Sowing Small Seeds in Open Terrariums: Depth, Size and Coordinating Several Projects

Open terrariums suit small, slow-growing plants that tolerate average room humidity: ferns, pileas, fittonias, small begonias, creeping figs and compact succulents in a freely draining mix. Raising these from seed is possible, but it is the part of the terrarium hobby where a rushed start costs the most. Seed size sets sowing depth, and sowing depth sets whether a project succeeds. When you are running several small terrariums at once, the practical solution is a routine that keeps seed size, moisture and timing consistent across every container rather than treating each one as a separate experiment.

Match sowing depth to seed size, not to the terrarium

Seed depth is a property of the seed. As a working rule, cover seed with a layer of substrate roughly two to three times its own diameter. Fine seed — the dust-like seed of many ferns, begonias and gesneriads — is sown on the surface and pressed in, because a covering layer blocks light and buries a seedling that has almost no stored energy to push through. Medium seed, such as fittonia or pilea, sits under a thin pinch of sieved substrate. Larger seed, including many succulents and larger houseplant species, tolerates and often benefits from a covering proportionate to its size.

Two conditions matter as much as depth. First, light: some fine seed needs light to germinate, so surface sowing is a requirement rather than a shortcut. Second, moisture contact: surface-sown seed dries out fast, which is why open terrariums need a different approach than open seed trays.

Why an open terrarium changes the method

A closed terrarium holds humidity; an open one exchanges air with the room. That airflow is good for plants that dislike stagnant, soggy conditions, but it dries the surface of the substrate steadily. Fine seed sown on the surface in an open vessel can fail simply because the top millimetre dries between waterings.

The workable compromise is to germinate in a small covered unit — a lidded seed tray, a clear plastic tub with vents, or a plastic bag over a pot — and move seedlings into the open terrarium once they have true leaves and a root system. Alternatively, sow directly into the open terrarium and cover the surface with a clear dome or glass sheet until germination, then remove it gradually. Both routes let you control humidity at the stage when it matters.

Set up each small project on the same pattern

Coordinating several terrariums is mostly about removing variables. If every container uses the same substrate, the same sowing rule and the same labelling, you can compare results and spot what actually failed.

• Substrate: use a fine, free-draining mix for small seed. Sieve the top layer so seed is not lost in coarse bark chunks, and moisten it before sowing so you are not watering seed into place.
• Sowing: tap fine seed from a folded paper or a small shaker; space it thinly. Dense sowing produces damping-off losses that look like a seed problem but are a spacing problem.
• Labelling: record species, sowing date and whether the seed was surface-sown or covered. With several containers, memory is unreliable after a fortnight.
• Watering: mist the surface rather than pouring, and bottom-water where the container allows it. A small mister with a fine spray is the single most useful tool here.

A dedicated fine-spray mister, a small sieve for substrate and a set of clear domes are modest purchases that make the whole multi-project routine repeatable. The sieve matters more than it appears: it gives you a consistent covering layer for medium seed instead of a guess.

Stagger the projects so they do not collide

Several small terrariums started on the same day all reach their demanding stage together. Stagger sowing by one to two weeks. This spreads out the period of daily misting and inspection, and it means a mistake in one batch does not wipe out the whole set. Keep faster-germinating species and slower ones in separate batches so their care stages do not overlap.

Choose plants that suit the format

Compact ferns, pilea, fittonia, small begonias and creeping fig are sensible candidates for open terrariums, and several are available as seed. For succulents, sow into a gritty, sharply drained mix — a standard potting mix holds too much moisture for them — and keep the dome brief, removing it soon after germination to prevent rot.

Move from germination to display

Once seedlings have two or three true leaves, thin them so the strongest plants have room. In a small open terrarium, three or four healthy plants read better than a crowded carpet, and airflow around each plant helps prevent fungal problems. Keep the substrate evenly moist but not saturated, and give bright, indirect light; direct sun through glass can overheat a small container quickly.

Treat the several terrariums as a small collection with a shared routine: one misting round, one inspection round, one note in the log. That consistency, more than any single technique, is what makes sowing depth and seed size decisions pay off across a group of projects.