Seaweed is something we often treat as a bit of a nuisance, stepping carefully around it on the beach on our summer holidays.
Yet this plant material can provide valuable nutrients and minerals to our plants, as a mulch or liquid to boost root health.
But how does it work and could you forage it from the beach if you don’t want to fork out on expensive extracts from the garden centre?
What’s in seaweed?
Seaweed comes in a category of biostimulants, which is different from regular fertilisers which comprise NPK (nitrogen, phosphorus and potassium) and other nutrients. It does contain a small amount of NPK but not enough to replace regular fertiliser, explains plant biologist David Gale, gardening advisor at Maxicrop.
It’s the trace elements which make it so valuable.
“So, in the same way we have vitamins in our diet, plants have trace elements in tiny quantities, like iron, molybdenum and manganese, whereas often fertilisers don’t have trace elements in them.”
Seaweeds contain alginates which are thought to improve soil structure and the defence characteristics of the plant, helping to ward off diseases and pests, says Gale, director of specialist consultancy Garden Ethos.
It contains plant growth hormones and amino acids which are thought to alleviate plant stress relief to help withstand heat, cold and drought, he adds.
What can it do?
“At its simplest level, treating plants with seaweed stimulates the plants to produce a much better root system,” says Gale. “In the ground or in a pot they grow better and are able to withstand the sorts of things that we as gardeners do, such as forgetting to water things, or planting in poor soil.”
Does seaweed replace fertiliser?
No. You still need to use fertiliser on your plants with the exception of large established shrubs and trees, says Gale.
“Seaweeds and other biostimulants are like the food supplements that we have as humans. It’s about having a better root system. “
How do you apply it?
You can buy it in dried or liquid forms from garden centres and only apply it in very small quantities. Some seaweed products have added fertiliser.
Most people add a little liquid seaweed extract to their watering can, dilute it accordingly and water into the soil.
Can you harvest it from the beach?
“Absolutely. You could collect bladderwrack (edible brown seaweed) from the shores of the UK, boil it up, macerate it and then put it on the soil.
“Crofters in Scotland still collect seaweed in the winter and lay it on the soil where it breaks down. Worms drag it underground and it’s direct.
“It wouldn’t be as pure (as a product you buy) because you’ll be collecting a whole mixture of seaweed and there might be bits of grit in it.”
However, the RHS warns that there is no public right to collect seaweed from the beach unless you own the beach and it is not designated a site of special scientific interest or a special area of conservation.
If you plan to gather seaweed, check with the land owner and the local council first, the charity advises.
How would you prepare it?
Put it through a blender and then boil it up until it breaks down, Gale suggests.
If you don’t want to boil it up, you could use seaweed as a mulch over beds and borders.
When should you apply it?
From the end of March, when the plants are starting to grow and during the growing season, when it will stimulate the roots to do well, says Gale.
“Plants only grow related to the soil temperature, so do it when the soil starts to warm up.”
Apply very diluted solutions every one to two weeks, depending on the plant.
Bedding may benefit from it once a week, while mature shrubs might only require it two to three times a year, and veg every two weeks, he suggests.
Stop applying seaweed extract from mid-September, when the plants are starting to slow down, with the exception of houseplants.
Will the results be visible?
“In my experience you get slightly bigger plants but also much healthier-looking plants which stay looking healthy and performing, whatever the weather and the environment throw at them,” says Gale.
“You also get better flavour. I extensively use seaweed extract on my tomato plants.”
Which type of seaweed extract should you choose?
Dried seaweed products include seaweed meal (crushed and dried fresh seaweed), powdered seaweed extract and liquid extract.
“Look for the dilution rates because something that seems a better price might not be once you work out how much needs to be diluted. The concentration will tell you what the value of the product is. Also look for its organic status.”
Can you use it on houseplants?
“Yes. I have loads of houseplants and particularly like succulents. When I liquid feed and water, I will mix in a bit of diluted seaweed extract at the same time.
“I find that seaweed extract is really beneficial to anything grown in pots and the smaller the pot, the more beneficial it is.”
You can also foliar feed (apply to leaves) with seaweed extract to get trace elements into your plant, Gale adds.
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