Why a shell border suits a coastal garden

Few things say "seaside" as quietly and convincingly as a ribbon of shells curving along the edge of a border. Oyster, mussel, cockle, limpet, whelk and scallop shells catch the light, echo the colours of the strandline, and knit a planting scheme together without competing with it. A few practical benefits come with the prettiness: a shell band is free-draining, it suppresses weeds, and as the shells slowly break down they release lime into the soil, which suits many of the Mediterranean and coastal plants we love to grow near the sea.

There is one important caveat, and it runs through everything below. Shells are not just decoration. They are part of a living shore, and the way you gather them matters far more than the way you arrange them.

Collecting shells the right way

The golden rule is simple: take only empty shells, and only from places where collecting is allowed. Walk the strandline above the high-water mark, where the sea has already done the work of cleaning and bleaching. Anything below that, and particularly anything in a rock pool, is likely to be someone's home.

• Never take a shell that is occupied. Hermit crabs, small crabs, anemones and worm tubes all use shells, and a "slightly moving" shell on the sand is a live animal.
• Leave rock pools completely alone. Limpets, winkles, top shells and periwinkles cling to rock for their lives. Prising one off kills it.
• Check the rules before you fill a bucket. Local byelaws, Sites of Special Scientific Interest, nature reserves, council beaches and National Trust land all have their own restrictions, and some species are protected by law.
• Take a handful, not a haul. Scatter your collecting across a wide stretch of beach rather than stripping one patch, so the shore keeps its shell supply for birds, hermit crabs and the calcium cycle of the sand itself.
• Leave mermaid's purses where they lie. Those dark, horny egg cases can still hold developing skate and ray young. Cuttlefish bone is better left for the birds, too.

If you garden far from the coast, garden centres and craft suppliers sell bags of shell grit and decorative shells, and a mix of bought and beach-found shells looks perfectly natural.

Cleaning and preparing your haul

Give your shells a soak in fresh water for a few days, changing the water daily, then scrub them with a stiff brush and let them dry in full sun. This removes salt, sand and any lingering whiff of the shore. Skip bleach, varnish and sealants: they look artificial within a season, they flake, and they take the shells out of circulation as a natural material.

Run your fingers around every rim and discard or blunt anything with a sharp, broken edge. A shell border is often walked past in bare feet, and a jagged whelk fragment is a genuine hazard. If you are laying your border against a lawn where children play, turn the sharpest pieces face down or use them in the middle of the band rather than the front.

Laying the border, step by step

Mark your line with a hosepipe or a taut length of string, then lift the turf or scrape back about 5 cm (2 in) of soil along it. Loosen the exposed ground and fork in a little sharp sand or grit so water never sits under the shells. If you are working on heavy clay, a strip of weed-suppressing membrane beneath the shells will save you years of weeding.

Sink a shallow trench roughly 15 cm (6 in) wide and 8 cm (3 in) deep. Lay the biggest, flattest shells first, nestled edge-on like roof tiles, then fill the gaps with smaller ones. Work in two or three courses rather than one thin scattering, and firm everything gently with the flat of your hand. Do not mortar them in; a loose, slightly irregular line looks better and lets you lift sections when you need to.

As a rough guide, a generous 15 cm band takes far more shells than you expect — roughly a large trug per metre of border. Gather accordingly, or plan to top it up over a season or two.

Plants that partner a shell edge

Thrift, sea campion, lavender, rosemary, sea holly and stonecrop all look superb rising from a pale shell band, as does the silver foliage of curry plant or sea kale. Space them 30–45 cm apart so their skirts just brush the shells, and set them slightly back so the border reads as a deliberate line rather than a scatter.

One caution: over several years, decomposing shell raises soil pH. That is a gift for lavender and thrift, but it is bad news for acid-lovers such as camellias, rhododendrons, blueberries and most heathers. Keep the shell band away from those beds, or line the trench with a butyl membrane to separate the two.

Keeping it looking good year after year

Wind and winter storms will redistribute the top layer, so keep a bucket of spare shells in the shed and top up each spring. Rake off leaves and debris by hand, pull weeds while they are small, and never pressure-wash the band — you will simply blast it into the soil.

Before you tidy, remember that shells make excellent shelter. Ground beetles, slow worms, toads and the occasional overwintering butterfly all tuck themselves beneath a warm shell. Slide sections aside gently rather than scooping, and put everything back as you found it. Look after the shore, and your border will look after itself.