Seaweed field guide

Want to know your thongweed from your dabberlocks? Kate Rew introduces the simple joy of seaweed spotting

Niall Meehan

Whether you want to eat it, identify it, or press it for posterity, learning to name the seaweed we swim with is easy with this ID chart. It may be enough to swim over it, but you can also press it, use it in dyes, and find species to eat (sea lettuce scones or sugar kelp crisps, anyone?).


Seaweed hunting was a popular pastime in Victorian times, and specimen pressing is an easy skill to acquire. Collect from shorelines after storms, rinse and cut to size. The most natural shapes are achieved by floating the weed on to paper by bringing it up through the water underneath the weed. Then add a piece of paper on top (kitchen roll, brown paper, whatever is to hand) and place your seaweed and paper sandwich under something heavy. I have whole collections pressed under tent carpets and doormats in Wales and Scotland – high footfall areas where over a week the samples are both dried out and squashed. Can be framed or made into cards.


Sea Lettuce (Ulva lactuca)

Emerald green and fragile-looking, sea lettuce is found all around the UK on rocks and in rockpools. It is robust to pollution, thrives on nitrates and is not as tasty as it’s purple cousin, but if the traditional cream tea is not your thing, a Devonian swimmer dries it then adds a large handful to make ‘Gutweed and Sea Lettuce Scones’.

Purple Laver (Porphyra purpurea)

Silk-thin but strong, purple laver changes colour over the year reaching darkest purple at the end of the summer. Widely spread across our shorelines in shallow water , and popular as nori – dried edible seaweed.

Bladderwrack (Fucs vesiculosus)

The air bubbles or bladders of this common weed help it to float, and make it instantly recognisable around the UK coast, where it may buffer your bumps in shallow swims amongst rocks. Rich in vitamins, minerals and phytochemicals, there is a long folk history of it being used to treat joint pain, urinary tract infections and thyroid conditions.

To make bladderwrack tea, dry it and simmer 1tsp in boiling water for 10-15 minutes. I once collected armfuls after a storm for a skin-nourishing super-slippery bladderwrack bath, but couldn’t quite bring myself to slip in. Can also be used as a dye for wool, giving tans and rusts.

In bladderwrack the air bladders are opposed on either side of the frond. There are two other common wracks: egg wrack has bladders along each frond, like a snake that has swallowed a series of giant eggs. Serrated wrack, as the name suggests, has a saw like edge.

Sugar Kelp (Saccharina latissima)

Also known as sea belt or weather weed, these twirling solitary strands of conker-brown kelp look like crinkled leather belts tied to their rocks by a thin stipe (stem). Often found swaying lushly in shallow water and rock pools. Nailed in a sheltered spot sugar kelp can be used as a (not entirely accurate) poor man’s barometer: if the kelp stays dry, the forecast is sunny and dry, if it becomes wet and flexible, rain is coming.  When it dries out a sweet white powder (mannitol) comes to the surface. Can be chopped up and baked to make crisps, and the sweet and salty flavour is currently used to add a sweet and salty taste to artisan rum and gin.

Oarweed (Laminaria digitata)

Oarweed is a type of kelp, and like other kelps (such as sugar kelp) has a satin rich lushness underwater. It grows  in dense swaying forests attached to rocky seabeds by single strands that branch out into ribbony fingers. It provides a nursery for fish and home for crabs and brittle stars. Fingers shredded by storms may be replaced by new ones the following spring. Washed up after storms, once rinsed off (by rain), chopped up and dried out, it (and other seaweeds) can make a good garden fertiliser. Grows quickly and part of kelp reforestation projects designed to restore ecosystems and lock in carbon.

Sea Lace (Chorda filum)

Also called Mermaid’s Tresses and Deadmans Rope these single fronds are hollow so have sufficient buoyancy to stand upright underwater, like very tall brown spaghetti. Most often found in sheltered bays with rocky substrates, in relatively shallow (less than 20 metre) water– grows up to 8 metres but 2-6 metres is more common. Disappears in winter. Wrap them around a chosen pebble to make an unusual gift: as it dries, the lace will tighten.

Furbelows (Saccorhiza polyschides)

Seen from above in a swirly sea these look like hundreds of glossy galloping horses tails. Seen torn from the sea and marooned on the shoreline they look like fancy kelp trees. The holdfast is like a scrotum with a bad case of goose bumps, giving way to a flat stipe that has, to each side, the ornate ruffles like a flamboyant dress shirt, and the whole thing is topped off with flat ribbons. The stormier the location, the longer and more divided the fronds, and the whole kelp can reach 4 metres tall. The ruffles are there to dissipate currents. Fast growing, annual species.


Want to learn more about seaweed and contribute to a major citizen science project that helps how we understand the influence of climate change and non-native species across the UK’s coasts? The Big Seaweed Search is a collaboration between the Natural History Museum and the Marine Conservation Society. To take part, and find all the information you need to submit your own seaweed survey, visit the website.

lookbook outdoor swimmer's handbook


The Seaweed Marine Field Guide is an extract from The Outdoor Swimmers’ Handbook by OSS Founder Kate Rew. Signed copies are available in the OSS Shop, but it is also widely available through independent bookshops, mainstream book stores and international suppliers.

Kate Rew