How to Build a Large Terrarium | Step-by-Step for a Thriving Mini Ecosystem

A large terrarium — a container at least 12 inches deep — needs a solid drainage base, the right soil mix, and slow-growing plants chosen for the ecosystem type you’re creating, whether open for succulents or sealed for tropicals.

A large terrarium turns a glass container into a self-sustaining garden, but the bigger the vessel, the less room there is for error. Getting the layering right and picking plants that won’t outgrow the space within months separates a thriving setup from a moldy mess. Whether you’re building from a glass jar, an acrylic cube, or a fish tank, the same principles apply. For readers ready to buy rather than build, we’ve rounded up the best extra large terrariums for plants on the market today.

What’s the Best Container for a Large Terrarium?

Any clear vessel at least 12 inches deep works — glass jars, fish bowls, clear plastic tanks, or custom acrylic builds are all fair game. The opening must be wide enough for your hand and tools to fit through comfortably. You can even place a large vessel on its side with the lid facing sideways for an unusual viewing angle.

Terrarium Layering: Drainage, Charcoal, Barrier, and Soil

The substrate stack is what keeps the ecosystem alive. Base layers — gravel, charcoal, and soil — should fill one-quarter to one-third of the total container volume.

Drainage layer: Add 1 to 1.5 inches of coarse gravel, pea gravel, sea glass, or beach stones. Small containers can get by with 0.5 inches, but large builds need the extra depth to keep roots out of standing water.

Charcoal layer: Scatter 0.5 inches of horticultural or activated charcoal on top of the gravel. Never use barbecue charcoal — the additives introduce toxins that will kill your plants. If you prefer to mix, use a ratio of 3/4 gravel to 1/4 charcoal. The charcoal prevents fungi, filters water and air, and absorbs odors.

Barrier layer: Place a thin separator between the charcoal and soil — sheet moss, synthetic window screen, weed cloth, or even a nylon hosiery patch. This keeps the soil from sifting down and clogging the drainage layer.

Soil layer: Add at least 1 inch of soil (2–3 inches for large containers) using a sterile potting mix. A good homemade blend is equal parts peat moss and rich garden soil. Moisten the soil until it clings together in a ball when pressed but no water drips out.

Choosing and Placing Plants for a Large Terrarium

Only slow-growing, small species belong inside a confined container. Pick plants that fit the ecosystem you’re creating: open/dry setups suit succulents and cacti, while closed/humid environments need tropicals like ferns, mosses, and peperomias.

Water the plants in their original pots a few hours to a day before construction. If your terrarium is viewed from all sides, place the largest plant in the center; if viewed from two or three sides, put the tallest in the back. For large builds, set tall plants in the center and shorter ones along the outside. Dig holes with a pointed stick or trowel, remove plants from pots, break up root balls, and separate them if multiple share a pot. Ensure 0.5 inches of soil covers the roots, then add decorative stones, bark, or moss patches. Use a dry paintbrush to sweep any soil off the container walls above the soil line — dirt there will rot and look unsightly.

Closed vs. Open Systems and Maintenance

After that, mist every couple of weeks only when the soil feels dry to the touch. Check for excessive condensation and wipe the glass if needed.

As the Missouri Botanical Garden’s guide on terrariums notes, common mistakes include using fast-growing plants that quickly hit the glass, skimping on drainage depth in large containers, and overwatering a closed system until mold takes hold.

FAQs

Can I use regular charcoal from the grill?

No — barbecue charcoal contains chemical binders and accelerants that release toxins harmful to plants. Only horticultural or activated charcoal is safe for use inside a sealed terrarium.

How often should I water a large closed terrarium?

Water very sparingly — only when the soil feels dry to the touch, typically every two to three weeks. Overwatering is the most common cause of mold and plant death in sealed ecosystems.

What’s the easiest maintenance mistake to make?

Allowing soil to stay on the glass walls above the soil line. It traps moisture against the glass, rots, and becomes a breeding ground for fungus gnats. Sweep it off with a dry brush immediately after planting.