When I built my first terrarium, I was hooked. It looked great on day one… and completely collapsed by day twenty. Mold, rot, sad little plants—I’d followed a tutorial, but still got it wrong.

After years of experimenting, I’ve learned there are a few key rules that make the difference between a thriving terrarium and one that slowly (or quickly) falls apart. Whether you're building your first or your fiftieth, these three principles are the foundation of any healthy, long-lasting setup.

Let’s get into it.

Rule #1: Right plant, right environment

Most beginner terrarium fails start here. You’ve got a beautiful glass container, a fresh bag of soil, and a trip to the nursery under your belt—but you’re mixing plants that were never meant to live together.

Why it matters:

A terrarium is a tiny, closed (or semi-closed) environment. That means you’re in charge of recreating the right balance of light, airflow, humidity, and moisture. If you put a desert plant like a cactus next to a humidity-loving fern, one of them (or both) is going to suffer.

What I wish I knew:

Terrarium success starts with plant compatibility. Always group plants with similar needs. It sounds obvious, but it's easy to ignore when you're focused on how things look.

Two plant families I recommend for beginners:

• Tropical/humidity lovers – fittonia, pilea, ferns, mosses, peperomia

• Dry-adapted plants – succulents, cacti, haworthia, jade

These two categories need opposite conditions. Keep them separate.

If you want to build a high-humidity closed terrarium, stick with tropicals and mosses. If you’re making an open one for a sunny spot, succulents can work—but only with excellent drainage and airflow. Don't try to mix both in one container.

Rule #2: Less water than you think

This one hurts, because it feels like you’re helping when you water. But overwatering is the number one terrarium killer. Especially in closed or semi-closed systems where moisture doesn’t evaporate as quickly.

Why it matters:

In a small space without drainage holes, excess water has nowhere to go. It sits in the soil, drowns the roots, and invites mold and rot. Even in open terrariums, too much water throws off the balance fast.

What I wish I knew:

You can always add more water—but you can’t take it out once it’s there.

Here’s how I approach watering:

• Start with a small amount. Just enough to make the soil damp, not soaked.

• Watch the condensation. In a closed terrarium, a little fog on the glass is good. If it’s fully fogged up 24/7, you probably overdid it.

• Feel the soil. Use your finger to test moisture before you water. If it’s still damp, wait.

• Use a spray bottle or dropper. This gives you more control than pouring from a cup.

If your terrarium looks too dry, it probably isn’t. Terrariums hold moisture longer than you’d expect, especially in humid climates or sealed containers.

Rule #3: Good layers = long-term health

The soil and hardscape aren’t just there to look nice. They’re functional—and skipping steps here is another common mistake.

Why it matters:

A healthy terrarium needs airflow around the roots, moisture control, and a structure that prevents stagnation. That’s where the layers come in.

What I wish I knew:

Every terrarium, open or closed, should have at least three basic layers:

1. Drainage (gravel or stones)
   This goes at the bottom to catch excess water and keep it away from the roots. About 1 inch is usually enough.

2. Activated charcoal
   A thin layer of charcoal helps filter the water and keep things fresh. It’s especially important in closed terrariums, where airflow is limited.

3. Soil
   Use a well-draining mix that suits your plant types. For tropicals, standard potting soil or aroid mix works. For succulents, go with cactus soil.

Optional fourth layer? Moss or decorative top layer. This holds in moisture, finishes the look, and discourages gnats and mold.

If you skip the charcoal, or skip the drainage, the system gets out of balance fast. Everything might look fine for a week or two—but you’ll run into issues sooner than later.

Bonus: light is everything (but not all light is equal)

Okay, this one’s not technically a “rule,” but it’s so important I had to include it.

A lot of people put their terrarium on a dark shelf or in a window with harsh sun—and then wonder why their plants aren’t thriving.

Quick lighting tips:

• Bright, indirect light is ideal for most terrarium plants

• Direct sun can overheat a closed container and cook your plants

• Low light = slow growth and long-term decline

• Artificial grow lights work great if your space doesn’t get natural light

If you’re not sure, try placing your terrarium a few feet from a window with filtered light (like through a sheer curtain). It’s a safe zone for most setups.

Final thoughts: start simple, stay surious

If there’s one thing I’ve learned building terrariums, it’s this: the basics matter more than the accessories.

A simple setup done right will outlast a fancy one with the wrong conditions. Follow these three rules—compatible plants, careful watering, and proper layers—and you’ll be surprised how well your terrarium holds up.

And the best part? Every terrarium teaches you something. Even the ones that don’t make it help you build better ones in the future.