Improving Your Soil

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 loses 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 fertilizers just as 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. admin

      Hi Brenda. It is generally accepted that the amount of salt on seaweed is relatively small. It is also very soluble so washes through the soil quickly. The freshest seaweed you can get will be the best, if you are worried about the salt you could rinse it off with a hose before spreading it but I don’t bother and have never had an issue. You will find masses of worms under the seaweed after a week or two, worms won’t like too much salt so I think we can assume it is OK. I hope this helps. Andrew

      1. Green

        In Hawaii we used a lot of seaweed from the cliffs and it did kill back plants the first couple of weeks if you didn’t rinse the salt. I think the spray may have built up from the waves and sun, or the ocean is saltier there, but here in Alaska it doesn’t happen. We would use it to suppress grass and weeds temporarily.

  1. Robert Hill

    Wonderful piece on seaweed ; I use loads of it ,maybe up to a foot deep as a mulch in November .
    Salt is never a problem,and You are right Spudz love the stuff. I understand it is o.k.to collect off the beach but you must never cut it .

    1. admin

      Hi Robert
      I am glad you enjoyed reading our piece on seaweed and to hear you have also had success. It is very satisfying to add a foot of seaweed to the beds in late Autumn and know it is protecting and nourishing the beds over the winter, wonderful stuff!

      No doubt you will have an excellent growing season, I wish you all the best.


  2. Sheana Griffiths

    Close to the Northumberland coast, we have all the seaweed we can use. The garden loves it. We mulch raised beds, earth up tatties, mulch shrub borders plus anything else where there is bare ground. Added directly to the compost heap if there is any surplus. Garden magic! And no worries about scorching tender plants.

    1. admin

      Hi Sheana, thank you for getting in touch.
      Yes, seaweed is fantastic stuff for the garden. There is a common misconception that it will add too much salt but I have never found this to be the case. You will know if you lift up your mulch there are numerous earthworms under it who don’t like salt, if it made the ground too saline they wouldn’t be there.
      Thanks for reading our posts!

    1. admin

      Hi Shashi. Unfortunately you won’t be able to get bulk seaweed to use as a mulch unless you live near the coast but there are many dried seaweed products on the market so you can benefit from it. We have a very handy seaweed and poultry manure pellet which is excellent (Seafeed) as well as liquid plant tonics. I include some links below for your convenience.


      I hope this helps


  3. Ray Hulm

    This is the third season that we have been using Seamungus/Seafeed. It’s a great product and results have been very good but I have a couple of questions. Firstly, why did you drop the large (20k) bags? These were very handy for allotment growers. Also although you provide a very detailed chemical breakdown of the product why not give us a rough idea of the proportion of seaweed to chicken manure and fishmeal? Sorry to nitpick but that’s the English for you! Regardless, Seafeed is top stuff and I would recommend it to anyone.

    1. admin

      Hi Ray. Yes, we will add the proportions of seaweed and manure mix to the product page, apologies for that. We had to drop the 20kg bags because the company that makes the product no longer makes retail packs and now sells direct to the agricultural sector in bulk loads. We buy bulk and re-pack ourselves but don’t have a bagging machine so have to us the largest shipable tub we can get our hands on (12kg). We are looking into solving this problem but for now we can only handle the 4kg and 12kg tubs, sorry! I hope this helps Ray, good to hear from you. Andrew

    1. admin

      Hi Helen

      Thank you for getting in touch. Can I ask if you have something growing in a 30L bucket and you want to add a seaweed mulch?
      All the best

  4. Richard Lightbown

    I started using seaweed on my vegetable patch (some raised beds and some not) last year just about the time I switched to a no-dig system. Generally vegetables thrive on it and it has other benefits such as slug deterrence and is largely immune to disturbance by blackbirds which generally delight in throwing other mulches about, particularly when they have young broods in the spring. (I was told authoritatively that they are looking for worms, but this is a lazy and inaccurate assumption since there are virtually no worms in my soil.)

    I had one bad experience when I composted some seaweed (I am not sure which species now, but probably bladder wrack) with kitchen waste and put the raw compost, then only a few weeks old, onto one end of a raised bed. The other end was mulched with old cow manure, and I planted the whole bed with rows of onions (4 rows of yellow onions and one of red). Onions in the dung mulch have thrived really well. Onions in the seaweed/kitchen compost, without exception, flushed poorly or not at all. They did finally start to grow after some heavy rain. At one stage I did taste the compost and it tasted salty. Most of the onions eventually sprouted and I planted gaps with young leek transplants and stuck a solitary seed potato under the mulch to see what happened. Apart from one leek that was too small, everything has now grown and looks healthy, but is very late and small. Initially many of the potato leaves had a yellow edge, but the plant seems to have outgrown this stage. (It was not organic seed so perhaps this was seed treatment.)

    What happened? I don’t know. I have put freshly harvested bladder wrack on top of my raised beds and not had this effect. Perhaps if I had let the compost mature this may not have happened. But it was almost as if there is some reaction between the seaweed and the kitchen waste.

    1. admin

      Hi Richard. Thank you for your comment and interesting information. I have not tried composting seaweed with kitchen waste and am curious why you have had problems here. If the compost had only a few weeks to break down it would likely be taking nitrogen from the soil to break down. Also, onions will like a firm footing and I expect the compost/seaweed mix may not have been dense enough at that stage of decomposition. I tend to use the seaweed as a mulch, especially in Winter as rain will wash any salts through the soil before planting in Spring. I hope this helps. Andrew

    1. admin

      Hi Rajab

      Yes, seaweed should be fine as a mulch in a pineapple farm. I would get your soil pH tested first though as pineapples like a pH of neutral to mildly acidic. If your soil is alkaline already the large amounts of seaweed will raise (male more alkaline) the pH slightly over time which you don’t want.

  5. Dave Rees

    Great article.

    I took over my uncles garden when he was unable to do so, he had for decades been a highly regarded vegetable exhibitor and in latter years was a National Vegetable Society Judge.
    I had excellent results this year but do not intend to exhibit, nor do I intend to do so.
    One of the finest vegetable growers in North Wales keeps his growing secrets to himself, I did however have a sneak look at his garden a few years ago during November to see it covered in seaweed!!!!
    I this year have coved the soil with a thick cover (15cm +) of seaweed covered with a similar layer of old horse manure full of red worms. We will have to see!!!
    I contacted my local authority’s Harbour Master and was granted permission to gather seaweed but only for my own use.
    Years ago Kinmel Bay suffered a severe flood when ground was covered in seawater for a few weeks, I saw the result of a soil test indicating 7,000 times (seven thousand) that regarded as suitable for normal cultivation!!

    1. admin

      Hi Dave. Sorry for the late reply your comment. Yes, seaweed is the magic sauce. If like me you are lucky enough to have a ready supply them make sure you use it. It does absolute wonders for your soil.
      Have a great season in 2018!

  6. Dave Rees

    Thank you for your reply Andrew.

    I intend this year to grow International Kinney potatoes, this is the Jersey Royal variety and I look forward with interest to see in seaweed is the answer to their very special flavour.


    1. Andrew

      Hi Dave. Best of luck with the potatoes. They will love the seaweed. In fact, I have seem potatoes grow very well just placed on the soil with piles of seaweed on top, makes them very easy to harvest too! I hope it all goes well for you this year!

    1. Andrew

      Hi Dave
      Don’t stress yourself about the pineapples, those who can grow them easily will be green with envy at the stuff you can easily grow in your garden!

  7. Dave Rees

    Hi Andrew

    I apologise for being a pain! But! I have a garden containing an extensive collection of ericaceous shrubs, Rhododendrons, Camellias, Azaleas, plus other species. I apply a mulch of ericaceous compost to these each year. I understand seaweed has a Ph of between 5.5 & 6.00 (I stand to be corrected).

    Could I apply with benefit a mulch of seaweed to the above?

    I look forward to hearing from you.


    1. Andrew

      Hi Dave
      You are not being a pain at all, we like questions as they help us improve what we do. I would be careful with pH of seaweed as figures can be vague and dependent on the variety of seaweed. Personally I have avoided regular seaweed mulches in beds where I want a low pH as I had understood it to raise pH. I will have to do some research on this for you. I would suggest a mulch on a small area to see what the effects are but would not recommend mulching the whole garden on a regular basis. This is something I am interested in so will follow up and get back to you.
      Sorry to be so vague!

  8. John

    I would like to grow a row of garlic in seaweed is this possible, i have covered the ground with about 4″ to 5″ of fresh seaweed

    1. Andrew

      Hi John, Garlic will grow perfectly well with a seaweed mulch. It acts as an effective weed suppressant, will deter inquisitive wildlife and provides valuable trace elements as it breaks down.

  9. Chrissie

    I live on the north east coast and have been gathering seaweed for my allotment for around 4 years. Thank you for the extra advice; very confidence building, especially information about freshness and taking only loose seaweed – nothing still attached to rocks.

  10. Catherine Collins

    I have a question about the folding of turf over seaweed through which you grow potatoes. How deep do you plant the potatoes, snd if you use this technique do you still need to build the ridges up, or leaves the potatoes to grow through?

    1. Andrew

      Hi Catherine. The potatoes are planted 4-6 inches deep. The potato foliage grows through the flap of soil and, as far as I know, it would be good practice to ‘earth’ up, this is traditionally done with more seaweed. I hope this helps. Andrew

  11. Debbie McFadden

    I am making a vegetable raised bed I have put cardboard in the bottom over turned over turf . Can I put a layer of washed up seaweed under a layer of shop compost to plant my vegetables straight into.

    1. Andrew

      Hi Debbie. Yes you can but you might be better off using the seaweed as a mulch on top of the compost rather than underneath.

  12. Linda McCanney

    Hi Andrew, I love this post and have created a version of this for my potatoes. Just a quick one!! In my part of the country, County Louth, you need shore rights to collect seaweed. I collect from my cousins shoreline and always have, It might be worth a mention. Linda

    1. Andrew

      Thanks Linda, great point. A license is required for collecting or harvesting seaweed other than small amounts for personal use. I still wouldn’t harvest any though. The best time to collect dead seaweed is after a big storm – you’re still going to get it nice and fresh.

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