Guide
A mason jar terrarium, built in an afternoon.
A 32 oz jar, four layers, and a handful of moss from the yard. Children can do most of this themselves, and the jar keeps going for months with almost no attention.
What you need
A wide-mouth 32 oz mason jar with its lid. Pea gravel, horticultural charcoal, potting soil, and a palm of moss. A spoon works better than fingers for the soil layer, and a chopstick helps set the plants without smearing the glass.
The four layers
Layer 1 · About one inch
Drainage
Pea gravel or small creek stones at the bottom. Water that sinks past the soil collects here instead of rotting the roots.
Layer 2 · A thin scatter
Charcoal
Horticultural charcoal keeps a closed jar from turning sour. Skip it and the jar will still work, but it will smell after a month.
Layer 3 · Two inches
Soil
Plain potting soil, damp enough to hold a shape when squeezed. Slope it higher at the back so you can see the layers from the front.
Layer 4 · Whatever fits
Plants and top layer
Moss pressed down firmly, then a small fern or a seedling. Finish with bark or a stone the child picked out.
Where to find moss
The north side of a shed, the shaded edge of a driveway, a rotting log at the back of the yard. Lift a piece the size of a palm with a butter knife, keeping the soil that comes with it. Leave the rest of the patch alone so it grows back in.
Common questions
- How much water does a closed terrarium need?
- Two tablespoons at setup, then almost none. A closed jar recycles its own water. Add more only when the glass stays clear for several days and the moss feels dry.
- Why is the glass foggy?
- Fog every morning that clears by afternoon is normal. Fog all day means too much water: leave the lid off for a few hours.
- Where should it sit?
- Bright indirect light. A windowsill in direct sun cooks a sealed jar within an hour.
- Can we put a bug in it?
- For one night, yes. A slug or a pill bug found after rain can stay overnight and go back where it came from in the morning. A sealed jar is not a home.
- Is moss collecting allowed?
- Take a palm-sized piece from your own yard or from a spot where you have permission, and leave the patch it came from intact. State and national parks do not permit collecting.
If you would rather not gather the parts
We put the same build into The Family Terrarium Kit, with the jar, the layers measured out, and the instructions we wrote for our own kitchen table. The overnight slug terrarium is a shorter version of the same idea.