Open vs Closed Terrariums: Which One Should You Build?

The single biggest decision in planning a terrarium isn't the jar or the plants — it's whether the container will be open or closed. That one choice shapes almost everything downstream: how humid the inside gets, how often you'll water, and which plants will actually thrive there. Neither style is objectively better; they're suited to different plants and different amounts of attention, and mixing the wrong plant into the wrong style is the single most common way a new terrarium fails in its first few weeks.

What "open" and "closed" actually mean

An open terrarium has no lid, or has one that doesn't seal — a wide-mouth bowl, an uncovered glass tank, or a jar with a loose-fitting top. Air moves freely in and out, so moisture that evaporates from the substrate and plants escapes into the room instead of staying trapped. A closed terrarium has a tight-fitting or sealed lid, so the moisture cycle happens almost entirely inside the glass: water evaporates from the substrate, condenses on the cooler glass walls, and drips back down, over and over, in a small, self-contained version of the water cycle.

There's a middle ground worth knowing about too: a lid propped open a crack, or a jar with a loosely fitted cork, behaves somewhere between the two extremes — more humid than fully open, but with enough air exchange that heavy fogging and mould are less of a risk than in a fully sealed jar. It's a reasonable way to hedge if you're not sure yet which way a particular plant list will lean.

A sealed vessel also holds its temperature more steadily than an open one, simply because there's less air exchange with the room to pull it back toward ambient. That's mostly a minor detail, but it means a closed terrarium sitting near a draughty window or an air-conditioning vent is somewhat more buffered from those swings than an open display in the same spot would be — one more small way the two styles behave differently beyond humidity alone.

Humidity and watering

This is where the two styles diverge most. A well-established closed terrarium can often go weeks, sometimes over a month, between waterings, because so little moisture actually leaves the container. The trade-off is that you need to watch for the opposite problem — a persistently fogged-up interior or mould developing on the substrate means too much moisture is trapped, and the lid needs to come off for a while to let some out.

An open terrarium behaves much more like a regular houseplant: it dries out at a pace closer to normal room conditions, so it needs more frequent attention, especially in a heated room in winter when indoor air runs dry. In exchange, it's far more forgiving of a slightly overzealous watering, since there's nowhere for excess humidity to get trapped.

Putting numbers on it helps: this site's own humidity guide targets roughly 80–95% relative humidity for moss and ferns in a closed vessel, against 60–80% for the same plants left open. Air plants want less humidity either way — around 60–75% closed and 40–60% open — which is part of why they're usually grown open rather than sealed. In a room running a fairly typical 40–45% ambient humidity, that works out to misting a closed moss-and-fern jar roughly two to three times a week, against closer to four times a week for the same plants left open, since the open vessel is losing far more of that moisture to the room.

Which plants suit which style

Closed terrariums are the natural home for plants that come from consistently damp, humid places — mosses, many ferns, and other shade-loving plants that would struggle in a dry room. They rely on that stable, high humidity to look their best. Open terrariums suit plants that prefer to dry out between waterings and don't want constant dampness around their leaves — succulents, cacti, and epiphytic air plants generally do far better without a lid trapping moisture against them.

Mixing the two groups in one container rarely ends well: a succulent in a closed, humid jar tends to rot, while a moss carpet in a dry, open dish tends to crisp up and brown. Picking the vessel type before the plant list, rather than after, saves a lot of frustration.

It's worth knowing, while you're choosing, that a few genuinely popular terrarium and houseplant options in both categories are toxic to cats, dogs, or small children if chewed — pothos, English ivy, and philodendron are common trailing plants that sometimes get added to a closed jar for extra greenery, and a few open-terrarium succulents people reach for, including jade plant, string of pearls, and aloe, carry the same warning. None of that means avoiding those plants outright, but a terrarium sits at coffee-table height in most homes, exactly where a curious pet or toddler can reach it, so it's worth checking a specific plant before it comes home if pets or small children share the space. The true ferns, most mosses, and most Tillandsia air plants covered elsewhere on this site are generally considered non-toxic by comparison, which is one more reason they're such common starting points. If a child or pet does chew on something and you're unsure whether it's a problem, a vet or poison control line is the right call, not a guess.

Reading condensation once it's built

A closed terrarium's glass becomes its own instrument panel once it's planted. A light, even mist that mostly clears within a few hours of a light cycle is the sign of a healthy, balanced water cycle — evaporation and condensation roughly matching each other. Heavy condensation that never clears, running down the glass in streams and never drying back, means the system is holding more moisture than it can comfortably manage, and it's worth cracking the lid for a few hours to vent some out before mould has a chance to take hold. A jar with clear, dry glass and a substrate surface that looks pale and dusty, on the other hand, is under-humidified and due for water — not a sign the vessel is "working" the way a consistently misty closed jar is.

An open terrarium doesn't build the same kind of condensation, so it's read differently: the substrate surface itself, and how a plant's leaves look, are the better signals. Curling or crisping leaf edges point to too little humidity for that particular plant; consistently soggy substrate that never dries between waterings points to too much for an open build, even though open vessels are generally more forgiving of that than closed ones.

Maintenance day to day

A closed terrarium, once established, is genuinely low-maintenance — the main task is watching the condensation pattern and cracking the lid open occasionally to vent excess moisture or let in fresh air. An open terrarium needs more regular watering and, because it isn't sealed, dust and debris can settle on the surface over time, so an occasional light clean-up keeps it looking its best.

Both styles benefit from a spot with bright, indirect light rather than direct sun, which can heat the glass and cook whatever's inside far faster than the same light would affect a plant in open air. That effect is worse in a closed vessel than an open one, since a sealed jar in direct sun can build up heat with nowhere for it to escape — a genuine risk to the plants inside, separate from the humidity question this article is mostly about.

Airflow issues show up differently in the two styles too. A closed terrarium that's rarely vented and stays overwatered is the classic setup for mould and fungus gnats, since still, damp air and decaying plant matter are exactly what both thrive in; regular brief airings and not overwatering in the first place do more to prevent this than anything applied after the fact. Adding a small population of springtails — tiny, harmless soil-dwelling arthropods sold specifically for bioactive terrariums — gives a closed jar an ongoing cleanup crew that grazes on mould and decaying organic matter before it builds up. An open terrarium, with more air movement by default, runs into this problem far less often.

Making the call

If you want something that mostly takes care of itself and you're drawn to moss, ferns, and other lush greenery, a closed terrarium is worth the slightly fussier setup. If you'd rather keep an eye on things regularly, enjoy air plants or small succulents, or want easy access to groom and rearrange the display, an open vessel is the more practical choice.

A couple of practical scenarios make the decision easier in the moment. If the container is going somewhere you'll walk past daily but rarely handle — a hallway shelf, a desk you don't often clear off — a closed jar's low-maintenance rhythm suits that kind of passive spot well. If it's going somewhere you're already tending other plants regularly, like a windowsill you water every few days anyway, folding an open terrarium into that same routine costs you almost nothing extra. And if you're planning to travel for more than a week or two at a stretch, a well-balanced closed terrarium is far more likely to survive the trip untouched than an open one will.

Either way, the humidity guide calculator on this site will give you a target range and a suggested misting cadence once you've settled on a style and a plant category, and it's worth revisiting that number if you move the terrarium to a different room — ambient humidity varies more from room to room than most people expect, especially between a kitchen and a heated bedroom in winter.