If I had to pick one material to put in every invertebrate enclosure, it would be cork bark. I buy it in bulk now because I use it in almost every setup I build. Flats for hides, tubes for climbing, small chunks mixed into substrate. It does a lot of jobs well, and unlike most natural materials, it lasts for ages without rotting away to nothing.
What cork bark actually is
Cork comes from the bark of the cork oak (Quercus suber), a tree native to the western Mediterranean. The bark is harvested every 9-12 years without killing the tree, which makes it one of the more sustainable materials in the hobby. Portugal produces about half the world's cork supply.
The bark itself is light, waterproof, and has a complex surface texture with plenty of crevices and folds. It does not decompose quickly, it does not leach harmful chemicals, and it is safe for all the invertebrate species commonly kept in the UK hobby.
Types and how to use them
Cork bark flats
These are flat or slightly curved pieces, usually a couple of centimetres thick. Lay them on the substrate as hides, lean them against the wall to create sheltered spaces underneath, or stack them with gaps between for climbing species. Isopods love congregating under cork flats, and I find most of my isopod colonies cluster on the underside of cork rather than in the substrate itself.
Cork bark tubes
Hollow cylindrical pieces, ranging from a few centimetres in diameter up to 15-20 cm for the big ones. Tubes make excellent hides for larger invertebrates and work well as vertical climbing structures in taller enclosures. Stand a tube upright in a millipede tank and they will use it as a retreat. Lay it on its side and you have a tunnel.
Tubes also hold moisture on their inner surface, creating a slightly more humid microhabitat inside. This is useful for species that appreciate a damp retreat without the whole enclosure being wet.
Cork bark chunks and granules
Smaller pieces can be mixed into substrate or scattered across the surface. In bioactive setups, cork chunks provide surface area for beneficial fungi and bacteria. Springtails colonise the underside readily. Some keepers use cork granules as a drainage layer instead of LECA, though I have not tried this myself.
Why it works so well for invertebrates
The obvious reason is that it provides hides and climbing surfaces. But there are a few less obvious benefits worth knowing about.
Cork bark is mould-resistant. Most hardwoods placed in a humid terrarium will sprout white mould within days. Cork resists this because of its chemical composition, particularly a compound called suberin which makes it naturally hydrophobic. Mould will eventually grow on cork in very wet conditions, but it takes far longer than on raw wood.
It insulates. Cork has a low thermal conductivity, which means a piece of cork bark sitting on warm substrate will not get as hot on its surface as glass or stone would. This gives animals a temperature-buffered hiding spot.
It does not acidify substrate. Some woods release tannins that lower pH, which can be an issue in enclosures where you are also growing plants or maintaining specific soil chemistry. Cork is relatively pH-neutral.
Preparing cork bark
Fresh cork bark from a supplier is generally ready to use. Some keepers soak it in hot water for 20-30 minutes to kill any hitchhikers and to waterlog it so it sinks in wet substrate. I usually do this with larger pieces to remove dust and let them absorb some moisture, but I have also used cork straight out of the bag without problems.
Do not bake cork bark in the oven. It is flammable, and the heat can cause it to smoulder or crack. A hot water soak is sufficient for sterilisation.
Species-specific uses
- Isopods: Cork flats on the substrate surface are the primary hiding spot for most colonies. Isopods shelter underneath during the day and emerge to forage at night. In breeding colonies, females carrying mancae in their marsupium tend to stay under cork where it is dark and humid.
- Millipedes: Cork tubes and flats provide retreats. Large species like Archispirostreptus gigas need correspondingly large pieces. Millipedes will also nibble on very old cork, though it is not a primary food source for them.
- Jumping spiders: Cork bark in an arboreal enclosure gives them grip surfaces and anchor points for web sacs. The textured surface is much easier for them to walk on than smooth glass or plastic.
- Beetles: Adult beetles climb on cork bark surfaces, and females may explore cork pieces on the substrate when looking for egg-laying sites. For larvae, cork bark is not particularly relevant since they live in the substrate.
- Mantids: Cork bark provides grip surfaces for molting. Since mantids hang upside-down to shed their exoskeleton, having textured surfaces near the top of the enclosure is helpful alongside mesh.
How long does it last?
In a typical invertebrate enclosure, cork bark lasts a couple of years before it starts to soften and crumble. Even then, it breaks down slowly. Compare that to raw hardwood branches, which can go soft within months in a humid setup. When a piece of cork does eventually deteriorate, just replace it. At the prices it sells for, it is one of the cheaper recurring costs in the hobby.
I keep a box of mixed cork pieces in various sizes so I always have something to hand when setting up a new enclosure or replacing old bits. Buying in bulk from reptile wholesalers works out cheaper than buying individual pieces from pet shops, and you get a better variety of shapes.