The 13-Centimetre Rule: Small Openings That Make a Solid Fence Wildlife-Wise

Most garden boundaries are designed for people: a clean line, a bit of privacy, something that keeps the dog in and the world out. Hedgehogs don’t read those plans. To them, a row of tight-fitting fence panels is a wall across a neighbourhood, separating feeding grounds, mates and shelter. The good news is that you don’t need to swap a solid fence for a scrappy hedge to help. A small, well-detailed opening at ground level can do a lot — and it won’t cost you your privacy.

Why 13 centimetres keeps coming up

The figure you’ll see attached to hedgehog-friendly fencing is 13cm. That’s the size of a square or arch at the base of a fence panel that lets a hedgehog pass through comfortably while staying small enough to remain unremarkable. It is not a cat flap, a drainage channel or a gap you leave to chance. It is a deliberate piece of detailing.

At ground level, 13cm feels tiny. It sits below the sightline, below the gravel board in many designs, and below the place where most of us look when we’re judging a boundary. That’s why it can work on a smart garden fence without turning the whole thing into a wildlife project. The aim is not to make the fence look broken. It’s to make it porous in one considered spot.

Plan the route before you cut anything

A gap on its own is just a hole. A gap that connects to planting, a log pile or a quiet corner is a route. Before you get the jigsaw out, walk the boundary and think like a small, prickly animal. Where would a hedgehog arrive from? Is there cover nearby? Does the gap lead into a lawn with nowhere to hide, or into a border with leaf litter and shrubs?

If you share the boundary with a neighbour, this is a conversation worth having early. Two gaps that line up across a fence are far more useful than one gap facing a paved patio. In a terrace or a row of gardens, a handful of well-placed openings can link a whole street into a usable corridor. That’s the hedgehog highway idea in practice: not a motorway, just a series of small, safe connections.

Keep the opening away from busy roads where you can. Connectivity matters, but so does road risk. A gap that funnels wildlife straight towards a fast road is not a win. Put it on the quieter side of the garden, near beds and shrubs, and let the planting do the guiding.

Detail the base so it stays neat

The fence base is where wildlife-friendly design usually goes wrong. A ragged hole cut into a panel will fray, collect damp and look like damage. A gap that runs the whole length of the fence is a draught, a weed trap and, frankly, a security weakness. What you want is one clean opening in an otherwise sound base.

If you’re installing a new fence, ask for a gravel board with a neat cut-out, or have one panel detailed with a small arch at the bottom corner. Treat any cut timber with a suitable preservative and keep the edges smooth. A metal edging strip or a small paved threshold can stop soil and gravel spilling into the gap. It also makes the opening look intentional rather than accidental.

Don’t cut into a retaining wall, a concrete post or any part of the structure that holds the fence up. The gap belongs in the infill panel or gravel board, not in the skeleton of the boundary. If the fence sits on a low plinth, you may need to plan the opening around it rather than through it.

Adapting a fence you already have

You don’t need a new fence to make an old one wildlife-friendly. On many panel fences, the simplest approach is to remove a short section of gravel board at one end of a bay and replace it with a purpose-cut piece that leaves a 13cm opening. On closeboard fencing, a neat cut at the base of one board — with the cut edge treated and supported — can work, though it’s a job for someone who knows how the fence is put together.

If the fence already has a gap under it, don’t assume it’s doing the job. A continuous 10cm gap may let hedgehogs through, but it also lets in wind, weeds and unwanted visitors. Better to close the long gap with a gravel board and create one controlled opening where you actually want it.

Privacy isn’t the problem you think it is

A hole at ankle height does not undermine a six-foot fence. It doesn’t open up a view, and it doesn’t make a garden feel exposed. What it can do is make a boundary feel less brutal — a solid line with one small, thoughtful weakness in it.

If the opening is in a very visible spot, screen it lightly with low planting rather than blocking it. A clump of hardy geraniums, a small fern or a strip of gravel and stepping stones can draw the eye without sealing the route. Avoid putting the gap right beside a seating area if the idea of it bothers you. Put it behind a shrub, at the end of a border, or on the side of the garden you don’t stare at from the kitchen window.

Make the corridor worth using

A gap is only as good as the garden it opens into. Hedgehogs need cover, food and water. Native shrubs, a messy leaf pile, a log stack and a shallow dish of water all help. So does leaving a corner a little wild. If your garden is all lawn, porcelain paving and clipped topiary, the gap may lead to a place with nowhere to shelter.

Climbing plants on the fence can add cover without blocking the opening. A hedge alongside the fence — even a short run of hawthorn, hazel or field maple — makes the boundary feel softer and gives wildlife a route to follow. Just keep the base of the gap clear. A hedgehog highway that ends in a dense thicket of ivy and a pile of bricks is not a highway.

When to pause

There are times to think twice. If the boundary is listed, rented, shared or retaining something, check before you cut. If the fence is in poor condition, fix the structure first. And if the gap would lead straight onto a road, a steep drop or a pond with no escape route, choose a different spot.

The 13cm gap is a small detail, but it’s the kind of detail that makes a garden feel part of something larger. A solid fence can still be a good neighbour — to the people on either side and to the wildlife that needs to get past it.