Making Seaweed Fertilizer: with added Nettles, Comfrey and Borage

| Appropriate Technology, Foraging, Gardening, Permaculture | comments | Author :

Making Seaweed Fertilizer: with added Nettles, Comfrey and Borage

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...

See the comments

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One response to “Making Seaweed Fertilizer: with added Nettles, Comfrey and Borage

  1. We’ve just been given the go-ahead on a plot at the Camden Community gardens (exciting!) so we’ll be able to use some of the nettle that’s already growing in the bed and we’ll do a seaweed collection on our next beach visit. Thanks for the post, perfectly timed with our new garden project.

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