Fertilizers Gardening Mulching

Using Seaweed as Mulch and Fertilizer

By Larry Hodgson

Question: I live in the Magdalen Islands. This year, I prepared my first vegetable garden. At the bottom of the beach near where I live, there is a lot of dried seaweed, especially eelgrass. It’s been there for a long time and I’m assuming it is therefore partially desalinated. I’d like to know the strengths and weaknesses of using it as a soil amendment. Can it also be used as a mulch? Is it going to too acidic or alkaline? 

Antonin Arseneau,
Magdalen Islands

Eelgrass is found in cooler ocean waters almost everywhere in the Northern Hemisphere. Photo: Brew, John, iNaturalist.org

Answer: Seaweed, composed of both algae and marine plants (eelgrass [Zostera marina], for example, is actually not an algae, but an aquatic flowering plant) makes an excellent mulch and an equally good soil amendment. 

Seaweed mulch
Seaweed mulch. Photo: www.bvi.gov.vg

In seaside gardens where it is readily available, it is popularly used to mulch vegetable beds where, in addition to moderating the temperature of the soil, reducing water loss due to evaporation and preventing the germination of weeds, it enriches the soil in nitrogen, potassium, magnesium, calcium and several trace elements. Seaweed mulch is also said to repel slugs, although that doesn’t always seem to work (results might depend on the types of algae and marine plants that make up the mulch). You can apply a layer of 3 to 4 inches (7.5 to 10 cm) of seaweed as mulch over the soil throughout the vegetable garden. And you can also use it as a mulch in a flower border, at the base of trees and, essentially, anywhere you would use an organic mulch.

As to using seaweed as a soil amendment, you can mix in 1 inch or even 2 inches (2 to 5 cm) of seaweed into garden soil like just as you would compost or manure. Seaweed will help to enrich the soil in minerals. After all, generations of gardeners have been purchasing concentrated seaweed fertilizer! It also helps lighten heavy soils while allowing thin soils to better retain to water and minerals.

Also, when you’re able to harvest enough seaweed locally, it makes a great addition to your compost pile.

Using Seaweed in the Garden

Dried seaweed
Dried seaweed. Photo: Titus Tscharntke, pixnio.com

As you suggest, seaweed that has been exposed on land for a while will have been rinsed enough of salt to be readily useable in the garden, but you could still use fairly fresh seaweed too by letting it undergo a few sessions of rain or by rinsing it with water from the garden hose before using it. Unless the local soil is already saline, any residual salts shouldn’t cause any problem: they’ll be diluted and leached away as you water your garden.

As for acidity and alkalinity, usually seaweed is neutral or slightly alkaline (a pH of 7 or slightly above). Indeed, in areas where the soil is highly acid, seaweed is sometimes incorporated in large quantities to reduce their alkalinity. Still, the alkalinizing effect of seaweed is very weak. There is no risk of substantially altering the pH of typical garden soil (usually in the 6 to 7 pH range) by incorporating seaweed at normal rates, unless, of course, the soil is already too alkaline (a pH of well over 7).

It goes without saying that you should only use seaweed carried on shore by waves and certainly not harvest it from the sea so as not to upset the often fragile ecological balance of the seabed.

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, laidbackgardener.blog will continue its mission of demystifying gardening and making it more accessible to all.

4 comments on “Using Seaweed as Mulch and Fertilizer

  1. Pingback: Survival Gardening Pro Tips: Unleashing Seaweed for Soil Enrichment - Survival Seeds for an Urban Area

  2. I would think about how much seaweed I was taking off the beach, as it is cover for important invertebrates that birds and other shore creatures feed upon. Don’t take all of it. Some places regulate this, so check first to make sure what you are doing is legal.

  3. Seaweed is heavy while fresh, then becomes lightweight as it dries.
    We used to be able to purchase it near here, but I am not certain if it is still obtainable. For a while, I used aquatic vegetation that needed to be removed from an irrigation pond, just because I needed to get rid of it anyway. I never determined if it was comparable to seaweed.

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