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By October 21, 2017 0 Comments Read More →

Seaweed on the Garden

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One of the most commonest seaweeds on the UK shores.

Frank Beswick gives advice on collection and processing seaweed to apply to your plot.

WHEN MY SON returned to England after a year abroad, he needed to put a bit of weight back on. Needless to say, maternal instinct kicked in and his mother cooked him a full roast dinner, with all the vegetables coming from my allotment. After he had finished he commented on how flavoursome the potatoes were. ‘What have you been doing to them?’ he enquired. I explained that I had been fertilising them with seaweed meal and that this had boosted their nutrient content.

Seaweed is an underused organic fertiliser, probably because many of us live some distance from the ocean and we are accustomed to using manure and compost, but in overlooking seaweed we are neglecting a major source of nutrients. Plants need a whole range of minerals, which come into the categories of major, minor and micronutrients. All gardeners should know the major nutrients NPK, nitrogen, phosphorus and potassium, and we are also aware that there is a need for calcium, iron and magnesium, to name but a few. Many of our soils now are depleted in minor and micronutrients, but seaweed can help to replace them.

Giant Sea Kelp – found in huge quantities in the Hebrides and other Atlantic facing shores.

Farmers near the coast have been applying seaweed for thousands of years. They did not know the chemistry, but they recognised the results. On the Atlantic fringe of the Hebrides there is a rich strip of land known as a machair. This is composed of a thick deposit of white shell fragments, which overlay the mineral-poor soil of the land. Such a highly alkaline soil should grow nothing, but it is in fact rich grazing land that provides juicy grasses for the highland cattle. For centuries the Hebrideans have applied seaweed and thus transformed a shell desert into rich land. They also used seaweed in lazybeds. These were raised beds on waterlogged ground unfit for growing. A mixture of turfs, sand, manure and seaweed enabled them to produce potato crops on barren land where no vegetables would otherwise grow.


Seaweed is still applied at Heligan Gardens, where the gardeners take a tractor down to Mevagissey beach after storms to reap the sea’s harvest. Heligan is organic, so it produces crops by applying vast amounts of organic material to the land. Much of this is manure, dug in at the double digging every four years, but an essential extra ingredient is seaweed. If you are wondering if the salt that it contains can be bad for the land, as no one wants a salt accumulation, don’t worry. Seaweed contains no more salt than land plants. The extra salt is only deposited on the surface of the plant. Many gardeners hose it down before they apply the weed, and others let it stand for a while in rainy conditions to let the salt drain away.

Brown kelp.


Another question is how long it takes to decay. The answer is not long. Spread a layer of seaweed on your ground and within a month or two it will have decayed into a thin layer an inch or so deep with no traces of the original weed. You could even add it to the compost heap and let it decay there.


Seaweed contains at least 60 plant nutrients, amino acids and growth hormones. If your cauliflowers have suffered whiptail, with thin, strap-like leaves and deformed curds, they lack molybdenum, an element necessary for protein production and nitrogen uptake. Applying nitrogen is no good unless you have sufficient molybdenum but the problem is easily remedied by adding seaweed meal and a liquid seaweed spray to the leaves, which acts as a foliar feed. You should though, accompany this by applying lime to raise the pH, as acidic soils prevent molybdenum uptake. If you see unusually small leaves, lack of zinc is responsible. If boron is lacking your celery will have a brown heart and your brassicas hollow stems. If copper is deficient your leaves will be bright green but prematurely wither. Again, an application of powered seaweed meal or a liquid seaweed feed will start to put things right. The same applies to iron, manganese and magnesium deficiency, all of which result in chlorosis, a condition in which leaves are pale and anaemic. This serious deficiency impedes photosynthesis and can kill the plant. Apply a seaweed spray immediately as a foliar feed, though nettle tea is an alternative. Seaweed can be applied in powdered form to improve the general fertility of the soil.

However, seaweed should not be applied only in emergency. My lovely potatoes were produced by regular applications before any deficiencies set in.


You can do an awful lot of damage to a beach by collecting seaweed willy-nilly. There are three types of algae growing on the beaches of the UK: red, brown and green. The green ones grow where fresh water leaks onto the beach, via either a sewage outfall or drain or even a small stream. Rivers, which drop their silt as soon as the water becomes salty, are not good places for green algae because they do not grow too well in muddy places.  

The brown algae are found attached to rocks. The large ones, well over 1m (3ft) long and attached to a pebble of some sort are called laminarias and come in three different species. The other brown seaweeds, well – dark green, are the fucoids found clinging to rocks. You will find some with serrated edges, some with twisted fronds and some with bladders or bubbles.

Beautiful red seaweed found on a beach on North Uist, Outer Hebrides.

The red algae are much smaller and beautifully coloured. These are some of the rarest seaweeds on the beach and should never be taken. It may well be illegal to take seaweeds anyway as the littoral zone (the point between high water and low water) traditionally belongs to the Crown, and you don’t want to go messing about there as a stay at Her Majesty’s pleasure might inconvenience. But the laminarias that you find on the beach are a spent force. They have been washed inshore from their huge journeys around the world. You will see the great fronds attached to rocks that act as a weight, and thus they sail on the ocean’s currents. You will not damage the beach in any way by taking these spent, once great sailors.

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