Building a Hibernaculum

Frogs, toads, newts, slow worms and common lizards are ectotherms: their body temperature tracks whatever is around them, and they can't generate their own heat to survive a Scottish frost. Left exposed, a hard freeze can kill them. To get through winter they need a spot below the frost line that stays cool but never freezes, and is damp enough that their skin doesn't dry out during torpor, the barely-active dormant state they pass the winter in. A hibernaculum is a garden structure built to provide that: a stack of rubble, logs and soil, covered in earth, with entrance tubes at ground level.

This isn't a hedgehog home or a pond guide. For pond design and general advice on attracting frogs, see the frogs page.

Where they'd otherwise shelter

In autumn, amphibians look for shelter on land, in log piles, hedge bottoms, compost heaps and under stones or paving slabs, though frogs and newts may instead overwinterKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → at the bottom of a pond. A hibernaculum is a purpose-built version of the log pile or rubble heap they'd otherwise have to find for themselves.

Which species will use one

Not every animal people picture is a realistic Scottish garden visitor, and the species you build for should shape where and how you build.

A hibernaculum near a pond suits amphibians and slow worms; a sunny bank version away from the pond suits common lizards. Whether either group turns up depends largely on whether they're already in the local area and can reach your garden: a hibernaculum improves the odds for animals already nearby.

Where to site it

Pick a corner that won't get dug over, walked on or cleared out, because once built it needs leaving alone through the winter. Choose a spot where the soil drains well: a waterlogged hibernaculum defeats the purpose, since the animals need damp air, not standing water.

A log pile near a pond gives frogs, toads and other amphibians a convenient bolt hole and hibernation site close to where they spend the rest of the year. If you have more than one pond, avoid moving spawn, tadpoles or pond water between them: transferring them risks spreading amphibian diseases such as ranavirus. Using separate nets and tools for each pond, and letting them dry before they touch another pond, is a sensible precaution.

For lizards, site it differently. Common lizards favour south-facing banks within easy reach of somewhere to escape to, so a reptile-focused hibernaculum wants sun and a nearby retreat, not shade. That matters in Scotland, where there are fewer strong sun hours for a lizard to warm up in. Keep either version clear of anywhere due for digging, planting or path repairs, and away from the middle of the lawn or an open run: a quiet corner behind planting or a shed gives some cover from cats and foxes, and vegetation growing over the mound adds more.

Step-by-step construction

You can build it as a dug pit or as an above-ground mound, and either is an afternoon's job once the materials are gathered. Most of it needn't be bought new: broken bricks, paving rubble, prunings and off-cuts of pipe all work, so use whatever's already lying around first.

1. Dig the pit (optional). For an in-ground version, dig a hole about 50cm deep and 1.5 metres across, in a sheltered spot with free-drainingSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition → soil. For an above-ground pile, start stacking on the surface.
2. Build the core. Fill with logs, branches, bricks and rocks or brick rubble, leaving plenty of gaps between them rather than packing everything solid.
3. Pack the gaps. Fill the spaces loosely with wood-chip so the pile isn't a series of open voids, but keep it loose enough for animals to move through.
4. Add entrance tubes. Insert two or three drainpipe or cement pipe off-cuts at ground level as you build, so animals have clear access into the core. If the tubes are plastic, roughen the inside with sandpaper first, because smooth plastic is too slippery for a frog or slow worm to climb.
5. Cover with soil. Mound excavated soil over the whole structure as a cover about 45 to 50cm deep, and to about the same height if building above ground. On colder inland and upland ground, the deeper end of that range is probably the safer choice.
6. Plant it over. Sow a native wildflower or meadow seed mix, or let grasses and other plants grow over the mound. It soon blends in with the rest of the garden.

Use logs with the bark still on where you can: the damp under peeling bark attracts woodlice, spiders and beetles for amphibians to eat, and treated timber such as fence posts and tree stakes is not as beneficial. Mixing log sizes helps, since a pile of thin twigs tends to rot down and collapse faster.

For a smaller commitment, Froglife suggests an upturned plant pot with a hole knocked in the side as a door, with old leaves or grass inside for extra protection, sometimes called a "toad home." It gives an amphibian somewhere to shelter without the dig.

Build in late summer or early autumn if you can, before animals start looking for winter shelter, and avoid disturbing it through winter. If you do uncover a hibernating lizard, Froglife advises gently re-covering it or moving it to similar habitat nearby rather than bringing it indoors, because once warm it will need to eat and finding food will be very difficult.

The legal position

Common frog, common toad, palmate newt, smooth newt, slow worm and common lizard are listed on Schedule 5 of the Wildlife and Countryside Act 1981 as "section 9(5) only." That covers the sale offences: it's an offence to sell, offer for sale, or transport any of them for sale. The offences of killing, injuring or taking an animal (section 9(1)), and of damaging a place it uses for shelter or disturbing it there (section 9(4)), apply to Schedule 5 species generally, intentionally or recklessly in Scotland, but not to species listed "9(5) only". So building, moving or dismantling a hibernaculum that shelters any of them isn't, on its own, restricted by that Act.

Great crested newt sits in a different category. As well as being on Schedule 5, it is a European Protected Species under the Conservation (Natural Habitats, &c.) Regulations 1994, which make it an offence to deliberately or recklessly kill, injure, capture or disturb it, or to damage or destroy a breeding or resting place it uses. It isn't a realistic occupant of a garden hibernaculum across most of Scotland, because its range is limited to parts of the central belt, the Borders and around Inverness. If one did turn up, the legally relevant point would be disturbing it while it uses the structure, not having built the structure.

The adder is also "section 9(5) only," protected from sale, not from disturbance. A garden hibernaculum shouldn't be built with the intention of attracting one. Leaving a hibernaculum alone through the winter is best practice even where disturbing a species isn't an offence.

Common questions

Sources