How to Make a Small Terrarium for Plants | A Simple Step-by-Step

Building a small terrarium takes about twenty minutes and a handful of supplies you can find at any garden center. The appeal is that these tiny ecosystems need almost no care once sealed. The method breaks down into five layers and one planting step, and the whole thing works in any clear glass jar, bowl, or vase you already own.

The exact layering matters more than most tutorials admit. Water needs somewhere to drain inside the container, and the drainage layer of pebbles at the bottom is what keeps roots from sitting in moisture and rotting.

What You Need Before You Start

Gather the container, the base layers, and the plants first so assembly flows without stopping. The plant choices matter as much as the container, since a closed terrarium traps humidity while an open one dries out.

• Container: any clean glass jar, vase, bowl, or bottle with a wide enough opening to reach inside
• Drainage layer: small pebbles, gravel, or aquarium stones
• Activated charcoal: a thin layer to absorb odors and keep water fresh
• Barrier (optional): a layer of sheet moss or sphagnum moss to keep soil from sifting into the pebbles
• Potting mix: a soilless mix works best because it stays light and drains well
• Plants: small ferns, mosses, or moisture-loving tropicals for closed builds; succulents or cacti for open builds

Kew Gardens recommends the same core sequence: pebbles, charcoal, soil, then the largest plant first, finishing with decorative stones or moss. If you prefer a closed terrarium, you will also need a lid or a container that comes with one.

The Layer-by-Layer Build Method

Wash and dry the container completely before adding anything, since a dirty or damp vessel invites mold from day one. Then build the layers in this order.

Step 1 — Add the drainage layer. Spread a layer of pebbles or gravel across the bottom. Guidance ranges from 1 to 2 inches depending on container depth, but a good rule is roughly one-quarter to one-third of the total height for the base layers combined. Enough for water to pool below the soil line is the actual goal.

Step 2 — Add the charcoal. Sprinkle a thin layer of activated charcoal over the pebbles, about a quarter to a half inch. This step is what keeps a sealed terrarium from going sour, though it is optional if you are building an open container that dries out regularly.

Step 3 — Add the soil barrier. If you want to keep the layers visually distinct, lay a sheet of sphagnum moss over the charcoal before adding soil. This step is optional but makes the terrarium look cleaner and prevents soil from migrating into the drainage zone.

Step 4 — Add the potting mix. Oregon State Extension recommends 3 to 4 inches of soilless potting mix for most builds, while Proven Winners suggests about 1 inch of sterile mix for smaller containers. Match the depth to the size of your container and the root length of your plants, then gently pat the soil down.

Step 5 — Plant, largest first. Set the biggest plant in first, then fill around it with smaller ones. Keep space between the roots and the glass so plants can grow without crowding the walls, where airflow disappears and rot can start.

Cover the roots with soil, add decorative rocks or moss on top if you like, and water lightly. For a closed terrarium, put the lid on now; for an open one, leave it uncovered. A misting every couple of weeks, or whenever the soil feels dry to the touch, keeps things healthy without overwatering.

Planting Layout and Care That Keeps It Alive

Taller plants belong in the back and shorter ones in front, with enough space between each so leaves never touch the glass. The same rule applies to the lid: a closed terrarium traps humidity, so it suits ferns and moisture-loving tropicals, while open builds work better for succulents and cacti that need drier air.

The single most common mistake is overwatering. A closed terrarium needs only a light misting and can go long stretches after sealing because the water cycles continuously. Direct sun is the other killer — these containers magnify heat and cook the plants inside. Bright indirect light is the sweet spot, and an east-facing window suits most choices well. The best small plants for a terrarium covers which species thrive in each build type.

Charcoal is worth including even when a tutorial calls it optional. It absorbs the byproducts of decomposing organic matter and keeps a sealed container smelling fresh, and the cost is trivial compared to rebuilding a moldy terrarium later.

Common Mistakes That Kill Small Terrariums

The failures that sink most terrariums come from rushing the setup, not from the plants themselves. Oregon State’s guide highlights a few specific problems to plan around.

• Skipping the drainage or charcoal layers in a sealed vessel traps water against roots and invites rot
• Overwatering — closed terrariums need only light misting and may go months after sealing
• Crowding plants so they touch the glass and block airflow
• Direct sun, which magnifies heat inside the glass and cooks the plants
• Mismatched plants — moisture lovers in open builds dry out, while succulents in closed builds rot

If you build a plastic-bottle version that requires cutting, that step needs adult help because the cut edge is sharp. Glass containers avoid the cutting issue entirely, but always wash and dry them before planting.

FAQs

How Often Should I Water a Small Terrarium?

Mist every couple of weeks or whenever the soil feels dry to the touch. Closed terrariums need far less water because the sealed lid cycles moisture continuously, so check the soil before adding any. Overwatering is the fastest way to kill a terrarium, so err on the dry side.

Can I Use Regular Potting Soil for a Terrarium?

Standard potting soil works, but a soilless mix drains better and stays lighter inside a sealed container. Heavy garden soil compacts quickly and holds too much moisture, which leads to root rot in a vessel with no drainage holes. Stick with a soilless potting mix for the best results.

What Plants Work Best in a Closed Terrarium?

Moisture-loving plants like small ferns, mosses, and tropicals thrive in closed terrariums because the sealed lid traps humidity. Succulents and cacti need drier, high-airflow conditions and belong in open builds instead. Always match the plant to the container type for the best chance of success.