Ferns in a closed terrarium: a comprehensive guide

Introduction

Ferns are amongst the oldest plants in the plant kingdom, renowned for their elegance and lush foliage. In a closed terrarium, they find an ideal environment: gentle warmth, constant humidity and soft light. This guide explains how to choose, plant and care for your ferns in a closed terrarium, to recreate a real mini-jungle under glass.

Why choose a closed terrarium for ferns?

A closed terrarium helps maintain high humidity and a stable environment, both of which are essential for the good health of ferns.
Unlike open terrariums, a closed terrarium limits evaporation and creates a tropical microclimate. This replicates the natural habitat of ferns, which are often found on the forest floor in damp woodlands, sheltered from direct light.

Advantages:

• Constant humidity without the need for frequent watering
• Mild and stable temperature
• Natural and attractive décor
• Low maintenance

Which ferns should you choose?

Not all ferns are suited to the confined conditions of a closed terrarium. Here are a few ideal species:

• Asplenium nidus: a tropical fern that thrives in shady spots with high humidity. Karnivores Nurseries
• Nephrolepis exaltata (e.g. ‘Green Fantasy’ variety): finely cut foliage, tropical feel, adapts well to terrariums. Karnivores Nurseries
• Dryopteris erythrosora: a fern with intricate fronds and a subtle coppery tinge – makes for an interesting visual feature. Karnivores Nurseries
• Athyrium spicatum (e.g. ‘Chiseled’ variety): a beautiful Asian fern with finely cut fronds and a striking decorative effect. Karnivores Nurseries
• Pyrrosia lingua ‘Cristata’: an epiphytic/epilithic fern, suitable for humid terrariums, adding textural variation. Karnivores Nurseries

Tip: choose young plants or plants with small root balls; large ferns are likely to touch the sides of the pot and become misshapen quickly.

How to set up a terrarium

1. The container

Choose a jar, a bottle or a glass dome with a lid. Clear glass lets light through whilst retaining water vapour.

2. Drainage

Place a layer of clay balls or fine gravel (2–3 cm) to prevent the roots from sitting in standing water.

3. The filter layer

Add a thin layer of filter moss or activated charcoal to prevent mould and purify the air.

4. The growing medium

Use a light, rich and well-draining mix. We strongly recommend using our special fern mix, which we produce ourselves (and use for our own plants), precisely because it ticks all these boxes.

5. Planting

Arrange the ferns, leaving a little space for them to grow. Cover the surface with natural Hypnum moss, live sphagnum moss and other fern mosses to retain moisture and maintain a natural appearance.

Light and location

Ferns prefer soft, diffused light, without direct sunlight.
Place the terrarium near a north- or east-facing window.
Avoid sources of heat (radiators, direct sunlight) which dry out the air.

Watering and care

• Watering: generally not necessary! The condensation cycle in a closed terrarium recycles moisture.
• Ventilation: open the lid once a week for 30 minutes to refresh the air and prevent excessive condensation.
• Cleaning: Wipe the sides if too many water droplets build up on them.
• Taille: cut back yellowed fronds to stimulate growth.

If the substrate becomes too dry, lightly mist it with soft water.

Common problems and solutions

Problem	Probable cause	Solution
Yellowing leaves	Overwatering, poor air circulation	Ventilate more often
Mould on the growing medium	Too much stagnant moisture	Remove the mould, ventilate, reduce watering
Dried-out leaves	Lack of humidity, excessive heat	Close the terrarium, move it away from direct sunlight

Combining ferns with other plants

Ferns go well with:

• Tropical mosses
• Fittonia
• Peperomia
• Small Tillandsias (epiphytes)
• And even some tropical carnivorous plants, provided the terrarium’s climate is well managed.

Conclusion

Creating a closed fern terrarium means recreating a small, self-contained ecosystem that is both soothing and decorative.
With a little care at the start, you’ll be able to enjoy a little piece of the tropical rainforest on your desk or in your living room for years to come.
Ferns are the unassuming queens of the closed terrarium — elegant, ancient and incredibly vibrant.