Don't have 4.5 billion years to make a planet? Make your own tiny world on the quick and cheap! After you throw in the basics, cap it off with an ozone, and let life take over. This terrarium is a great quick project to introduce kiddos (and adults, too!) to the growing world. And after the first soaking, it waters itself! This is one of my favorite projects to do in a classroom (making 30 at a time is no problem), and signs of life spring up from your work even within a few days. Let's grow!

What: Self-Watering Terrarium
Cost: ~ $0.30 per terrarium
Concepts: biology, ecosystems, plant life cycle
Materials:
- Recycled tennis ball container with cap (we ask at tennis courts, but other plastic containers will do)
- Sand
- Potting soil
- Water
- Seeds (scarlet runner beans and grass seeds are excellent)
- Small cup for pouring

Oh, I plant wait!

Step 1: Prep Your Container

Time to make the plant feel right at home! Remove any labels on your tennis ball container so that sunlight can get in, and we can see in the sides. Make sure to save your cap for later.

Depending on who you're working with, this is a great time to talk about why plants need sunlight, and the photosynthesis process for making plant food. Here's a great explanatory resource for that!

Step 2: Sanding. Literally.

Start by adding sand to your container, scooping a bit at a time. My guide for students is to make it about two finger widths deep, which they can measure by placing their hand along the side of the bottle. Tap the bottle on the ground to make the sand layer flat.

The sand will act as a drainage run-off for extra water in our terrarium to ensure that the soil doesn't get flooded. This will also increase our terrarium's overall capacity for water, which will eventually get converted in to plant material. We keep the layer thin because most plants' roots don't do well in sand as it is low in usable nutrients.

Step 3: Soil Your Plants!

Add in your potting soil on top of the sand layer, ensuring that there is no mixing. For our plants, any general potting mix at the store will do, or you could test out local soils to see how well they perform. Pour the soil about four finger widths deep, ensuring there is a good amount of space for roots below but also for stalks above.

Learning about the science of soils is fascinating and is a central topic in agricultural science. I found this page to be accessible and interesting if you want to go deep in to soil science.

Step 4: Spread Those Seeds!

For our terrariums, I often use scarlet runner beans (the big ones), and grass seeds (the wee ones), often throwing in brasicas or any other seeds or beans we happen to have around. You can find these at any gardening section, but also try out beans from the grocery store. They're often still viable, which is pretty fun.

For the big seeds, make a hole with your finger or a pencil, drop it in, and cover it up. A general rule is to bury seeds at a depth approximately half their longest dimension. You can place a second big seed in there, in case the first one doesn't germinate, but make sure to spread them apart. Don't want them competing too much!

For small seeds, you can just sprinkle them on top. For grass, it grows everywhere (that's what it's bred for!), so you can go heavy with this and they're pretty much guaranteed to grow.

If you want to see more on seed science, the wikipedia page on seeds is the best resource I've found with seed anatomy with the University of London posting this look deep into seed anatomy.

Step 5: One Time Watering

To begin the self-watering cycle, we're going to add some in slowly. Grab a small cup, and water your terrarium slowly. You can even tilt and pour down the side of the container to minimize impact with the soil. As you pour, if you look down the side, you can see water seeping down to the sand layer. You want to keep pouring until your soil looks like a nice, chocolate cake mix but without pools.

What's exciting is that after you add this water, you never have to water again. That's right, all the water that ends up in our plants is from that initial bit that you added. Pretty neat, huh? Even weeks or sometimes months out, we've had growing plants all from that first bit of water. It's a mini-biome in there!

NOTE: If you're doing this with young students, this is often the hardest step to get right. If they go too far, you can salvage by adding more dry soil on top to absorb the extra water, but it will make it harder for your seeds to grow.

Step 6: Cap and Place!

Oh joy! Your self-watering terrarium is ready to go. Already in the first photo, you can see the water cycle beginning with water condensing on the sides of the container. Cap your terrarium, and place your terrarium in a sunny place to start catching some rays. You can leave your terrarium capped forever.

The greenhouse effect of sunlight will continue to keep water cycling. As your terrarium heats up in the day, the air will become moist, condense, and sometimes precipitate back down to the top of the soil. Even if you can't witness it in action, your terrarium has all the water it will ever need.

Within a few days, life should start to spring up!

Here is a small explanation of the greenhouse effect, and another here of the water cycle.

Step 7: Let It Grow!

Life finds a way. You'll start seeing sprouts in no time, and here's our terrarium after only a week. You can see new grass growth that's still pale, and the older stuff that has started to photosynthesize. Often the grass will grow all the way to the top of the terrarium. Our scarlet runner bean has opened up, too, and you can see its taproot coming out and heading into the soil. So exciting, and with another week or so, this will become a full leafy plant!

After your plants have grown nice and big, you can keep them in there (maybe with a bonus watering if it's been a long time), or you can transplant them to your garden to find a new, bigger, and earthier home.

Here are a couple tips and experiments that you can do:

What Else Grew? Sometimes you may find that when you uncap your terrarium, an insect may fly out. A lot of potting soils start with insect larvae inside, so you grew an animal, too! You may also find that you grow some fungi that you don't want, so feel free to uncap occasionally to let new air in, which will suppress growth.

Different Environments: If you want to conduct a science experiment, try growing some in darkness, some in partial darkness, and others in full light. The results are really interesting. You'll still see grass growth in the dark environment, but they will be pale white because they're not photosynthesizing. Pretty neat!

Different Inputs: You can mix your own soil, add different amounts of water or sand, or different numbers or types of seeds. Make predictions and see what happens!

Have fun, explore, and keep growing!