Why Frogs Need a Hibernaculum

As the first frosts arrive, frogs, toads and newts begin looking for somewhere safe to spend the winter. In the wild they would creep into log piles, leaf litter, compost heaps and old mammal burrows. Gardens often lack these messy, undisturbed corners, so a hibernaculum gives amphibians a frost-free refuge. It is simply a mound or sunken chamber built from logs, stones, rubble and dead leaves. The gaps and tunnels stay cool but rarely freeze, and the damp conditions suit amphibians' skin. A good hibernaculum also shelters insects, beetles, queen bumblebees and maybe a hedgehog or slow worm. Building one is one of the most useful things you can do for garden wildlife.

Choosing the Right Spot

Position matters more than size. Pick a quiet corner that is shaded for part of the day, ideally near a pond but not in a spot that floods. Dappled shade under a deciduous tree or beside a hedge is ideal. Avoid full winter sun, because a mound that warms up too early can wake frogs during a cold snap. Keep it away from busy paths, patios and dog runs. A south-facing bank is fine for reptiles, but for amphibians, cool and damp is better.

The hibernaculum should sit on well-drained ground. If your soil is heavy clay, raise the mound slightly and add a layer of rubble at the base. Leave at least a metre of undisturbed ground around it. You can build it in a border, a wild corner, or beside a log pile. If you have children, involve them in gathering leaves and stones, but keep the finished mound out of play areas.

Gathering Materials

You probably already have most of what you need. Aim for a mix of materials that create gaps, hold moisture and insulate.

• Logs and branches: untreated wood, ideally with bark still on. Different diameters are best.
• Stones and rubble: broken bricks, old tiles, gravel, and stones. Avoid anything painted or treated.
• Leaf litter and leaf mould: dead leaves, moss, compost, and a little straw if you have it.
• Topsoil: the soil you dig out, kept to one side.
• Terracotta pots or pipes: broken pieces or short lengths of pipe can create entrance tunnels.

Avoid treated timber, plastic sheeting, and anything shiny or sharp. Do not use grass clippings, which turn slimy and airless. If you use straw, make sure it is not sprayed with herbicides. A wheelbarrow of logs, a bucket of stones, and a few sacks of leaves are enough to start.

Building Your Hibernaculum, Step by Step

Work on a dry day in autumn, before the first hard frost. The build should take an hour or two.

• Mark out the site: a circle or rectangle about 1 metre across. Remove the turf and keep it for the top.
• Dig a shallow depression: 20–30 cm deep. Keep the soil nearby. This forms the damp base.
• Add a rubble layer: broken bricks, stones and gravel. This improves drainage and creates small voids.
• Build up with logs: lay logs and branches criss-cross, leaving gaps between them. Add stones and more logs in layers.
• Pack gaps with leaves and moss: use leaf litter, leaf mould and a little compost. This insulation keeps temperatures steady.
• Create entrances: leave a few gaps at ground level, or add a short pipe or terracotta pot as a doorway.
• Cover with topsoil: cap the mound with the soil you dug out, then a layer of leaves. Aim for a gentle mound about 40–60 cm high.
• Firm gently: press down with your hands, but do not compact it. The structure should feel solid but still have air pockets.

If you have a spare log, half-bury it upright in the mound. It will rot slowly and provide extra hiding places. Do not worry about making it neat. A slightly untidy pile is exactly what wildlife wants.

Planting and Camouflaging the Mound

Once the mound is built, plant it with native ground cover. This hides the structure, keeps it damp and adds extra cover. Good choices include ivy, bugle, wild strawberry, wood anemone, ferns, deadnettle and native grasses. Avoid vigorous plants that will smother the entrances, such as bracken or Japanese knotweed. A few spring bulbs, like snowdrops or native bluebells, will look lovely but do not disturb the mound.

Leave a patch of leaf litter at the base. Do not tidy it away in autumn. Fallen leaves are part of the hibernaculum, not garden waste. If the mound dries out in a hot spell, water it gently, but avoid soaking it. A damp, cool mound is perfect.

Winter Care and What to Expect

From November to February, treat the hibernaculum as a quiet zone. Do not turn it, move logs or dig around it. If you must move a log, check underneath first and replace it carefully. Frogs and newts may be sheltering in the gaps. Never handle amphibians with dry hands; if one needs moving, wet your hands first and keep the encounter brief.

In spring, you may see frogs emerging and heading to the pond. Newts and toads may also appear, along with beetles, centipedes and queen bumblebees. Leave the mound in place year-round. Top up the leaf litter each autumn and let it settle. Over time, the wood will rot and the mound will become an even richer habitat. A hibernaculum is not just a winter shelter; it is a small, living wildlife bank that keeps giving.