If you live near the coast, seaweed can be a fantastic resource to forage for fertilizing your garden. There’s lots of easy ways to use it to increase the health of your soil, and your veggies and flowers too.

First of all, check the local regulations where you live around collecting beach-cast seaweed. Collection is fine in some places, and not so fine in others – this post has links to the regulations in Australia.

Once you’ve got the all-clear, always remember to practice responsible foraging and harvest lightly from within the tidal zone – there’s lots of animals, birds, insects and other organisms who also consider that beach cast seaweed valuable, so leave enough to go around.

We’ve used everything from shopping bags to a pull-behind trolley to harvest beach-cast seaweed. How you roll is up to you.

Some people consider the older seaweed stinky or dirty, but to me, it’s just organic material that’s decomposing – and it smells a lot better than plenty of other decomposing organics I’ve worked with!

Once you’ve got your seaweed home, there’s several ways you can use it in your garden. Which method you use will depend on your garden, and you. Generally we don’t wash our seaweed unless it’s going into seaweed tea.

Why use Seaweed in your garden

Seaweed contains ALL the elements (wow!) but most only in trace amounts – it does however typically contain useful amounts of: iodine, copper, manganese, potassium, phosphorus, iron and zinc. It’s typically used straight up, as a compost addition, or as a brewed seaweed tea.

In either form, seaweed is used as a great soil conditioner which helps build a healthy soil food web in your garden. It’s liquid form is also used as a foliar spray for ornamental and edible gardens alike.

Seaweed tea is known for starting strong seedlings and producing resilient vegetables (it’s a huge help against marginal frosts) as well as improving veggies transport and shelf life.

And as we all know, super healthy plants means less pest problems, longer fruiting periods and general garden goodness.


You can use seaweed to mulch around and underneath your plants straight up. It will decompose faster if it is underneath another layer of mulch, or go dry and crinkly and decompose slower if it’s the top layer – mulching with seaweed is good for:

  • Instant organic fertilizer solutions! Lay it down, and you’re done. Boom.
  • A great broad-spectrum, slow-release fertilizer for plants
  • As a dried-out spiky top layer it’s helpful for deterring snails, slugs and some household pets too
  • As a faster-decomposing under layer it’s also great for slug control, as the slugs dislike the small amount of salt
  • Weed free mulch! No embedded weed seeds here, unlike many straw mulches
  • Doesn’t blow away in the wind like some mulches can
  • Organic (ish) – if gathered from clean waters and not near ocean outfalls, your seaweed should be a healthy addition to your garden.
  • In sandy soils, the alginates in the seaweed (particularly bladder wracks) can really help as an additional wetting agent

Basic Seaweed fertilizer tea

Home made seaweed tea is a great addition to any garden that’s packed with plant-friendly nutrients – we love it because it can be made seasonally when the seaweed shows up, and then used throughout the year.

You can make straight-up, single ingredient seaweed tea, or combine it as we do with other nutrient and mineral-packed plants, for an all-round liquid fertilizer.

Either way, the process is basically the same. Here’s how we make our Seaweed tea, with comfrey, nettle and borage.

You will need:

  • A bucket or barrel with lid
  • As much seaweed as will fit in your bucket
  • Comfrey, nettle + borage leaves
  • Non-chlorinated water (rainwater is great! go catch some)
  • A stick for stirring
  • A shady spot to stash your bucket for the duration

Fill your bucket with lightly washed seaweed, and add your herbs and other leaves. Fill to top with water, and place somewhere out of the sun (maybe not next to the front door, it will smell at certain stages of brewing), with the lid on lightly, but not tight.

Stir the brew with your stick each day for a week or so if you remember.

Then wait, for about 3 months.

During this time, the tea mix will start off aerobic, which smells fine, then slowly go anaerobic, which is the stinky stage. Fear not, leave it alone, and push on.

After a few months or so (the timing depends a lot on your ambient temperature, this mix will progress faster in Summer than Winter) the anaerobic stage gives way to a second aerobic stage (as more good bacteria have moved in), at which point your seaweed tea will smell good again. And now it’s ready to use!

You’ll probably find that most, if not all, of the seaweed and leaves have broken down entirely, or there may be some sludge at the top, or bottom, of your bucket.

While this sludge is liquid (sludgy?) gold, it’s best put in your compost pile rather than straight on your plants, as it’s concentrated goodness and might be a bit much for them.

You can pour the liquid off into another vessel from the sludge for applying to plants, or leave it in the same vessel – its up to you.

Dilute your concentrated seaweed tea 1:10 with water, and apply to plants and seedlings weekly for very healthy veggies that are packed with extra goodness.

Extra goodness from the added herbs:

Comfrey + Borage: potassium, nitrogen, phosphorous | Nettle: Nitrogen + trace minerals

Variations: You can also aerate your seaweed tea throughout the brewing process with a small aquarium oxygenator, or a snazzy compost tea brewer – this will shorten the brewing time dramatically and cuts out the stinky stage, but obviously requires more energy inputs.

You can also add microbial inoculants to your seaweed tea to increase microbial activity and speed up the process that way, as well as possibly enhancing the result – these can be got at some garden stores or online.

Live somewhere where seaweed harvesting is not possible? You can buy dried kelp from most rural stores where it’s sold as an animal feed supplement, which works fine in the above recipe.

Using Seaweed tea

You can use Seaweed tea at the 1:10 ratio on the soil of your garden, and also as a foliar spray for plant leaves.

The seaweed tea can be helpful anti-fungicide against powdery mildew and some other fungal diseases, and it’s also a helpful pest deterrent.

Diluted seaweed tea is also great for seedlings, as it contains some natural hormones that aid plant growth. We use it frequently as the liquid in our soil blocking + seedling mixes, as well as in seedballs.

If you can’t access or make the home made stuff, there’s also liquid seaweed products like Seasol that are made of kelp from Bass straight and knotted kelp from the north atlantic – another option for happier plants.

A note about home-made compost + seaweed teas: Caution when handling the tea if you’re pregnant is a good idea, especially during the anaerobic stage.

As with any natural brew, use common sense, high-quality ingredients, and if it smells badly wrong at the end when it should smell great, don’t use it.

Do you make seaweed fertilizer at a small or large scale? What do you use? Got any recipe tips for us? We’d love to hear…

Keen for more seaweed info? Check out Foraging, Drying + Eating Seaweed in Australia…

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How to Make and Use Seaweed Fertilizer

I’m sold on using seaweed fertilizer, whether fresh or purchased—and as people try it for themselves, they’re also learning its benefits.

As a commenter on my YouTube Channel writes:

“Dude it worked!!! My plants have grown very well with washed seaweeds! I use or twice every week and it is working awesome! Ive never had such growth before!! wow! Thanks man! God bless you….never listen to those who say negative things on you…You are doing great! God bless you.”

Thank you! God has blessed me and continues to do so. And I count the abundance of free local seaweed as one of those blessings.

Over a year ago, I posted this video on making and using seaweed fertilizer in the garden:

If you live far from the beach or don’t feel like hauling bags of seaweed, you can get good seaweed fertilizer on Amazon. Neptune’s Harvest is a popular one and is really rich since it’s a mix of both seaweed and fish. Fish emulsion is like magic in the garden—and when you mix it with seaweed, you’re really adding the bounty of the ocean to your plants. They go crazy. In fact, my friend Jo the Master Gardener once told me that fish emulsion is the way to grow truly awesome organic strawberries in Florida. It greens them up and makes them fruit without encouraging leaf growth over fruit.

Another option that I used to use on my beds in North/Central Florida was kelp meal. It’s loaded with minerals and a little goes a long way. I don’t know if kelp is totally safe post-Fukushima, but I haven’t heard anything really scary lately.

I added kelp meal to the fertilizer mix I used to grow these amazing cabbages:

I followed the directions for making Complete Organic Fertilizer, which Steve Solomon writes about in Gardening When it Counts. Once I had my mix, I sprinkled it all down the beds, raked it in, put down a weed barrier, punched holes, then planted cabbage seedlings. They did better than any I’ve grown before or since—absolutely beautiful heads.

Seaweed was part of that. Consider it a multivitamin for your garden, loaded with micronutrients. The big three (nitrogen, phosphorous, and potassium) are the main course—and seaweed has those, but not in huge amounts. However, seaweed is really rich in the little things that add to the overall health of your plants.

How to Make and Use Seaweed Fertilizer

So, you have some seaweed and want to try it out? Here are three good options.

Option #1: Seaweed as Mulch

Take the seaweed, rinse it out, then use it as mulch. That works nicely and breaks down over time. Maritime Gardening agrees:

Option #2: Compost It!

Put seaweed directly into the compost pile. Consider it a “green” layer. I don’t bother rinsing it when I do this, figuring the salt on it will work its way through.

Option #3: Make Liquid Seaweed Fertilizer

You’ve seen me do this before with weeds, manure, kitchen scraps, etc.:

You can do it with seaweed, as well. It’s a great additive—or it can be used all by itself.

This is a very good video where a man uses the same method I do, but with comfrey and other northern leaves, along with seaweed:

Hey, that guy looks way more pro than me. I should send him a T-shirt.

Now, go—find yourself some seaweed!

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David The Good is a Grow Network Change Maker, a gardening expert, and the author of five books you can find on Amazon: Compost Everything: The Good Guide to Extreme Composting, Grow or Die: The Good Guide to Survival Gardening, Totally Crazy Easy Florida Gardening, Create Your Own Florida Food Forest, and Push the Zone: The Good Guide to Growing Tropical Plants Beyond the Tropics. Find fresh gardening inspiration at his website and be sure to follow his popular YouTube channel.

How To Create Your Own Seaweed Fertilizer

Seaweed fertilizer is known for its healthful benefits, like adding up to 60 trace elements to your mulch or compost. Seaweed also contains many growth hormones and disease control properties. Because of these factors, plants treated with this type of fertilizer have access to virtually every nutrient they could ever need. Seaweed also provides food for the beneficial fungi in your soil. Not only is it inexpensive to make seaweed fertilizer, but it is organic and eco-friendly.

Step 1 – Put It Together

The first thing you need to do is to put your seaweed into the bucket and add the warm water. Stir up your mixture well, and seal up the bucket.

Step 2 – Let It Rot

The benefits of seaweed fertilizer don’t come from using fresh seaweed. Rather, they come from using decayed seaweed. Let your mixture rot in the bucket, opening it to stir your mixture every few days. This process should take about a month. Once it is rotted, the water will turn brown and murky, and it will have a rotten plant matter smell to it. You’re already halfway through your process!

Step 3 – Dilute Your Concentrate

The rotted mixture in your bucket is your seaweed fertilizer concentrate. In order to use your fertilizer, you must dilute it with more water. The recommended proportion is 1:16, and so when you add the correct amount of seaweed fertilizer concentrate to water, it is ready to use.

Step 4 – Apply Fertilizer

The best way to apply the ready-to-use fertilizer is to put it in a garden spray bottle. These are tanks with long spray nozzles at the end of a hose. To use your fertilizer, put your diluted concoction into a garden sprayer, and spray it over your flowers and vegetables. Allow the mixture to sink into the soil as many of the nutrients can only be absorbed through the roots of the plants. This is especially important for root vegetables like potatoes, carrots, beets and onions.

Fertilize with a seaweed fertilizer once a month, or more if necessary. Rain can wash nutrients away, and if the soil is dry, then the vitamins are unavailable to the plants because they’re immobile. As always, fertilizer should be used at the beginning of your planting to ensure healthy germination for your seeds.

If you would like, you may add a fish emulsion to your seaweed fertilizer. This can add other trace elements and nutrients to your soil upon application. If odor is a problem, you can also supplement your fertilizer with molasses. The sugars in molasses help to control the rotting smell, as well as promote good microbial growth by feeding the aerobic bacteria with the simple sugars. The combination of all these steps can bring you a stunning flower garden or a healthy, robust vegetable garden every year.

The best plant tonics and supplements for your garden

Plant tonics – how they work

Many tonics are applied to foliage. They are absorbed mainly through the stomata (pores), which are predominantly on the underside of the leaf.

The best time to apply foliar feeds is when the foliage stomata are wide open, in the early morning or evening. In hot sun stomata close to prevent too much transpiration (loss of water).

It is best to apply tonic to your foliage in the early morning or evening (RM Floral / Alamy)

Relatively low concentrations are applied and nutrients are absorbed rapidly (usually within 6-24 hours).

Foliar feeding can act as a catalyst, increasing the plant’s nutrient uptake from the soil. According to Amigo Bob Cantisano, California’s foremost organic farmer, liquid feeds frequently give you produce with higher Brix levels. A high Brix level (measured with a refractometer) equates to high sugar content/sweetness, which equates to healthy plants more able to resist pests and diseases.

• Can a black spot spray restore your roses to health?

Leading plant tonics and supplements

Maxicrop, a leading seaweed feed, was next on Alex’s list. There are four main products: one contains iron (Maxicrop plus Iron) – useful for ericaceous plants; one is a basic seaweed tonic (Maxicrop Original); two have fertiliser added. Seaweed tonics have a big following amongst amateur gardeners. I am partial to deploying them when plants start to look slightly distressed. They are easy, safe and pleasant to use.

Garlic – this is another popular product amongst amateurs. Alex stocks two products – Aston Rabitof, a garlic foliar spray designed to control not only the bunnies but also deer and other mammals. Aston Garshield is another foliar spray designed to help stimulate natural plant defences.

I have used garlic in the past, but I am not convinced. As a slug deterrent I favour Grazers G2 formula (sprayed onto the foliage) which also strengthens and stimulates growth, and I use the original Grazers (sprayed onto the foliage again) for rabbits, deer and bigger fry.

Ground aluminium sulphate (17 per cent) is commonly used by gardeners who hanker after blue hydrangeas but garden on alkaline soil. It is a popular product but must be applied when the plants first go in. Try to acidify the soil, too, by adding bark mulch and sulphur.

Ground Aluminium Sulphate can help you grow blue Hydrangeas on Alkaline soil(Norsworthy Photography / Alamy)

• Top tonics for hydrangeas

Potassium bicarbonate (baking powder) for mildew is not just an old wives’ tale. Dejex sell it commercially, too. It has EU approval by virtue of being “a commodity with approval for use as a professional pesticide”. It acts as a mild fungicide and is very safe to use.

Magnesium sulphate (Epsom salts), another household product, is used to correct magnesium deficiency. Magnesium is a constituent of chlorophyll and if the plant is deficient the leaves may be pale or have yellowing between the veins. Thin soils exacerbate this deficiency; sometimes high applications of potash-rich fertilisers cause magnesium to be unavailable to the plant. Apply it when necessary and you will see almost magical, instant results.

Chelated iron is often applied to acid-loving plants that are growing in too highly alkaline soils, which causes a yellowing of leaves. Librel SP Iron is a form sold as a soil drench and Dejex say it is popular with growers of amenity plants, they especially use it for camellias, rhododendrons and citrus plants.

Latest developments

There are many other supplements that are popular. I am a big fan of Biochar and by adding it to my thin soil it helps the water holding capacity. I am also a fan of mycorrhizal fungi for use when replanting roses in soil where roses have been grown before, it saves moving mountains of soil.

I have usedeffective microganisms having been impressed by their use in agriculture and horticulture in Japan. This is a cocktail of fungi and bacteria which suppress harmful fungi and bacteria. I think perhaps they are of greatest use in impoverished soil. A client told me it doubled his rice crop (in Japan), but his was quite poor acid soil.

Another great tip I got from Japan was Neem oilThis is a very effective against many pests and fungi and is widely used in commercial crops across the world. Here, as with most of these non mainstream products, they legally cannot make claims for use as a pesticide or fungicide under EU law. But if you have a problem with aphids or other pests Neem oil is extremely effective.

Neem oil is extremely effective in helping with aphid problems

• Organic ways to fertilse the garden

Apply it once a week for three weeks to the foliage and/or use as a drench.

There is a relatively new Dutch product, Topbuxus which claims to cure box blight. It contains magnesium oxide plus nitrogen. On the website it claims to be 100 per cent effective in stopping fungal disease in box. If you do not want to go down the systemic fungicide route (which is extremely effective) this may be your best option, but I can not vouch for it.

Alex also stocks some biofungicides and insecticides that are only for use professionally – these are growing in popularity as the restrictions on chemicals such as neonicotinoids, are clamping down on their use.

These supplements are being used more and more as many pesticides and fungicides are increasingly withdrawn from the market. I rely on them increasingly and find them mainly pleasant to use. It is confusing though, especially as manufacturers legally can not always say what the products do because of the prohibitive costs necessary to register them. Whatever you use, make sure you keep a keen eye on your plants though and act early on.

The shadow of the gardener is the best tonic, after all.

Effect of seaweed concentrate on hydroponically grown spring barley


Spring barley (Hordeum vulgare cv. Triumph) was grown hydroponically over a 6-week period. Two treatments were incorporated either into the hydroponic solution or sprayed onto the plants at rates of 1 ml per 3 litres. The treatments applied were: (i) a seaweed concentrate prepared fromAscophyllum nodosum (L.) Le Jolis (marketed as Maxicrop Triple), (ii) a ‘Trace element’ treatment incorporating the micro and macro nutrients added to the seaweed extract base to produce the formulated product Maxicrop Triple and (iii) a control treatment. Irrespective of the mode of application, plants treated with Maxicrop Triple grew faster than plants under either of the two other treatments. Elevated growth rates were also found for the ‘Trace element’ treated plants when incorporated into the hydroponic solution. At the final harvest, plants with Maxicrop Triple incorporated into the hydroponic solution showed increases from 56–63% over the control treatment for the growth characteristics measured. ‘Trace element’-treated plants produced increases of between 25–45%. When the treatments were sprayed the effect was less pronounced. Maxicrop Triple increased growth characters by 35–38% and the ‘trace element’ treatment gave increases in the range of 2–13%.

Liquid Seaweed Fertilizer

Liquid seaweed is great nourishment for plants but is different from most N P K fertilizer treatments. In fact seaweed only contains a little K – potassium. But it has many other minerals, vitamins, and enzymes that are natural growth stimulants. Many of these are not found elsewhere. These nutrients fuel the plant cells’ growth, and this results in healthier, stronger, and more disease-resistant plants. Expect an increased uptake of nutrients from the soil, better resistance to frost and other stress conditions, increased resistance to pests, and best of all, increased yields!

University studies report that applications of seaweed improved yields in potatoes, tomatoes, peppers, strawberries and corn, and geraniums produced more flowers per plant; grapes were sweeter; gladiolus corms grew larger; and cucumber yields increased 40 percent and the fruits suffered less often from softening and rotting.

Seaweed extends the shelf life of fruits and vegetables if applied 10 days before harvesting. It lengthens the life of cut flowers if they are sprayed with liquid seaweed a day or two before cutting.

Liquid seaweed shouldn’t be used as your only fertilizer. Nitrogen (N), phosphorus (P), and potassium (K) should come from another source.

For several years, I have done seaweed fertilization with the powdered version of a product called Maxicrop. It is also sold in a liquid form, but mixing the soluble powdered form is much more cost effective. A 10.7 ounce container will make 65 gallons! It dissolves very easily in water and my plants enjoy the excellent source of nutrition that seaweed provides.

The downside to mixing it yourself is that it’s messy. When the powder gets wet, it can create stains that take some effort to clean. Also, don’t mix it up outside when it’s windy. The powder will blow away. If you buy Maxicrop in a pouch, it’s useful to move it into a jar with a screw top lid so you have more control and there’s less chance that it will spill.

For awhile I sold Maxicrop in our natural garden store. It sold very well. I had happy customers who would return for more on a regular basis. I just looked at the reviews and Amazon customers are happy with it too; over 100 reviewers average 4.6 stars (out of 5).

Maxicrop uses seaweed that is harvested along the Norwegian coastline, an excellent seaweed growing environment. There are thousands of varieties of seaweeds, and very few have value for plant life. This seaweed from Norway has been thoroughly tested and its value proven. Maxicrop is a non-polluting, renewable resource, and is OMRI listed, meaning that it is approved for use in gardens and farms that are certified organic.

Seaweed fertilizers can be used as a soil treatment. And since most plants absorb their nutrients through the leaves, applying it with a foliar method will benefit the plant even more. As the plant absorbs sunlight, it will also absorb the nutrients in the fertilizer.

Alginates (sponge-like starches found in seaweed), hold water droplets near the plant roots, making moisture available to them without drowning them. They also help enrich the soil by feeding a myriad of beneficial microorganisms.

Soaking seeds with liquid seaweed prior to planting will improve seed germination, root growth, and early seedling vigor. Liquid seaweed also can be used as a rooting solution. Place cuttings in a solution of liquid seaweed and water until roots develop, then plant. It reduces transplant shock and speeds root growth.

How to Use Seaweed to Mulch Your Garden

Benefits of seaweed for gardening

Gathering seaweed for the garden has always been a favorite outing for our family. We usually take a small skiff to a nearby beach and load up with as many sacks as we can safely transport home. It’s fun for children, as they can participate as well as an adult, or they can simply enjoy the beach while we gather the seaweed. As we fill up our sacks, our thoughts drift to the many benefits this will bring our garden.

  • Saves water, keeps soil moist at ground level

    The purpose of any mulch is to keep garden soil from drying out at the surface. And by preventing moisture from evaporating, mulch reduces the need for watering. The practice of mulching is essential in areas where conditions are hot and dry.

  • Eliminates the need to weed
    Mulch covers the soil and blocks new weeds from sprouting. Because the soil beneath the mulch remains moist, any weeds which do manage to sprout through the mulch are easy to pick.
  • Repels slugs and other pests

    Slugs are immediately repelled by two things – salt and sharp-edged materials. Seaweed has a natural salt content which repels slugs, and within a few days of application it dries and becomes quite crispy. Slugs do not like “crispy” surfaces, as the sharp salty edges cut into the soft body tissue. While some mulches may provide hiding spots for slugs, earwigs and other pests, seaweed mulch does not have this disadvantage.

  • Enriches the soil
    Seaweed is a broad spectrum fertilizer that is rich in beneficial trace minerals and hormones that stimulate plant growth. Seaweed is high in carbohydrates which are essential building blocks in growing plants, and low in cellulose so it breaks down readily. Seaweed shares no diseases with land plants.
  • Boosts lethargic plants
    Seaweed fertilizer contains an abundance of fully chelated (ready to use) micro-nutrients which can be readily absorbed by plants without any further chemical decomposition needed.
  • Helps lighten the soil
    Compacted soil can benefit as seaweed mulch breaks down. As the material becomes incorporated into the soil, aeration is improved and the soil becomes more crumbly and moist.
  • Does not contain weed seeds, unlike bark mulch
    Two years ago we used commercial bark mulch to cover our garden pathways for the purpose of blocking weeds. Today, these pathways are sprouting horsetail, an invasive weed which can be difficult to eradicate. Seaweed does not bring any foreign weed seeds into your garden.
  • It’s free!

But what about salt? Is this a problem?
We have been using seaweed as mulch for many years and have not seen any adverse effect, such as a salt overload in the soil. In our region we have plentiful rain. If you are concerned about salt, seaweed can be spread out over the driveway and rinsed with a hose. Of course this is not an issue if you are using freshwater lake weed.

I am always in awe of the fact that there is a garden under the sea; that plants can not only survive but thrive in a watery world that is turbulent and ever changing.

Seaweed is magical stuff in the water, and it’s somehow even more so when out of it. It’s incredibly healthful to us, to our animals, to our soils and our plants. It seems anyone who gets to ingest a little seaweed does better for it.

As seaweed breaks down into the soil, it encourages microorganisms whose activities help convert unavailable nutrients into forms that plants can use. It increases chlorophyll production and contains many micronutrients important for soil and plant health, as well as acting as a growth stimulant: it is rich in cytokinins, plant growth hormones that work above and below ground, improving root growth.

I recently went to Inagh Valley Trust, a seaweed research centre in Connemara, Ireland. They’re developing all sorts of interesting seaweed products, including adding seaweed to manufactured bread to increase its shelf life, and creating a seaweed feed for honeybees, to improve hive conditions and combat disease.

I spent a morning geeking out on seaweed spores swirling around in glass jars as they went through propagation, and munching on seaweed health bars. Then I went to the coast and wondered how much beach-strewn seaweed I could cram into my suitcase home – and whether Ryanair would complain about the smell.

It’s very important to collect only seaweed that has washed up; it’s not sustainable for everyone to go around the rocks pulling it off. Winter storms, however, often wash up great mountains of the stuff. There are brown, red and green algae, and all have different nutrient levels, so collect a variety, if you can. In general, seaweeds contain 10 times the mineral levels of land-based plants and are particularly rich in iodine and calcium. You can put them directly on beds; they will be salty, so you can’t plant direct into them, but a winter of rain will wash the excess salt away.

If you don’t have beds that are suitable for such methods, add the seaweed to your compost, or compost it on its own. Once broken down, it’s the magic ingredient for growing thin-skinned new potatoes, as well as for top-dressing pots, where it will act as a mulch while continuing to break down. Rotting seaweed is always hopping with life. Springtails (little jumping insects) seem to love the stuff, but don’t be put off; it’s part of seaweed’s magic that everyone wants a part of it.

If you are nowhere near the shore, you’ll have to buy some. And it’s well worth it: if I had the money for only one type of fertiliser, it would be seaweed. There are numerous brands; I like the Irish one called Seafeed, which does pure seaweed meal, just dried and ground. Whichever you use, your garden will thank you.

Seaweed Fertilizer — Using andComposting Seaweed for Your Garden

To make a long story short, I love seaweed. I mean the garden loves seaweed, and the garden is the ultimate boss, so I aim to please the garden!

The short version of this love fest for seaweed follows…

In edible terms, the many forms of aquatic plants are called ‘sea vegetables’, and you’ll find the likes of dulse, kombu, wakame and more in shops.

Like vegetables they are bursting with minerals and oh so good for you — even if you only have some wrapped around sushi sometimes.

Seaweed in the garden, how do I love thee… let me count the ways:

  1. Seaweed fertilizer is actually a bit mis-named. It is more of a tonic, due to the low quantity of nitrogen and phosphorus… although it does have the full range of properties in it to improve your soil. As well as supplying bulk to condition the soil, seaweed contains around 60 trace elements, growth hormones and nutrients, and fungal and disease preventatives. Interestingly any soil imbalances, such as a deficiency of nitrogen, will be corrected by adding seaweed which will balance the soil environment so that nitrogen fixing bacteria are helped along.
  2. Seaweed stays put if you put it on the garden. It doesn’t blow away or clump together or roll away.
  3. Seaweed deters pests. Birds don’t like to get hurt with it when it’s hard and scratchy and don’t like getting tangled with it when it’s wet and slinky.
  4. Same with dogs, cats and many other critters. It’s just too darned awkward, and for some animals the smell is off-putting.

What’s the best way to use seaweed on the garden?

Firstly, there is no need to wash seaweed because the sand and salt water clinging to it contains essential elements that will benefit plants. Unless you happen to have a high sodium content in your soil, remember, there is no need to wash seaweed before using it in or on your garden.

Secondly, don’t try cutting seaweed up with a mower because there are stones, sand and shells hiding in it.

Thirdly, dry and hard seaweed is just as phenomenal for plant growth as when it’s wet and soft. The older and harder it is, obviously the longer it will take to break down and supply nutrients to the soil for feeding your plants.

Fourthly, many countries have rules about protecting their marine coastlines, which includes the harvesting of seaweed. Commercial operators you are not, so it’s unlikely you will deplete this resource by strolling along the local beach and filling up a bag with seaweed.

However check beforehand, and if you can’t find any information about your area, or there are no notices on the beach, follow these guidelines:

It is fine to pluck floating seaweed and seaweed below the high tide mark. Seaweed that has washed up above the high tide mark often makes a valuable contribution to the biodiversity of the beach and surrounds. It helps stops sand erosion and provides a habitat for local plant and insect life.

And lastly, there is concern that polluted waters pollute the sea life also. Sadly fish certainly have accumulations of medicines, farming chemicals, heavy metals such as mercury and zinc. Be aware that seaweed could be also so aim to get it from unpolluted water.

Now get your creative juices flowing… because seaweed is so versatile, here are some tried and tested ways gardeners and farmers, have used seaweed. . .

Green algae is a form of seaweed

  • Put seaweed straight on top of the garden, wet or dry. Snake it around plants and chop the bigger kelp fronds first with a sharp spade or let them dry then break them up. Great for tree roots — no need to go close to trunk, put it further out and up to the drip line.
  • Composting seaweed is popular and produces wonderful compost—put in hand-deep layers between other materials. Seaweed acts a compost activator to help speed up… well, the compost making.
  • Many growers of potatoes like to dig out a trench and put seaweed in the bottom, then a layer of good soil or compost. The seed spuds sit on this and are covered with more soil. Putting seaweed in the bottom of holes before planting other plants, such as tomatoes, asparagus and rhubarb is an excellent way to supply seaweed fertilizer to the plants’ roots. Potatoes can also just be put down on good soil/compost and a very thick seaweed mulch wrapped around them in a large protective circle. The potatoes are kept moist and protected from the sun and wind and as they grow, pile up more seaweed to protect the growing plants. You get nice clean spuds and a rich bed of soil for the next crop.
  • Let seaweed go dry and crisp then break it up and sprinkle on soil.
  • Make compost tea. Put a large handful of seaweed in a bucket of water (preferably rainwater) and leave to soak for a minimum 3 weeks and up to a year. Put a loose lid on.

    On a larger scale, use a net onion bag or a porous sack to hold seaweed and put it in, or tie it to hang in a larger barrel or rubbish bin of water. After using the compost tea, you can reuse the seaweed several times with fresh water, then put the seaweed in the compost or use on your garden.

    The above method to make seaweed tea means you will need to keep it away from the house due to the strong smell. Try and spare the neighbors too… especially on those muggy, buggy summer’s nights… phwew!

    To make a more beneficial seaweed compost tea containing active organisms, and eliminate the whiffy smells you will need an aerator to oxygenate it, or buy some Microbial Inoculant to generate microbial activity.

    Both of these can be bought at garden centres or online. For simple, cheap oxygen machines try pet shops that sell fish tank equipment. Follow the instructions to set these systems up and you will produce supreme liquid seaweed fertilizer.

    If the liquid is too dark, dilute the seaweed tea with water until it is normal tea coloured, then spray on plants. Seaweed compost tea can be applied to the roots of plants or foliar fed on leaves. It’s not likely to hurt plants if too strong; it’s more a waste of good nutrients that can’t be utilized.

    As well as supplying nutrients for leaves to absorb, seaweed tea sprayed or misted onto leaves, inhibits pests, viruses and fungal problems such as mildew and blight.

What about other no dig garden materials? Learn about leafmulch; mull over manure ideas; would wood ash work wonders? Hurry here: No Dig Materials

HOME page: No Dig Vegetable Garden


When choosing which seaweed to buy, it helps to think of seaweeds as vegetables from a different realm, the sea. Like our land vegetables, seaweeds present a wide range of sensory experiences: colours, textures & flavours will vary from one to the next. Within a group, a family even, the variation is surprising in both culinary applications and nutritional benefits.

Because Seaweeds are photosynthetic, their colour and location in the water depend upon the sun. The familiar green seaweeds, highest in chlorophyll, thrive where the water is clear or shallow. The brown seaweeds grow in the subtidal areas, about 1-2 meters below the surface where red and orange wavelengths are absorbed. Red seaweeds mostly found much deeper – 100 meters – because blue & violet wavelengths are able to penetrate greater depth. So seaweeds are classified in 3 groups according to their pigment and their chemistry and although each seaweed has its unique profile, active ingredients and benefits, seaweed from the same group share some common traits:

  1. Brown seaweeds: known for their richness in iodine, include the largest sea plants, some of them part of large under-water forests. (There are about 160 species in New Zealand). Although grouped as ‘brown’, their colour may vary between orange, gold, khaki, dark green, brown and black brown.
    In addition to their iodine content, they also contain other unique substances that are being researched for their beneficial impact on degenerative diseases: alginates (detox radiations & heavy metals), fucoidan (cancer & infections), laminarin (cardio-vascular health) & lignans (oestrogen related illnesses). Brown seaweeds naturally contain a high concentration of minerals & trace elements, especially calcium, magnesium, potassium & zinc. They also contain Omega 3 & 6 fatty acids in a favourable ratio (1/2).
    Their main health benefits are: nourish thyroid gland & brain, balance hormones, improve metabolism & facilitate weight loss, detox environmental toxins from the body, inhibit cancer cell growth and facilitate bone & joint health.
  2. Red seaweeds: the oldest & largest group of algae with over 6000 species worldwide (roughly 570 species of found along the NZ coast). Their distinctive colour pigments allow them to survive at great depths. They vary greatly in size, shape and ecosystem, and despite their name, their colour ranges from pink through to crimson, purple and orange.
    Red seaweeds are an excellent source of minerals, carbohydrates, antioxidants, enzymes and generally very high in dietary fibre. They are also rich in agar and carrageenan and have a positive potassium/sodium balance. Red seaweeds are an excellent vegetarian source of high quality, complete protein – containing all the essential amino acids the diet requires. They are also a source of heme iron (a form of iron that can actually prevent iron-deficiency anaemia). Red seaweeds are a good source of iodine, well suited to maintaining healthy thyroid function.
    They have a tonic effect on the body – strengthening the immune system and facilitating re-mineralisation, promote cardiovascular health & regulate cholesterol, nourish the nervous system – improving resistance to stress, relieve congestion in colds & flu, reduce inflammation and are naturally anti-viral, anti-bacterial, anti-parasitic, anti-biotic, anti-septic and anti-fungal, and soothe the skin and the digestive tract.
  3. Green seaweeds: grow closest to the shore and are closely related to land plants. Often noticed because of their vibrant colours – various greens & yellows, they are fewer in number than the red or brown seaweeds (about 140 species in New Zealand).
    Green seaweeds are the emeralds of the sea and they contain a wealth of minerals & trace elements, many times greater than is found in land-grown vegetables. They offer a large amount of digestible vegetable protein (up to 70%) and are an important source of calcium, iron, enzymes and anti-oxidants. They are a top source of chlorophyll, fibre and Vitamin A, B & C.
    They are said to improve digestion and reduce sugar absorption, balance blood PH, antiviral against influenza, soothe burns, cuts & sores as a compress or poultice, toning, hydrate & nourish the skin, and clear intestinal worms.

Disclaimer: This material is provided for educational purposes only and IS NOT intended as a substitute for professional medical advice, diagnosis, or treatment. This information is generic and should be verified by a qualified health practitioner for specific & individual needs & requirements.

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