The best thing about terrariums is that they require very little care. You usually only need to tend to a terrarium between once a week and once a month. Although exact care instructions will be dictated by the types of plants you choose, as well as whether the container is open or closed, there are a few general guidelines that will keep your terrarium happy and healthy for a long time.
Most terrariums like to be kept in bright, indirect sunlight. Although many plants like moss and grasses like shady areas outside, when you bring them inside they require more light than you may think. Don't give them too much light though- direct sunlight reflecting through the glass container can make a magnifying glass effect, and scorch the plants. A few feet away from a sunny window, or the windowsill of a shaded window make a great home for your terrarium.
If your terrarium is closed, it will require very little water since almost none is lost through evaporation. Keep an eye on it for the first couple of weeks. The walls of the container should not be heavily fogged up or overly moist. If this is the case, open up the lid for a few days and let it dry out. Once you've achieved the right water levels, a closed terrarium can go a month or more with no water. Some people recommend opening the lid every week or so to allow fresh air to circulate, but that's not always necessary either.
For an open terrarium, you'll want to wait until it gets fairly dry before you water it. One of the most common reasons terrarium plants die is due to over watering. When the terrarium seems dry, add a few tablespoons of water, or mist with a spray bottle.
If your plant starts outgrowing it's container, prune some of the longer branches. Pruning the top of the plant has the added benefit of encouraging more growth around the outsides of the plant, which will help your terrarium look lush and full. You also want to clip off any leaves that are touching the sides of the container, since they are likely to trap water and rot.
You also want to periodically remove any dead leaves or rotting plants to keep the rot from spreading throughout the terrarium. If it looks like your plants are rotting, it's probably due to too much water in the container. Open the lid and let it dry out for a couple of days.
Lastly, do not fertilize the terrarium. Adding fertilizer encourages plants to grow bigger, which you generally want to avoid in a closed container. Instead, scrape off the top layer of soil and replace it with fresh soil every few months.
Although that was a long list of instructions, you'll find that caring for your terrarium is a cinch once you get the hang of it. With a little patience and TLC, you'll have a beautiful terrarium for years to come.