Terrariums: The Water Cycle

Terrariums are wonderful projects: they’re easy to plant, easy to care for and they look wonderful. They also recycle their moisture, so they rarely need to be watered, requiring almost no attention. Often, a closed terrarium can be left for a month or more between watering.

Discussing the water cycle is a great introduction for this project. What are clouds? What are they made of? What is rain? What does the sky look like when it rains? Why does it rain? Where does the rain go after it falls? What happens to puddles after it rains? These questions will start a discussion about evaporation, condensation, and precipitation. Discuss each of these things as you put your terrarium together. (You may want to check out the animated diagram of the water cycle from the US Environmental Protection Agency.)

Any clear container can be made into a terrarium; just make sure that your container is watertight. Choose something large enough to accommodate the plants, and has a cover, lid, or door to keep the moisture from escaping. Jars, bottles, and aquariums are commonly used and each works great. Whatever the container, you can now easily bring nature into the classroom.

Many plants do well in terrariums, and it is best to choose the ones that will fit the size of the container. Slower growing plants require less trimming, and are less likely to take over. If you are willing to pay more attention to them, you can experiment with more aggressive plants. They require more frequent trimming, but will allow you to have more variety in your terrarium.

Some plants suitable for terrariums are:
Pilea (Aluminum Plant )
Fittonia ( Nerve Plant )
Podocarpus ( Buddhist Pine )
Aeschynanthus ( Lipstick Plant )
Baby Tears ( Very aggressive grower! )
Very small ferns
Miniature African Violets
Coffee Plant
Creeping Charlie
Boxus (Boxwood)
Wandering Jew (Aggressive Grower)
Creeping Fig (Aggressive Grower)

Planting Instructions:

1. Place a 1/2 inch layer of small gravel in bottom.
2. You may choose to sprinkle activated charcoal on top of the gravel, but this is optional. It will help to filter the water as it drains through the layers.
3. Test your potting soil before using it by squeezing a handful. If it clumps easily, add some Perlite or Vermiculite to help with drainage. These can usually be found in garden shops. Add a 2-inch layer of potting soil, or possibly a little more depending on the size of your container and the size of the plants you intend to use.
4. Add your plants, again taking into account the size of the space you have to work with inside the terrarium. Be careful not to overplant – you need to leave plenty of room for your plants to grow. Push the soil aside, place a plant in the depression, and gently replace the soil around the roots of each plant. Water lightly.

Neglect It! Water lightly only when the soil is dry. You should only need to water, at the most, every couple of weeks, depending on conditions. Be very careful not to overwater! Place in a bright area, but not in direct sunlight. You should have enough light to read by. When plant gets as big as you want, pinch off the newest growth to encourage bushier growth.

Do not fertilize. As the nutrients found in the potting soil get used up, the plant’s growth will slow, helping to keep the plant from overgrowing the terrarium. Over time the soil can be “refreshed” by scraping off the top layer of soil, and adding some fresh potting soil. This will add a small amount of nutrient, and will spruce up the look of your terrarium as well.

Small rocks, moss and dried twigs make good decorations and add to the look of a micro-world of plant life. A terrarium can also be an ideal place to observe insects, but you will want to return them to the outside world after a few hours so they can survive in their natural habitat.

When your terrariums are finished, discuss the following: We only watered the soil in our terrariums once; how did the water get on the lid? Take your lid off the terrarium and feel the soil. Why is the soil still wet? Do you think that any water has evaporated from the soil? Why? If water evaporated, where did the evaporated water go? Did it ever rain in your terrarium? How do you know? Where did the rain come from? Is there anything in your terrarium that reminds you of a cloud or cloud drops?”

You may want to make a connection between the water cycle in the terrarium and in the real world with a discussion using the following: “If the terrarium is a model of the real world, what do you see outside that reminds you of the plant in our terrarium? reminds you of the soil in our terrarium? reminds you of the small water droplets on the lid? The soil in our terrarium stays moist, the ground outside never dries out completely. Why? What keeps it moist? Water collects on the lid of the terrarium, water also collects in the sky as clouds, where does the water in the clouds come from?

Keep your terrarium after the lesson is over and enjoy it for many months to come!