Sealing, Venting and Lids That Actually Fit

Three closure regimes

A full seal — a ground-glass stopper, a gasketed clip lid, or a cork pressed home — creates a genuinely closed system. Water cycles indefinitely, watering becomes an annual event, and the build succeeds or fails on how well balanced it was at planting.

A partial seal — a resting lid, an overlapping glass disc, a loosely fitted cork — loses a little vapour each day. Humidity settles somewhere around seventy to eighty-five per cent, moulds are far less troublesome, and you will top up every few months.

A vented arrangement — a lid with drilled holes, a mesh panel, or a deliberate gap — approaches room humidity with a damp microclimate near the substrate. This is the regime for hair cap mosses, most lichens and anything from exposed habitat.

Matching the closure to the planting

Tropical carpeting mosses, Fissidens, filmy ferns and Selaginella want a full or near-full seal. They come from environments where saturation is constant and they have no drought strategy to fall back on.

Temperate cushion mosses such as Leucobryum sit best under a partial seal. They tolerate high humidity but rot at the crown if condensation never clears, and a slow daily loss of vapour is exactly what prevents that.

Anything from a wall, roof, roadside or heath needs venting. If you are unsure, ask where the species grows and whether that place is wet at three in the afternoon in July. If it is not, do not seal the lid.

Testing a closure before you plant

Put two centimetres of damp substrate in the empty vessel, close it, and leave it somewhere with the light it will eventually get. Check after twenty-four hours.

Heavy condensation obscuring the glass all day means a full seal with a lot of retained moisture — fine for tropical planting, wrong for cushions. A light fog that clears from the upper glass by afternoon indicates a healthy partial exchange. No condensation at all means an open regime.

Weigh the vessel at the start and after a week if you want a number. Losing more than about two per cent of total water weight per week is effectively an open system, whatever the lid looks like.

Retrofitting ventilation

The cheapest fix is a spacer: a small cork wedge or a folded strip of stainless mesh that holds the lid a millimetre or two proud. It is reversible, costs nothing, and turns a full seal into a partial one in ten seconds.

For a permanent vent, drill the lid rather than the vessel. Glass lids can be drilled underwater with a diamond hole saw at low speed and gentle pressure; a cork or wooden lid takes a normal bit. Cover the hole with stainless or nylon mesh glued with aquarium-grade silicone to keep fungus gnats out.

Never drill a bottle or a curved wall. The stress concentration on a curved surface makes cracking likely, and a cracked vessel with an established build in it is a genuinely miserable afternoon.

Materials, gaskets and what fails

Cork breathes slightly, which is often a feature, but it grows mould on the underside in a saturated vessel and needs replacing every few years. Ground glass seals perfectly and can bind shut with mineral deposit — a smear of a little silicone grease on the joint prevents that.

Rubber and silicone gaskets on clip-top jars are excellent and outlast everything else, but check them for perishing annually. A hardened gasket loses its seal without any visible change to the lid.

Avoid metal lids in permanent contact with a humid interior. They corrode, and the corrosion products end up in the substrate. If you love the look of a metal closure, line the interior face with a disc of glass or food-grade plastic.

Extended notes

• A sealed vessel in direct sun becomes a greenhouse within minutes. The closure decision and the placement decision have to be made together.
• Condensation that never clears also blocks light. Many 'not enough light' problems are really 'too much lid' problems.
• If you want to photograph a build, vent it for an hour beforehand and the glass will clear on its own.

Frequently asked