Improving Your SoilSeasonal Articles

Seaweed and Potatoes

Seaweed in frostA Protective Seaweed Mulch
I took the opposite photo in early January, it’s frosty seaweed obviously but it’s in my garden rather than on the beach. I prepared most of my beds in December (even then a little late, early Autumn is best) with a hefty top dressing to protect from the bad weather and to make sure the worms and microbial life in my soil weren’t twiddling their thumbs all Winter.

It’s the life in the soil that processes the organic matter and releases the nutrients that feed your plants so feeding the soil should be rule number 1 of vegetable gardening. Last year I added plenty of manure and compost but this year I’ve really gone to town:

layers of manure, seaweed and garden compostI have built up layers of Envirogrind (1/2 barrow per sq. metre) rotted farmyard manure (1/2 barrow per sq. metre) and fresh seaweed (1 barrow per sq. metre). You can just about see the original soil in the bottom right corner of the photo, then black Envirogrind, bulky lumps of manure and finally seaweed on the left.

As you may know I’m going for ‘No Dig’ so I just layered all this on top rather than digging it in. The seaweed is now rotting down nicely with a layer of gooey mush underneath which worms, for some reason, seem to absolutely love.

Every cloud has a silver lining I guess and while no one can say they enjoyed the recent storms (apart from the surfers) one benefit is the coast is covered in washed up seaweed. Seaweed is better used fresh as it looses it’s nutrients very quickly so best picked soon after a storm. Don’t worry about the salt as it contains very little and won’t harm your soil, rummage under a rotting pile of seaweed you’ll find lots of worms who wouldn’t be there if the ground was too salty.

I don’t know if collecting seaweedit’s illegal to collect it, it is to cut it which is fair enough but the stuff on the beach is fair game as far as I’m concerned. Here’s Niall on one of our seaweed gathering expeditions pointing out possible escape routes should we get ‘disturbed’ by the law.

Seaweed & Potatoes
Potatoes are particularly fond of seaweed. The population explosion on Ireland’s West coast before the famine was pretty much based on the link between potatoes and seaweed where soils were ‘built’ over generations using seaweed, sand and manure. I have seen potato plants growing in piles of seaweed alone and still produce very good yields so this year I’m keen to do some experiments in my own garden.

I’d love to explain exactly why seaweed is so good for potatoes and plants in general but like a magician it seems reluctant to reveal its secrets. I spoke to Stephen Casburn of ‘Sea Nymph’ this morning (he supplies us with our Irish seaweed liquid feeds) and he explained it as follows:

spreading fresh seaweed on vegetable bedSeaweed is a balanced source of 58 trace elements, growth hormones, nutrients and fungal and disease preventatives. Essentially it’s a multi vitamin for plants.

Your garden soil only has room for a fixed amount of elements so if you have too much of one thing, like iron for example, you will push out others and make them unavailable for plants. The beauty of seaweed is it is a balanced source of a broad range of elements which is released slowly so won’t overwhelm your soil.

Seaweed also stimulates the good bacteria in the soil which process nitrogen and make it available to your plants. It also seems to bolster the plants immune system making them more able to resist attack from pests and diseases.

There is still a lot we don’t know about seaweed as the benefits seem to outweigh the measurable components. It’s about the sum of the parts, it’s a balanced bit of everything which seems to create that little bit of magic.

But back to the potatoes:

Lazy Beds
Dermot Carey had shown me how to create lazy beds in the traditional way by making ridges of seaweed and folding a flap of inverted turf over the top. My vegetable garden is all in raised beds so I can’t try it there but I do have a bit of a field where I’ll give it a go. I include a diagram below:

Making a lazy potato bedIt’s a bit of a shame to call it a lazy bed because it’s pretty clever really and doesn’t exactly build itself! The idea is to make a seaweed filled sandwich between sods of grass.  Here’s what you do:

  • Mark out your drills with a string line approx 750mm or 2.5 feet apart and make long mounds of seaweed along the drills.
  • Cut 3 sides of your sods to half a spade depth along the mound a bit like the keys of a piano leaving the edge closest to the seaweed uncut. Do this on both sides of the mound.
  • Fold the sod over the seaweed from both sides to create a long sandwich with the upturned sod grass side down on top of the seaweed.
  • Dig a further half spades depth from the trench to fill the gap left where the sods meet.
  • Make a holes in the top of the mound with a dibber and plant your seed potatoes 4-6 inches deep at a spacing of 11 inches apart for earlies and 13 inches apart for maincrop potatoes.

The seaweed sandwiched between the sods will provide a little bit of heat as it rots down which is helpful to get the plants off to a good start in early Spring. The grass above and below the provides nitrogen which will be broken down by the soil bacteria which are in turn fed by the seaweed, good eh?

I’m going to add a note of caution of my own here (even though I haven’t actually tried it yet): if your soil is a heavy clay I’m not sure if this will work so well as air may be cut off from the seaweed which will take longer to rot.

early morning frost on seaweed covered bedsSeaweed mulch
As I said my vegetable garden is all raised beds (it was very wet) so the lazy bed method can’t be applied. I’m keen to use seaweed here too though so my plan is to plant all my potatoes a little shallower than normal and use seaweed as a mulch to earth up instead of dragging soil up around the plants. I’ll do half and half on the beds and compare the yields and will let you know how I get on.

I figure the seaweed on the surface will break down nicely over the Summer not only feeding the potatoes but also improving the soil for a quick follow on crop in the Autumn.

It’s free, you might as well use it.
I get a real kick out of using natural inputs to improve my soil, seaweed is plentiful around our Island, it’s free and has so many advantages it’s a real shame not to use it. You will build the most fabulous garden soil by adding seaweed every year without damaging the environment by adding artificial fertilisers just as Irish coastal farmers did for generations.

Remember to get it as quickly as possible after a storm for the most benefit and please don’t cut it from the rocks as you will get in trouble with the boys in blue.



  1. caroline jolley

    Very interesting. We have a new ‘helper’ in the garden who is 81. He has been regularly bringing up bags of seaweed over the winter and mulching the soil. My other helper who is a City boy at 80, disapproves strongly but I’ve told him to hang on until we see the results. Another helper who is a botanist, used to shred the seaweed before treating the soil at her father’s farm. This was for large crop areas.
    I’ll watch your results with interest – we’re also doing some potatoes in the polytunnel with seaweed to see what happens.
    As you say – it’s free ! We also use lots of poultry manure and leaf mould, also free . 😉

  2. Jane Powers

    Excellent blog post. I particularly like the illustrations of the lazy beds. I’ve seen them made several times, but never more expert than when Dermot Carey (mentioned above) demonstrated them for me. He pointed out how important it is not to break the “hinge” on the flap of turf, as when it is intact, the grass can’t creep out to recolonize the beds. Such a well thought-out method of cultivation!

  3. Mark Peyton

    Living on the south coast of Puerto Rico allows me to collect seaweed conveniently. I have found two types of seaweed that wash onto the shore. One variety is sea grass that is consumed by Manatees. The other is a seaweed that looks like pine needles. Both varieties make an excellent mulch and years later they have not completely decomposed. It creates an excellent moisture sealant and allows easier weeding and less watering.

  4. John Johnson

    On Cape Breton Island, Nova Scotia…Just put my potatoes in the ground in raised beds (relatively shallow beds at 8 inches/20 cm) deep. Seed is at around 4 inches/10 cm depth. Should I mulch w/seaweed (aged) now or wait until they start to show? Any opinions? My gut says now and add as the plants grow…???

    1. Andrew

      Hi John. I would go with your gut there, add a mulch now and top up as they grow to prevent any tuber being exposed to light.
      I hope this helps. Andrew

  5. Bob

    I’m down in Cork & make an annual trip to collect seaweed for the spuds (furst early & mains) over the past 10 years or so. This has benefited the yield, soil texture and IMO reduces the impact of pests like slugs etc. I shove a few bunches into the water butt and this is used to water and feed other veg. My use of artificial fertilizer is practically neligible.

    1. Andrew

      Hi Bob. I am glad to hear you are enjoying the benefits of adding seaweed to your garden. It is interesting you mention slugs as my garden should be riddled with them but in practice I rarely have any issues. Like you I apply seaweed every year and find it has transformed my pretty awful garden soil into a dark and fertile oasis. Thanks for getting in touch Bob, I hope you have another great growing season.

  6. Kira Grygoryev

    i just got 4 coal bags full of seaweed and have been wondering how best to use them, i think i will mulch/earth up my potatoes with it, compost some and soak some to make liquid feed. I am attempting to grow some of my potatoes using the Ruth Stout method this year, partly as an experiment and partly to improve the soil in my garden which sadly is very poor, its encouraging to hear adding seaweed has helped improve your soil!

    1. Andrew

      Hi Kira, Seaweed is a fantastic additive for the garden. Use it as a mulch with any of your vegetabless and simply dig it in at the end of the growing season. It will break down and give great structure and nourishment to the soil.

  7. Killian O Donnell

    Any suggestions about how to deal with a layer of not-quite rotted seaweed in a raised bed I intend to use for carrots? Might make the soil a bit too rough for the fine stuff carrots seem to prefer.
    Maybe just pop it in the compost?

  8. Tony Reddin

    hi Kira, Make sure your potato sets are very well covered as they grow so sunlight doesn’t turn them green and inedible

  9. Richard

    I brought in soil for my garden a couple of years ago and sowed potatoes. Lovely crop of spuds, Only full of wire worm. 🙁 Would the addition of seaweed help remedy this? I have everything in raised beds at the moment, but i would like to use the ground again.

    1. Andrew

      Hi Richard. I am not sure, all I can say is I used to have a wireworm problem but not anymore. I have also used a nematode in the past which helps rid the soil of wireworm but unfortunately the company who makes the one we sell has just ceased trading.

  10. Howard Brown

    This year I have decided to have my potatoes grow in mostly Sea Weed
    I added a layer of sea weed
    I added a later of black soil
    Then I mulched sea weed with my lawn mower and mixed it with my soil

  11. Brid Martin

    A novice here Can I put my potatoes sitting on the seaweef and cover with clay.Eill the seaweed cause any damage or burn the sprouted potatoes. I am using small sprouted potatoes. Thanks Brid . Great article and info.

    1. Andrew

      Hi Brid. I would do it the other way around, put the potatoes in clay and cover with seaweed but either way they will work.

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