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Seaweed mulch and fertilizer

Updated on November 4, 2010

Seaweed can do things other fertilizers can't

Seaweed is one of the great unsung heroes of gardening. It’s a cheap and highly effective way of mulching and it’s a good problem solver for troublesome patches in the garden. As a fertilizer it’s just plain brilliant.

Seaweed is basically a broad spectrum fertilizer. It contains all the elements required by plants. All terrestrial plants are descended originally from seaweeds, and retain some of the same biology.

Seaweed mulch

It looks like high quality potting mix, but it’s a lot more effective than any potting mix will ever be.  It’s a black, homogenous soil-like mix, and it’s extremely easy to handle.

As mulch, spread on a garden bed, it looks positively elegant. It’s a pleasure to work with. If you’re using local topsoil, it’s a big boost to the nutrient levels of the soil, and can be mixed in with any loose soil quite easily with a fork when planting.

It’s quite economic, as a soil improver, and on 10 litre bag can cover the average garden bed. A quick watering-in will activate it.

Note: if you live in a dry climate, it can dry out, which slows down its effect in the soil. It’s advisable to cover it with a layer of bark mulch or something similar to cut down evaporation. (Bark cover is a useful protection for areas prone to drying out anyway. If you’ve got annuals which are looking sorry for themselves, that’s why.)

Potting with seaweed mulch

As a potting mix, it can be used on its own or with peat to provide a very reliable medium, particularly for plants which are picky or under performing.

It’s particularly useful for indoor plants, which are often inflicted with standard potting mixes that just can’t do the job as growing mediums.

I’ve never lost a plant in a seaweed potting mix, which is a pretty impressive record in view of the fact that most other media I’ve used have all been name brands.


If you’re having problems with seedlings, seaweed mulch is definitely a good option. It’s a good, soft, consistent material which allows seeds to sprout and doesn’t have the issues that seed raising mix can create, like being too sandy, or in my opinion lifeless.

A growth medium which is too wet can affect seedlings, so the seaweed mulch should be left in a “moist”, but not wet, condition. Nothing grows well in mud.

Transplanting seedlings from their punnets to a seaweed mulch pot is also very effective in kick starting their growth spurt. Seedlings are vulnerable at this point, and I’ve found the seaweed mulch to be the safest medium for that first transplant. 

Seaweed fertilizers

Seaweed extracts are powerful things. A teaspoonful in a watering can, or even less, in some cases, is the usual industrial level. The dosage, however, is appropriate. Plants take up relatively small amounts of nutrients. More usually isn’t better, in fact it’s often quite wasteful, not to say expensive.

Seaweed works on all types of plants. It’s particularly useful during the flowering season, when plants are expending a lot of nutrients in the flowering effort.

For gardeners, the seaweed fertilizers are a good, cheap, way of maintaining plant nutrient levels and covering all possible needs of the plant. Although standard NPK (nitrogen, phosphorus and potassium) mixes will do this job, some are really one trick wonders as foods. They’re designed to cover three basic requirements of the plants, but the fact is that nitrogen, phosphorus and potassium have to work with other elements to function properly, and these can be lacking, either in the soil or the fertilizer mixes.

An undernourished plant can’t make full use of fertilizers because of the lack of essential trace elements. Seaweed, which is almost all nutrient, is perfect for plant maintenance.

It’s also very good as a cure for lethargic looking plants which are acting like they’ve hit a brick wall, doing nothing and not producing new leaves, etc. That’s a classic symptom of a plant in limbo, can’t grow, but otherwise normal. Broad spectrum fertilizers are the best option, to cover any possible deficiencies, and 5c worth of seaweed extract can help out a whole garden bed.

For a few bucks, you can have a first aid mix in your arsenal. The seaweed extract is excellent value, and a small bottle can last for ages. It’s non toxic, just remember this is potent stuff, and too much of any fertilizer isn’t good practice.  


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    • jackinabox profile image

      jackinabox 4 years ago

      Thank you for sharing this.

    • Paul Wallis profile image

      Paul Wallis 6 years ago from Sydney, Australia

      Skully- Sorry for slow reply. I'm in Australia, so I can't give you a useful supplier, but you should be able to get wholesale quotes, at the very least. Check out your local suppliers online, compare and talk to the best options so you can be sure you're getting good quality.

    • profile image

      Skully 6 years ago

      Do you know where I can purchase 1t bulka bags from at a good price?

    • Paul Wallis profile image

      Paul Wallis 6 years ago from Sydney, Australia


      Sorry for slow reply, not on the site as much as I'd like. This is a true mulch, no cutting required. I actually grow seedlings in the mulch, which is much like good black loam, but with many more nutrients.

    • profile image

      michael booth 6 years ago

      Hi paul

      as a potting mix, do you cut it up and put on top after you have drilled the seed? or do you put it around the seedling after they have growen a bit?

      Great article by the way

    • Paul Wallis profile image

      Paul Wallis 7 years ago from Sydney, Australia

      My pleasure, Dirt Farmer.

    • The Dirt Farmer profile image

      Jill Spencer 7 years ago from United States

      Thanks for a good read! Lots of good information.

    • mistyhorizon2003 profile image

      Cindy Lawson 7 years ago from Guernsey (Channel Islands)

      Excellent article and I too have hubbed about the benefits of liquid seaweed, although I also use fresh seaweed as a mulch on my veg allotment because we have a limitless supply on the local beaches that the local authorities encourage the farmers and gardeners to remove from the beaches to make the beaches less smelly and more appealing to tourists.