From Coastline to Compost Bin – Using Seaweed As A Garden Fertiliser and Soil Improver


I may be a composting virgin, but now that I have an allotment I am getting serious about the dark art of decay.

Since the New Year I’ve been swotting up and assimilating all manner of advice about building, filling and maintaining a successful heap. Composting, like brewing and train spotting, attracts a certain degree of ardent geekiness. Whether ‘aerobic’ or ‘anaerobic’ it’s not a subject to be trifled with: you’re either deeply into composting, or not bothered about it in the slightest, there’s no middle ground. Two months into my allotmenteering career I can sense myself morphing from a bin-it bystander into a composting commando faster than you can say ‘microorganism’.

Within a week our first wooden bin was already full to the brim with weeds, vegetable peelings, fallen leaves, prunings and shredded cardboard, layered carefully so that the mix was neither too sludgy nor too dry. I suspect I may not have got it right first time, but in three or four months I will know if I’ve triumphed or embarrassed myself. Either way, this is likely to be the start of a new obsession.

Bladder wrack, Fucus vesiculosus

Living by the coast it is hard to escape seaweed. Its slippery fingers poke at our feet when we walk the dogs along the shore, flail limp around our ankles when we paddle and waft salty notes up our nostrils. Seaweed appears on menus in the town’s finer establishments (Wyatt and Jones serve the best miniature sourdough loaves with seaweed-infused butter) and we overhear visitors complaining when it blocks their path to the waves. On summer evenings heaps of decaying seaweed crackle and pop as tiny springtails perform Cirque du Soleil in miniature.

Since starting my composting adventure I have wondered whether I am permitted to collect seaweed and if it would make decent compost. Turns out I can and it does.

One of the ‘gaps’ cut through the cliffs by our ancestors in order to gather flints and seaweed from the shore

Farmers on the Isle of Thanet where I live harvested seaweed from the shore and spread it on their fields for hundreds of years. That much I already knew. Indeed most of the access routes to our beautiful beaches, known as ‘gaps’, have been cut through the chalk for this purpose alone. The marks made by cart wheels can still be picked out in some locations, for example at Botany Bay (see above).

“The sea furnished an inexhaustible supply of manure, which was brought up by the tides to all the borders of the upland, quite round the island, and most probably was liberally and judiciously applied by the monks and their tenants; and their successors to the present time have not neglected to profit by their example. Owing to these circumstances, Thanet always was, and most likely always will be famous for its fertility”

Edward Hasted ‘The island of Thanet: Introduction’, in The History and Topographical Survey of the County of Kent: Volume 10 (Canterbury, 1800)

Two hundred years later the fields of Thanet remain hugely productive – those that have not been built over – but the vitality of seaweed wasn’t exclusively harnessed here in East Kent. Known locally as vraic (pronounced ‘vrak’), seaweed in the Channel Islands was ploughed into the earth before planting potatoes in late winter and early spring. (Some farmers credit seaweed with encouraging the development of thin-skinned spuds, whilst others claim it enhances their flavour – why not try for yourself?) Further back in history, seaweed was dried then burned before the ashes were spread on the land. In Scotland ‘lazy beds’ created on otherwise unpromising, rocky ground were enriched by lifting up sods of peat and nesting seed potatoes on desalinated seaweed underneath. Ingenious and effective. Again, potatoes were grown in this manner until the arrival of potato blight. From the Falkland to the Faroe island, seaweed has been used for generations as a mulch, soil improver and fertiliser.

Diagram of a lazy bed from the Quickcrop Blog

Not only is seaweed free and plentiful, but also incredibly high in potash and nitrogen, making it of high value to gardeners. A recent report in the International Journal of Environmental Science and Technology points to other plant benefits including stimulation of seed germination, enhancement of plant health and growth, improved water and nutrient uptake, enhanced frost, saline and disease resistance and remediation of pollutants in the soil.

As seaweed breaks down in the soil, it promotes microorganisms that unlock unavailable nutrients for plants to use. Seaweed also increases chlorophyll production and contains a wealth of micronutrients important for soil and plant health. Cytokinins, plant growth hormones that work above and below ground, improve root and shoot growth. In general, seaweeds contain ten times the mineral levels of land-based plants and are rich in iodine and calcium. Farmers of yore may not have appreciated the scientific detail, but they knew they had unique access to a precious resource.

Porphyra dioica, the black laver seaweed beloved by the Welsh

Perversely, given all the benefits, modern farmers don’t tend to use seaweed on their fields any more. Some suggest that this is because supermarkets don’t list it as an acceptable fertiliser for their producers to use, but I suspect it’s also because it’s bulky, in erratic supply and neither as ‘controllable’ nor as consistently formulated as more expensive artificial fertilisers. Whatever the case most of the seaweed that’s washed up on Thanet’s beaches these days is either left or gathered up and disposed of by the local council before, and sometimes after, it has started to rot. Unlike terrestrial plants, marine seaweeds start their growth cycle in winter and die in summer, which is why summer storms tend to wash vast quantities onto beaches just as the tourists want to spread their towels on the beach.

According to the Crown Estate, which manages around half of the foreshore (the land between mean high water and mean low water) around the coast of England, Wales and Northern Ireland, seaweed collection is permitted provided it’s in small quantities for personal use. The same rules apply in Scotland. In designated conservation areas harvesting of seaweed is not allowed and if collection is for commercial gain then a license must be obtained . The Crown Estate stress that would-be collectors should consider the sensitivity of collecting anything from the wild, even if it’s not alive. However, provided you follow a few common sense rules and act responsibly you are more likely to benefit the environment by using seaweed rather than harsh chemical fertilisers.

We did our first shoreline foraging walk tonight and came home with two large carrier bags brimming with assorted seaweeds. Tomorrow our haul will be interred beneath the early potatoes we’ve been chitting upstairs, hopefully giving them a great start in life. Any excess will be added to the top of our compost heap. Here’s my advice if you fancy following our lead:

10 Top Tips for Collecting and Using Seaweed in Compost

  1. Do your homework – Check the appropriate local authority’s website before collecting seaweed that’s become detached on any beach – this is also referred to as ‘drift’ or ‘cast’ seaweed. If the beach is private, you will need to ask the landowner’s permission. If the beach is part of the Crown Estate and otherwise unprotected, you do not require further permission
  2. Everything in moderation – Take only what you need and a small proportion of what’s been washed up. You’ll tend to find more dead seaweed is washed up on beaches following a storm or rough weather. Avoid old and dehydrated seaweed as salt concentrations may be high. Different seaweed species have different nutrient contents so try to harvest more than one type if you can.
  3. Dead not alive – never collect living seaweed – that is seaweed that’s still attached to rocks or the seabed. If you move any rocks in the process, they must be replaced the same way up so as not to damage any attached creatures.
  4. Mind your step – avoid trampling on living seaweed or disturbing any other wildlife in the process, especially birds and seals that might be sensitive to humans. Importantly, never put yourself at risk of rising tides or slippery rocks. No amount of seaweed is worth cracking your head open or drowning for! Never drive a vehicle onto a beach unless you have permission from the landowner.
  5. Keep it clean – collect your seaweed early in the morning or as the tide is going out to reduce the chances that it may have been tainted with dog mess. Remove any non-compostable flotsam and jetsam before adding to your heap. A mesh bag or sack will allow any residual water and sand to drain out; not that either are an issue for the garden, but they will make your load heavy!
  6. No Prewash – there is no benefit to washing salt or sand from seaweed unless you live somewhere extremely dry or plan to apply vast quantities in one go. Farmers have been using seaweed on the same land for centuries without causing a harmful build up of salt. However, avoid collecting seaweed from polluted beaches or close to sewage outlets. If salts bother you, then hose your seaweed down on the drive or a patio before adding to your garden or compost heap.
  7. Chop Chop! – cut seaweed up with a sharp spade before composting to speed decomposition. Layer it with drier materials to prevent it from becoming a sludgy, stinky mess as it starts to break down.
  8. Dig in – if you don’t have a compost heap, you can still dig seaweed into the ground, use it to line trenches or apply it as a mulch. Slugs and snails do not like dry, crisp surfaces or salt, so used as a mulch seaweed can deter these perennial pests.
  9. Lighten up – avoid adding seaweed directly to heavy soils. Seaweeds have little or no cellulose and are rich in water-soluble gels. If worked into heavy soils they may form an impermeable layer and cause waterlogging. On heavy soils like clay it’s better to compost your seaweed first.
  10. Wet and dry – If you don’t have ready access to fresh seaweed you can water your compost heap with liquid seaweed or apply it directly to your garden or allotment. Alternatively, a dried seaweed product such as Quickcrop’s Seafeed, improves soil structure at the same time as increasing yields. It’s completely organic too.

There might seem to be a lot of caveats around using seaweed in the garden or on your allotment, but ultimately collecting your own for composting is a sustainable activity and perfectly permissible in most circumstances. Not only will you be returning natural goodness to the earth and helping your plants to thrive, but you’ll be saving yourself a bob or two in the process. TFG.

Sources of Further Advice and Information

Photographs by John McKenna, aka The Beau.

The beach at Dumpton Gap, Broadstairs, in August. The black areas are ‘cast’ seaweed