the d?cor of your home or office.
Through every step of the process, be careful not to introduce pests and pathogens into your terrarium, as this humid enclosed environment is a perfect breeding ground. Wash your hands or wear clean gloves while building and maintaining your terrarium. Start with a very clean container. Wash it with soap and hot water. Rinse well in cool water and air dry. If you feel the need to use a commercial glass cleaner, keep the lid removed and wait a few days before your initial planting. This will allow time for all fumes to escape.
You will need a soilless potting mix with relatively low fertility. Garden soil is much too heavy and will compact. Before the potting soil is placed in the container, mix in some charcoal. Just a handful or two is all you need, depending on the size of your container. Charcoal will add organic matter and absorb any chemicals or odors. The tiny pores in the charcoal hold water and nutrients and later make them available to the plants.

See more at Penn State Extension
Image source: Garden Therapy