Substrate is the most important thing in a millipede enclosure. More important than temperature. More important than which vegetables you offer. Millipedes live in their substrate, eat their substrate, moult in their substrate, and lay eggs in it. If the substrate is wrong, nothing else you do will keep them healthy long-term.
What goes into a good millipede substrate
The standard recipe that works for most commonly kept tropical species breaks down roughly like this:
- 50% well-rotted deciduous hardwood - Oak, beech, and birch are the most commonly used. This needs to be properly decomposed: soft enough to crumble in your hand, dark in colour, and earthy-smelling. Fresh sawdust or woodchips are not a substitute. The wood should be well past the stage where it holds any structural integrity.
- 30% coco coir or organic topsoil - Coco coir holds moisture well and gives the substrate a workable texture. Organic topsoil (from a garden centre, the bag kind, not random garden soil) works too. Either one. Some keepers use a mix of both. Avoid anything with added fertilisers or pesticides.
- 20% leaf mould or dried leaves - Decomposed leaves that have been breaking down for a season or more. This adds nutrients and mimics the forest floor conditions these animals come from. If you can't get leaf mould specifically, a thick layer of dried whole leaves on top of the substrate achieves a similar effect over time as the millipedes consume and break them down.
Where to get the wood component
This is the bit that trips people up. You can't just grab a branch from the garden and crumble it into the enclosure. The wood needs to be well-rotted, ideally by white-rot fungi that break down the lignin and make it digestible for millipedes. In woodland, look for fallen oak or beech branches that are soft and spongy, easily pulled apart by hand, and covered in fungal growth. That's what you want.
You can also buy pre-prepared rotted wood or "white rot wood" from invert suppliers. It costs more than collecting your own, but it's consistent in quality and you know it's the right stuff. If you're collecting from woodland, stick to deciduous trees. Never use wood from coniferous trees (pine, spruce, cedar, yew). The phenols and terpenes in softwood are toxic to millipedes. This is one of the most commonly repeated warnings in millipede keeping, and for good reason. It kills them.
A question people ask: what about wood from the garden centre, like the decorative bark chips? Generally avoid these unless you're certain of the wood type and that it hasn't been chemically treated. Dyed or preserved bark is obviously out. Plain, untreated hardwood bark mulch is probably fine, but dedicated invert-grade wood is a safer bet.
Calcium
Calcium is not optional. Millipedes have calcified exoskeletons that need to be rebuilt with each moult. Without adequate calcium, moults fail. A failed moult is usually fatal or permanently debilitating, and it happens quietly, underground, where you can't intervene.
The most common calcium sources are crushed cuttlefish bone (available cheaply from pet shops, sold for birds and reptiles), limestone chips, and crushed oyster shell. Scatter it across the substrate surface and bury a few pieces as well. You'll see the millipedes actively feeding on it.
Some keepers also add powdered calcium carbonate directly to the substrate mix when preparing it. A tablespoon or two mixed through a large batch of substrate provides a baseline, though surface supplementation should still be offered.
Leaf litter
A layer of dried leaves on top of the substrate is both food and habitat structure. Millipedes graze on decomposing leaves, and the layer also helps maintain surface humidity.
Oak and beech leaves are the standard choices. Both break down slowly enough to last a while but are readily consumed. Avoid walnut leaves (contain juglone, which is toxic to many organisms) and anything from beside busy roads (pollutant contamination). Collect from parks, woodland, or your own garden if you don't use pesticides.
Some keepers bake their leaves at a low temperature (100C for 30 minutes) to kill off any unwanted hitchhikers before adding them to the enclosure. Others don't bother and have no problems. It depends on your tolerance for the occasional stray woodlouse or springtail turning up.
Moisture
The substrate needs to stay consistently damp. Not wet, not dry. The "wrung-out sponge" comparison gets used a lot because it's genuinely the right texture. If you squeeze a handful of substrate, it should feel moist and hold together but not drip water.
Mist the surface regularly. How often depends on your enclosure ventilation, room temperature, and ambient humidity. In a well-sealed tub, you might only need to mist every three or four days. In a terrarium with a mesh lid, potentially daily. Check the substrate surface. If the top centimetre is dry, mist.
The deeper layers of substrate should stay damp naturally if the enclosure retains moisture well. If the bottom of the substrate is dry, you either have too much ventilation or haven't been misting enough over a longer period.
Depth
For large species like Archispirostreptus gigas, aim for 15-20 cm. For medium species (10-15 cm body length), 12-15 cm is usually sufficient. Smaller species can manage with 10 cm. The millipedes use the depth for burrowing, moulting, and egg-laying. If your substrate is too shallow, they'll spend more time on the surface than they'd naturally prefer, and females may struggle to find suitable laying sites.
Replacing substrate
Over time, the substrate gets consumed and broken down. The wood component gets eaten, the leaves are consumed, and what's left is mostly frass (millipede droppings) and depleted soil. You need to refresh it periodically.
A full substrate change every 6-12 months is reasonable for most setups. Don't replace all of it at once, because the millipedes rely on the microbial communities and fungi in the established substrate. Replace about half to two thirds at a time, mixing fresh substrate with the old. This also preserves any eggs or tiny juveniles that might be buried in there if your millipedes have been breeding.
Things to avoid
- Softwood in any form (pine, cedar, spruce) -- toxic
- Commercially treated or dyed wood products
- Garden soil from areas treated with pesticides or herbicides
- Peat (acidic, poor nutritional value, environmentally problematic to harvest)
- Pure coco coir as the entire substrate (lacks the wood component millipedes need for food)
- Sand or gravel (no nutritional value, abrasive, impedes burrowing)