Building a Hibernaculum

Frogs, toads, newts, slow worms and common lizards are ectotherms: their body temperature tracks whatever is around them, and they cannot generate their own heat to survive a Scottish frost. Left exposed, a hard freeze kills them outright. To get through winter they need to reach a spot below the frost line that stays cool but never freezes, and humid enough that their skin doesn't dry out during the months they spend in torpor, the dormant, barely-active state they pass winter in. A hibernaculum is a garden structure built specifically to provide that refuge, a stack of rubble, logs and soil engineered to hold a frost-free, damp core through the coldest part of the year.

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

What they'd otherwise be looking for

RHS guidance on garden amphibians notes that, in autumn, amphibians seek sheltered places away from ponds, 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 → in the bottom of a pond rather than on land. A hibernaculum is a purpose-built version of the log pile, hedge bottom or rubble heap they'd otherwise have to find for themselves, built properly, rather than left to chance.

Which species will actually use one

Not every animal people picture when they hear "hibernaculum" is a realistic Scottish garden visitor, and the choice of species should shape where and how you build.

In practice, a hibernaculum sited near a pond is built for amphibians and slow worms. A sunny bank version, sited away from the pond, is aimed at common lizards. Froglife's guidance on garden lizards notes that whether either group actually turns up depends on whether the species is already present locally, a hibernaculum improves the odds for animals already in the area, it doesn't summon a species that isn't there.

Where to site it

Pick a corner of the garden that won't get dug over, walked on, or cleared out, once built, this feature needs to be left alone through the winter. Froglife's wintering-sites guidance specifies choosing a spot where the soil drains well; a waterlogged hibernaculum defeats the purpose, since the animals need damp air, not standing water.

RHS guidance on dead wood and compost heaps notes that locating 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 more than one pond is involved, use separate tools and nets for each and let anything that touches the water dry out before it goes near another pond, ordinary hygiene that limits the spread of amphibian disease between water bodies.

For lizards, site it differently: Froglife notes that common lizards favour basking on south-facing banks within easy reach of an escape place, so a reptile-focused hibernaculum wants sun exposure and a nearby retreat, not shade, worth taking seriously in Scotland specifically, where a lizard already gets fewer strong sun hours than one further south to warm up in before retreating. Keep either version clear of anywhere due for digging, planting or path repairs, and tuck it away from the middle of the lawn or an open run, a quiet corner behind planting or a shed gives some natural cover from cats and foxes, and letting the surface vegetation establish over the mound adds more once it's grown in.

Step-by-step construction

It can be built as a dug pit or as an above-ground mound, depending on how much digging you want to do, either version is a job for an afternoon rather than a weekend, once the materials are gathered. None of it needs to be bought new: broken bricks, leftover paving rubble, prunings and off-cuts of pipe all work, so it's worth using whatever's already lying around the garden or shed before buying anything.

1. Dig the pit (optional). For an in-ground version, dig a hole roughly 50cm deep and about 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. Skip this step for an above-ground pile, simply start stacking on the surface.
2. Build the core. Layer in logs, branches, bricks and rocks or brick rubble, leaving air 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 pipe or drainpipe 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, smooth plastic is too slippery to climb.
5. Cover with soil. Mound excavated soil over the whole structure to a depth of roughly 45 to 50cm, and to about the same height if building above ground. On colder inland and upland ground, staying toward the deeper end of that range is the safer choice.
6. Plant it over. Sow a native wildflower or meadow-seed mix, or let long grass establish over the mound. Vegetation cover develops naturally over time and helps the structure blend into the garden.

A couple of points are worth carrying into the build: logs that still have their bark on are more useful than stripped ones, treated timber such as old fence posts or tree stakes is less beneficial for wildlife than untreated wood, and it's worth mixing log sizes, too many thin twigs rot and collapse faster than the structure needs.

For a smaller commitment, Froglife's guidance also covers a scaled-down option: an upturned plant pot with a hole knocked in the side as a doorway, or a small wooden structure, with old leaves or grass packed inside for extra insulation, sometimes called a "toad home." It gives an amphibian somewhere to shelter without the dig.

The legal position

Most of the species likely to use a garden hibernaculum, 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 means it's an offence to sell, offer for sale, or transport any of them for sale, but the Act itself does not make it an offence to kill, injure or disturb these species. Building, moving or dismantling a hibernaculum that shelters any of these animals isn't, on its own, restricted by this Act. Scotland's Nature Conservation (Scotland) Act 2004 extended the Act's disturbance offence to cover reckless as well as intentional acts, which is why that wording carries both.

Great crested newt sits in a different category: alongside its Schedule 5 listing, it's protected as a European Protected Species under the Conservation (Natural Habitats, &c.) Regulations 1994, which does 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, given how restricted its range is. If one ever did turn up in a hibernaculum, the legally relevant point would be disturbing it while it's using the structure, not the act of having built the structure in the first place.

The adder is also Schedule 5 "section 9(5) only," protected from trade, not from disturbance, but as covered above, a garden hibernaculum shouldn't be built with the intention of attracting one.

Common questions

Sources