Every once in a while you run into something that sounds too good to be true but isn't ... like terrariums.

Terrariums are indoor tropical gardens anyone can create. Total plant nerds, casual gardeners or parents who just want to create a simple and inexpensive science project that the family can enjoy. The bonus is that once you put in the last plant, terrariums are maintenance-free — which is the heart and soul of the too-good-to-be-true part.

Erica Doud, a floraphile auto mechanic — a plant lover who also happens to know her way around the inside of an automobile engine — teaches a terrarium-building class. The class was held at GardenHood, an independent Atlanta retail nursery.

Here are Doud's guidelines to creating and maintaining a terrarium, in five easy steps:

Step 1: Collect the materials

Terrarium materials on a gardening bench Gathering all the materials for your terrariums is almost as exciting as making the terrarium itself. (Photo: amy gizienski/flickr)

Items you'll need include:

  • A clear glass vessel. This can be almost any type of container that appeals to you, from a simple, large Mason jar to something of artistic or architectural interest such as an interestingly shaped apothecary jar you might find at a flea market or antique mall. The vessel can be open or have a lid. Unless you have a special interest in miniatures, taller vessels tend to work better than shorter ones.
  • Small rocks. These can range from bagged pea gravel sold at plant nurseries or the nursery section of box stores to expanded shale such as permatil available at some nurseries.
  • Activated charcoal. Horticultural charcoal is readily available at nurseries and box stores.
  • Peat or sphagnum moss. This is also available at nurseries and box stores.
  • Potting soil. Don’t scrimp! Use a good quality soil developed especially for containers. Fafard’s Ultra Container Mix with Extended Feed is an excellent choice.
  • A small spade
  • A mist bottle
  • Moisture-tolerant plants. There aren't many hard and fast rules in creating a container garden, but the selection of the type of plant materials is one. You'll want to use the type of plants that grow naturally in tropical conditions. Some excellent choices include, but aren't limited to, a small palm such as Neanthe Bella, Fittonias, Peperomias, almost any small-growing fern, prayer plants, mother-in-law tongues and even small-growing Phalaenopsis orchids often seen for sale at groceries and box stores. Don't use succulents. That's the one hard and fast rule. These plants are from arid regions and even an open terrarium will trap too much moisture for them to survive long term.

Step 2: Preparation

One of the goals is to create an environment as free of bacteria as possible. To do that, wash the vessel either by hand or in the dishwasher and, after assembling your materials, wash your hands with soap and hot water. You’re now ready to start creating a tropical indoor garden!

Step 3: Planting

Men placing soil and plants in a terrarium There's a process to building the layers of your terrarium. (Photo: Rawpixel.com/Shutterstock)

First, create a layer of substrates and soil.

Start with rocks. Depending on the size of your vessel, the rocks should be 1/2 to 2 inches deep. This is important to the health of the terrarium because this is where overflow water will collect.

Add a thin layer of charcoal. This is a little messy, so use a small spade to add the charcoal to the container. You won't need much. Just a thin layer. Horticultural charcoal is a "sweetener," meaning that it will help keep bacteria and mold from growing in the terrarium. This is why you sterilized the container and washed your hands before starting.

Add the moss. Create a firmly packed layer 1/2 to 1-inch deep. If you are using Sphagnum, it will be matted and bone dry. Break it up, place it on top of the charcoal and moisten the moss by spraying it with a mist bottle. Once the moss is damp, pack it down to form the 1/2 to 1-inch layer making sure you have no gaps in the moss. Using distilled water is best, though not necessary. You can "distill" tap water by filling mist bottles 24 to 48 hours before planting your terrarium and just letting the bottles sit. The moss layer serves two purposes. One, it acts as a second filter to the charcoal and, two, it helps the soil absorb water.

Add the soil. This will be the thickest layer. Make the thickness equal to the depth of the root ball of your largest plant. Here, again, it's easiest to add the soil using your small spade.

Next, the fun part: adding the plants!

Insert your plants. Once again, there are no rules, just a few guidelines. Choose a mix of plants with different heights, colors and textures that appeal to you and place them in a manner that you find aesthetically pleasing. Tall plants, for example, don't have to go in the middle. You can even divide the plants as you pull them from the pots, but don't break them into more than thirds. One thing to think about is that underplanting is better than overplanting. Remember, the plants will grow! When adding the plants, massage the root mass a little to break up the roots, which will help stimulate new root growth. Then nestle the plants into the soil, keeping the soil even with the top of the root ball.

Water in the plants. Use a mister to avoid creating a stream of water like you would get from a watering can or cup. The mist will help settle the soil. A stream of water, on the other hand, will dislodge the loose soil, form puddles and cause some soil particles to splatter up onto the sides of the terrarium, essentially creating a mess! If you can, avoid misting the foliage, though this may not be possible. In any case, be patient. This will require repeat mistings. This is also a great time to "rinse" the sides of the terrarium with the mister to remove any soil or other potting medium residue you may have gotten onto the interior of the glass. If you feel like you should add more water that the mister is providing, you can take the top off the mister, hold your thumb over the opening and gently sprinkle water into the terrarium.

How much water should you add? The goal is to evenly saturate the soil. The color of the Sphagnum moss (if you used that instead of peat) will give you a clue about whether you are achieving this. As the water seeps through the soil and into the Sphagnum the moss will turn from a light tan color to caramel. The idea is to moisten the soil and the moss, but not form a "pond" in the rocks. When the soil and moss are dampened, the terrarium will create its own self-sustaining environment. Put the top on if you've made a closed terrarium, and you're finished making the terrarium. But you're not quite done.

Step 4: Placing the terrarium

A terrarium box shaped like a house Terrariums like a certain amount of light. (Photo: Mr Nai/Shutterstock)

Now, if you haven't already, you need to find a spot in your home for your terrarium. Even with low-light plants, finding a good place where the plants will thrive may present a challenge. That's because many people tend to overestimate how much sunlight comes into their homes. Choose a location where the terrarium will receive good, indirect light.

Locations near an east-facing window tend to be the best choices for optimum growing. Morning light offers sufficient light without being too strong. In addition, plant metabolism is more active in the morning than in other parts of the day. Your plants will get good light at a time when they will benefit from it the most. South-facing windows offer the next best light; then west-facing windows, but be sure not to place the terrarium too close to the window where the strong afternoon light might be too bright. North-facing windows generally offer poor light for growing even low-light plants. Remember, "low light" doesn't mean "no light."

Two other considerations for placing your terrarium are:

1. Avoid placing them near air vents.
2. Think about height. Place the terrarium at eye level or higher to avoid looking down onto the top of the terrarium, especially if it is sealed.

Step 5: Maintaining the terrarium

A terrarium with small stone animals You can add little flourishes to your terrarium, like small stone animals. (Photo: tarapong srichaiyos/Shutterstock)

Give the terrarium a couple of weeks to see if it is too dry and needs more water or if water is pooling in the bottom, rock layer. If too dry, add more water following the method described previously. If too wet, just unseal the container for a day or so and let some water evaporate. An indication you may have too much water is if the interior of the terrarium becomes foggy. Condensation on the interior of the glass is normal and desirable. As the plants transpire through photosynthesis in the enclosed, moist environment they will create a sort of rain cycle in which trapped moisture will condensate on the interior of the terrarium and drip down the inside of the glass. When this happens, it indicates you have created an enclosed rainforest tropical plants should love. Too foggy probably means too much water has accumulated in the terrarium.

The only other thing you need to do is rotate the terrarium a quarter turn every few weeks. Plant foliage will orient toward a light source. Rotating the terrarium will keep the plants from all "leaning" in one direction. Other than that, your little science experiment should not require any maintenance. The record for plants growing in an unsealed terrarium, for example, is said to be 50 years!

Don't worry if you lose a plant or two. Simply replace ones that don't make it with another of the same type or a similar variety. And don't feel guilty. "After all," said Doud, "no one kills more plants than the experts."

Editor's note: This story was originally published January 2015 and has been updated with new information.