You can make them stunningly modern, or use traditional techniques and terrarium jars and cloches.
You can really use anything for a terrarium, as long as it is glass. Look for a jar or container with a wide mouth. While it is possible to use something with a small opening, it is much easier to add plants if your container has a wide mouth. You don’t want your plants touching the sides of your jar; so the wider the bowl the more plants and soil you’ll be able to fit.
A large container will allow for a greater choice of types and size plants as well as the option of fitting in decorative elements.
When considering terrarium plants, look for plants that like low to medium light. Try to get a mix of sizes, leaf textures and leaf colors. Make sure they are small enough to fit in your terrarium, preferably without touching the sides. You’ll also want to buy terrarium plants that don’t mind a humid environment. This leaves out most succulents and cactus. However, if you have your heart set on succulents, you can make beautiful terrariums with these plants, but you will want to make an open terrarium and add clean, course sand to your potting mix.
The next step is to add moss on top of the stones and charcoal. This is to keep your next layer, potting soil, from mixing in with the charcoal and stones. This is an aesthetic decision so this step is optional.
With a large spoon or small shovel, add sterile soil-less potting mix on top of your moss or if you aren’t using moss, put the potting mix right on top of your charcoal. Add as much potting mix as you can – at least a couple of inches. You will want to make sure your plants will fit into your terrarium with the top on, so you might have to hold your plants up to measure as you add soil.
At this point it’s important to think about the design of your terrarium. Will the terrarium have a back and a front? If so, you probably want to put your tallest plant in back, or in the middle. You can also contour your soil so that it mounds and sinks to create interest.
Using a large spoon, or your fingers, dig a hole in the potting mix. Place your terrarium plant in the hole and gently pat the soil around it. If your terrarium has a narrow neck that you can’t fit your hand into, you can use cooking chopsticks, tongs or long tweezers to place your plants and pat them in.
You do want to make sure that there are no air pockets between the roots of your terrarium plants and the soil.
A good trick from Tovah Martin’s fabulous book, The New Terrarium, suggests that you put a cork on the end of a skewer or chopstick to tamp down soil, particularly for a terrarium with a small opening.
Caring for your terrarium is easy. Check every couple of weeks to see if your terrarium needs water. If your terrarium is closed, take off the top at least once a month to air it out. If you see lots of condensation or have added too much water, leave the top off until it has had a chance to dry out.
Pull off any leaves that show signs of yellowing or damage and prune plants if they grow too large.
Don’t fertilize your terrarium because you don’t want to encourage growth.