Terrarium cork lid: measure the neck and choose the right cork

A terrarium cork lid closes a bottle and keeps the humidity inside. To choose one, first measure the inner diameter of the neck. A slightly tapered cork makes up for a small gap, and you can sand it down if it is a little too wide. Nothing can be done if it is too narrow.

What the lid is for

A bottle with a lid becomes a closed terrarium. The University of Illinois points this out: bottles closed with a cap, a cork or a tight-fitting lid keep the humidity inside. The water evaporates, condenses on the glass and runs back down to the soil.

Without a lid, the same bottle becomes an open terrarium. It dries out faster and needs watering. So the plants are not the same: our guide closed or open terrarium helps you choose.

The material matters too. The RHS advises against metal screw caps, which can rust. Cork does not rust.

Why cork

Cork is the bark of the cork oak. The University of Washington Botanic Gardens describe it as light, waterproof, elastic and a good insulator. It is harvested from living trees, without felling them.

These qualities serve a terrarium. The elasticity lets the cork grip the neck. Being waterproof, it keeps the water inside.

Our natural cork lids come from Portugal. The cork is not treated. According to the manufacturer, it withstands temperatures from about -30 °C to 120 °C. It cleans with a damp cloth.

Measure the neck

Measure the neck before you order the lid.

1. Take a rigid ruler or a caliper.
2. Measure the inner diameter of the neck, at the rim, from one side to the other, through the center.
3. Measure a second time, at a right angle. The manufacturer points out that every container has a slightly different opening.
4. Note the larger value.

Then compare it with the lid. Our three natural cork lids have a slightly tapered shape: as a rule, the top is wider than the bottom. The product page of each lid gives three measurements, in millimeters:

The manufacturer does not say what each figure refers to. Measure the lid when it arrives, at the top, at the bottom and in thickness, before you try it on the bottle.

Adjust a lid

The lid is a little too wide. Sand or file the edge yourself. Remove a little at a time, turning the lid, to keep an even shape. Try it often. According to the manufacturer, this is a common step with raw cork.

The lid is a little too narrow. The tapered shape makes up for a slight gap: the lid goes lower into the neck. If the gap is large, nothing can fix it. Take the next size up.

The lid will not go in. Never hammer a lid in. The glass can crack. Sand the lid instead.

The LED lid

An LED lid closes the bottle and lights it from above in the evening. It shows off the moss and the plants.

Our large-format LED lid measures 13.2 x 11.9 x 3.5 cm. It runs on two AAA batteries, not included. The manufacturer offers it for our Sven XL, Milky, Bolder Bob, Fat Joe and Teddy bottles. The fit can vary a little from one bottle to another: measure the neck if you already have the bottle.

Some bottles come with their own LED lid. This is the case for the Little Joe bottle: 20 cm in diameter, 22 cm tall, about 5.5 liters, and a 10.5 cm neck. The batteries for its lid are not included.

The LED is decorative. To know if your plants have enough light, see our guide to terrarium lighting.

Open and close

A corked terrarium is not opened every day. The glass tells you what to do.

• Light condensation: all is well, change nothing.
• Water streaming down the glass all the time: open it for a few hours a day, until the condensation is light again. The RHS warns that too much condensation can make the plants rot.
• No condensation at all: the terrarium lacks water. Mist a little soft water, then close it again.

Remove the lid by turning it gently, without pulling it out in one go. Lay it flat, on a clean cloth. Our guide to condensation explains how to read the fog on the glass.

The lid is not a canning seal. According to the manufacturer, it is not suitable for sterilizing or hot canning.

The cork tamper, a separate tool

The cork tamper closes nothing. It is a 22.5 cm handle with a cork head. You use it while building the terrarium, before planting.

In a bottle with a narrow neck, your hand cannot reach the bottom. The tamper presses down each layer of sand, activated charcoal or potting soil, without mixing them. Penn State Extension suggests the same step with a cork fixed to a stick. Our bottle terrarium tutorial shows the whole build.

In short

1. Measure the inner diameter of the neck twice, at a right angle.
2. Measure the lid when it arrives: the manufacturer does not say what each figure refers to.
3. Too wide: sand the cork little by little. Too narrow: take the next size up.
4. For an LED lid, check the list of intended bottles and plan for two AAA batteries.
5. Open only if water streams down the glass, for a few hours a day.

Sources

• DIY bottle terrariums make great projects and gifts, University of Illinois Extension
• Bottle gardens and terrariums, RHS
• Plant Profile: Quercus suber, University of Washington Botanic Gardens
• Creating a Closed Terrarium, Penn State Extension
• Manufacturer data sheets (dimensions, origin and temperature resistance of the lids, batteries and compatibility of the LED lid, Little Joe bottle, cork tamper)