Everything You Ever Wanted to Know About Seaweed Fertiliser

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So I did the research, and it turns out you don’t really need to know much abut seaweed fertiliser.

Now seaweed fertiliser is VERY weak so there are a lot of people out saying not to use it. Whilst I get where they’re coming from, I used exclusively seaweed fertiliser for the first two years of my houseplant…career (?) and my plants grew really well.

So whilst it’s not the greatest option going, it does work.

What’s the NPK of seaweed fertiliser?

This varies a lot depending on the seaweed used (it’s usually kelp) and how it’s prepared, but I don’t think I’ve ever seen one that had an NPK that was higher than 1. And I’ve never seen one that didn’t have at least element be a 0. So 0-0-1 is quite common. I also saw 0.9-0.6-0.

To be honest, most of them don’t list NPK, which…probably tells us what we need to know.

Can you make your own seaweed fertiliser?

Yes, but I wouldn’t bother unless you want your home to smell like, in the immortal words of Kill This Plant, bootleg Seaworld.

You could make a tea by steeping the seaweed in water and leaving it to steep for a few weeks and then straining it out. Don’t though! It’ll smell like death! The stuff that’s been processed to get rid of the smell smells bad enough.

Which plants like seaweed fertiliser?

Ok, I’m just going to advise that you don’t use seaweed fertiliser as a fertiliser – especially if you have plants in water and/or semi-hydro. It’s so weak that it’s basically not worth the money.

I use General Hydroponics Flora Series, which is a three-liquid system that you mix together in water. The bottles are 0-5-4, 5-0-1, 3-1-7.

It’s more expensive then seaweed – actually the seaweed is £11.99, each GH bottles is £10 BUT you’ll go through GH quicker.

Benefits of using seaweed fertiliser

  • It’s natural and can be organic (if you choose an organic one)
  • It’s very gentle and won’t damage your plants
  • It’s AWESOME for helping certain plants root, so can be great to add to propagation water, 2ml per litre of water. Apparently good for Monstera, if you’re struggling.
  • It can improve the health of the plant – apparently seaweed contains growth stimulants
  • It can help increase microorganisms in your soil and improve its health
  • It can ward off pests, such as slugs because they don’t like the saltiness of it. Apparently said salt isn’t going to leach into your plants and kill them
  • It’s rich in trace elements

Problems with using seaweed fertiliser

  • It smells. Not horrific, like fish emulsion, but…bad
  • It doesn’t usually contain all the essential nutrients. This is fairly common in cheap houseplant fertilisers, which is why I use hydroponic ones
  • Unless you pay to have it tested, you won’t know which trace elements it’s rich in (except iodine, which is pretty much a given), so you won’t know what you’re lacking

Final thoughts

As I said at the beginning, I don’t have a problem with people using seaweed fertiliser – in fact, I recommend it for beginners because aside from the smell it’s fine. It’s easy to get hold of, there’s not much difference between the brands and it lets people experiment with finding a fertilising schedule that works for them.

It’s not the greatest, but unless you’re growing your plants in something super low nutrient, it won’t really matter. A lot of beginners use store-bought houseplant potting mix which usually contains plenty of nutrients to supplement the seaweed.

Caroline Cocker

Caroline is the founder and writer (and plant keeper) of Planet Houseplant

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