Open or closed? Choosing the first jar
Published Sep 1, 2026 · 7 min read
Closed and open terrariums are two different climates, not two styles — the lid decides everything else.
Before gravel, before moss, before anything, there is one decision that shapes the whole jar: will it have a lid? A closed terrarium and an open one are not two styles of the same thing — they are two different little climates that want different plants, different watering and different places in the house. Get this choice right and the rest of the notes fall into line. Get it wrong and you'll be fighting your jar for months.
What a lid actually does
Seal a jar with damp soil and plants inside and you've built a water cycle: moisture evaporates from the soil and leaves, condenses on the cool glass, runs back down, and is drawn up again. The air inside stays near 100 percent humidity, the temperature barely swings, and the plants inside — if they're the right plants — live as if in a rainforest floor. That's why a well-built closed jar can go months without watering. Take the lid off and the cycle breaks: the jar dries like any pot, needs regular watering, and suits plants that want air moving and soil drying between drinks.
Closed: the easier first jar
We recommend a closed jar to every beginner, for one reason: it removes the watering decision, which is where most indoor plants die. Once the moisture level is right (Jar 05 shows how to read it), a sealed jar keeps itself right. Its plants — mosses, tiny ferns, creeping fig, nerve plants, baby's tears — are cheap, slow, and content in a humid box. Its needs: bright indirect light and to be left alone. Its failure modes are gentle and visible: too much fog, a bit of fuzz, a yellow leaf, all covered in Jar 07.
Any jar that closes will do. A large pickle jar with its own lid, a cookie jar with a glass top, a cork-stoppered bottle, a demijohn, a lantern with a door, a cheap fishbowl with a saucer on top. Two things matter more than beauty: an opening you can get a hand or chopsticks through (wider is easier), and glass that's clear rather than tinted, because tint steals light. Start big; a jar the size of a small bucket is far more forgiving than a jam jar, whose tiny volume swings wet-to-dry in a day.
Open: the dry-lovers' bowl
Open terrariums — bowls, wide vases, lidless jars — are for plants that would rot in a sealed one: succulents, small cacti, air plants, some tillandsia arrangements. They want the opposite climate: airflow, quick-drying soil, more sun than a closed jar could survive. They're beautiful and they're genuinely a different hobby: watered sparingly but regularly, placed nearer a window, and less self-sufficient. If you already keep succulents happily, an open bowl is a natural extension. If you've killed a few, the closed jar will treat you better.
The plants decide, not the jar
The most common beginner mistake is mixing climates: a succulent under a lid (it rots in a fortnight) or a fern in an open bowl on a sunny sill (it crisps in a week). Pick the plants you love first, then let them dictate open or closed, then choose glass to match. Jar 03 sorts the plants by climate so you can't accidentally cross them.
Three questions to settle it
- Where will it live? A bright spot out of direct sun suits a closed jar; a sunny sill suits an open bowl (Jar 06 explains why the reverse cooks or crisps).
- How much attention do you want to give? Closed: minutes a month. Open: a few minutes a week, forever.
- Moss and ferns, or succulents and stones? Green and damp says lid. Grey-green and architectural says open bowl.
Questions from the counter
Can I do a "half-closed" jar — a lid I take off sometimes?
Yes, and it's a good beginner middle ground: a closed jar whose lid gets cracked for an hour when fog is heavy. But it's still a closed-climate jar with closed-climate plants; the lid-off time is ventilation, not a change of regime. Don't use it to justify a succulent under glass.
Does the lid need to be airtight?
No — a loose glass top or a saucer over a fishbowl works fine. Some air exchange is good for the jar and reduces fuzz. What you want is enough closure that the glass fogs in the morning; if it never fogs, the jar is effectively open and should be treated as one.
Plastic containers?
Clear plastic works for the water cycle but scratches, yellows and clouds within a year, and the whole pleasure is looking in. Glass from a charity shop costs about the same and lasts forever. If you must use plastic, use it for the first practice jar.
Lid decided — now what goes in the bottom: the five layers.