If you've been looking at terrariums for a while, you'll have noticed that not all glass cases are the same. Some are sealed — little ecosystems that barely need touching. Others are open at the top, filled with succulents or cacti that crave dry air and direct sun. The two types need different plants, different placement, and a different approach from you as a plant owner. Get it right and either one will sit happily in your home for years. Get it wrong and you'll have a struggling plant and a nagging sense that you're missing something obvious.
This guide covers what each type actually is, how they differ in practice, and which one suits your home — and your patience.
What is a closed terrarium?
A closed terrarium is a glass container with a lid — usually a cork, a fitted glass stopper, or a hinged top — that creates a sealed, humid environment for tropical plants. Moisture evaporates from the soil, condenses on the glass, and drips back down to the roots. The plants cycle the same water, almost indefinitely, with very little input from you.
The plants inside tend to be ferns, mosses, pearlwort, and other tropical species that thrive in warmth and humidity. They grow slowly, which is part of the point: a well-set-up closed terrarium can hold its shape for years without much intervention.
A closed terrarium needs bright, indirect light — a well-lit room, but out of direct sun. Direct sun through glass turns the interior into an oven.
What is an open terrarium?
An open terrarium has no lid. It's typically a wide glass bowl, a geometric planter, or a shallow dish — something with a generous opening that lets air circulate freely. The plants inside are succulents, cacti, and air plants: species that evolved in dry, bright conditions and would rot quickly in the humidity of a closed container.
Open terrariums need direct sunlight — a south- or east-facing windowsill is ideal. They're watered occasionally and sparingly, and they're more forgiving if you forget them for a week or two.
Open vs closed at a glance
Closed terrarium
- Sealed glass container with a lid
- Tropical plants: ferns, moss, pearlwort
- Needs bright indirect light — no direct sun
- Self-watering cycle; minimal intervention
- Humid interior; slower-growing
- Suits most UK rooms with reasonable natural light
Open terrarium
- Open glass container, wide mouth
- Succulents, cacti, air plants
- Needs direct sunlight — south- or east-facing window
- Water occasionally and sparingly
- Dry interior; tolerates neglect well
- Suits bright, sunny spots; struggles in north-facing rooms
Which suits your home?
Light is the deciding factor
This is the single most important question: how much natural light does the spot you have in mind actually get?
In the UK, light is a real constraint — particularly from October to March. North-facing rooms can go weeks with little more than grey ambient light. South- and west-facing rooms get genuine sun, at least in spring and summer.
If you're placing a terrarium in a north-facing room, or anywhere that doesn't get direct sun for at least a few hours a day, a closed terrarium is almost always the right call. The tropical plants inside tolerate lower light far better than succulents or cacti, which will stretch, pale, and eventually give up without sun.
If you have a bright, sunny windowsill — especially south- or east-facing — an open terrarium with succulents is a strong choice. The light suits the plants, and the dry air near a window suits the open container.
Heating, humidity and UK winters
UK homes present a specific challenge for open terrariums in winter: central heating drops indoor humidity significantly. For succulents and cacti, that's actually fine — they like dry air. For closed terrariums, the sealed environment handles humidity internally, so your heating system doesn't affect the plants inside.
The practical upshot: closed terrariums are more resilient to the UK's variable climate indoors. They sit well in heated living rooms, north-facing spare bedrooms, and the kind of low-light corners most UK homes actually have plenty of.
Pets, children and curious hands
Open terrariums with succulents are generally the safer option around pets, but it's worth checking individual plant species if you have cats or dogs that graze. Succulents vary — some are fine, others aren't.
Closed terrariums, being sealed, keep the plants out of reach and reduce the chance of soil being scattered. If you have a curious cat, a sealed glass container is less of a temptation than an open bowl of interesting-looking soil.
Which suits you as a plant owner?
If you want low effort, choose closed
A closed terrarium asks very little once it's set up. Place it in bright indirect light, open the lid for an hour every week or two to let fresh air in, and mist lightly if the glass stays completely clear for more than a few days. That's largely it.
The self-sustaining water cycle does most of the work. You're maintaining rather than managing. For anyone who wants living plants without a caregiving routine, a closed terrarium is the more forgiving choice. Our how to build a closed terrarium guide walks through the build from scratch if you want the full picture.
If you want to tinker, choose open
Open terrariums reward attention. Succulents and cacti are slow-growing but they do change — a new pad on a cactus, a rosette that tightens in strong light, an air plant that pushes out a flower spike if you've been misting it well. They're satisfying to arrange and rearrange, and the dry soil means you won't cause damage if you move things around.
Watering is simple — a light pour every two to three weeks in summer, less in winter — but it does require more active judgement than a closed setup. If you forget, most succulents will forgive you. If you overwater, they won't. Our step-by-step open terrarium guide covers plant placement and soil layering in detail.
Common mistakes with each
With closed terrariums:
- Placing in direct sun (overheats the interior quickly)
- Overwatering before sealing, leading to mould early on
- Using the wrong plants — succulents and cacti will not survive in a sealed humid container
With open terrariums:
- Placing in a low-light room and wondering why the plants stretch and pale
- Overwatering — the most common cause of succulent failure indoors
- Choosing plants with incompatible water needs and mixing them in the same container
Avoiding these three in each category accounts for the majority of early problems. For a deeper look at what thrives inside a sealed container, our guide to plants that do well in a closed terrarium is worth a read before you build.
Our verdict — and a starting point
For most UK homes, a closed terrarium is the more straightforward choice. The light requirements are easier to meet, the maintenance is minimal, and the self-sustaining water cycle means you're not guessing at watering schedules. It's also better suited to the variable light and heated interiors that define most UK rooms for a good chunk of the year.
An open terrarium is the right call if you have a genuinely sunny windowsill and want plants that reward a bit of hands-on attention. Succulents and cacti are tough, interesting plants — they just need the right conditions to show it.
Decided which suits you? Browse our closed terrarium kits or open terrariums to find the right starting point.
