6 Tips for Using Seaweed in the Garden

Heading to the beach? While you’re there, collect some seaweed to use in your garden. For generations farmers in coastal communities like the Channel Islands have been amending their gardens and fields with it. Rich in nutrients and trace elements like potassium, magnesium, nitrogen, and phosphorous, seaweed can help amend garden soil, and the benefits don’t stop there.

When fresh seaweed is applied to garden beds as mulch, the application can help mitigate weeds, and won’t introduce new weeds or pests the way some bark mulch can. As the seaweed breaks down, it contributes to a lightweight loamy soil. Seaweed can even be brewed into a nutrient-rich tea. Here are six tips for collecting and using seaweed:

Photograph by H Matthew Howarth.

How to Use Seaweed in your Garden:

1. Collect seaweed mid-beach: Says EarthEasy’s Greg Seaman, collecting seaweed from the middle of the beach is your best bet. Seaweed that’s mid-beach is far enough from the water to have had an opportunity to dry out somewhat, but it’s not so dry that land-lubbing bugs have had the chance to get to it.

2. Don’t over-harvest: Seaweed has a crucial role in the ecosystem of beaches, so it’s important not to strip beaches of it entirely. Greg suggests picking no more than a third of the seaweed from any one patch.

3. Use fresh seaweed: According to the Royal Horticulture Society, incorporating fresh seaweed into the garden can be a good substitute for farmyard manure. There’s no need to allow seaweed to dry before adding it directly to garden beds.

4. Layer thickly: Seaweed should be added to gardens in relatively substantial quantities. Greg Seaman recommends two applications of seaweed, each about 4 to 6 inches deep. The Royal Horticulture Society recommends a barrow load per square foot of garden. If you aren’t able to find seaweed in these large amounts, even a small application mixed with compost or other amendments will be beneficial.

5. Add it to your compost pile: According to the Rodale Book of Composting, digging fresh seaweed into your existing compost pile can speed up composting. Existing compost bacteria will feast on the alginic acid in seaweed leaves, kickstarting the process.

6. Make a tea: Fresh seaweed can also be used to brew a nutritive tea. Fill a large bucket with rinsed seaweed and fill with fresh rain or hose water. Allow the “tea” to brew for several weeks, stirring occassionally. Strain the nutrient-rich liquid into a spray bottle and use as an organic (and free) plant food! More details on The Hedge Combers.

Above: A 2011 study at the University of Rhode Island tested the effects of green seaweed (ulva spp) on sweet corn, proving its potential as an affordable resource in coastal agriculture. Photograph by Kqedquest.

Above: Gather seaweed into large trash barrels or net bags to transport to the garden. Photograph courtesy of Farm for Life Project.

If you’re not near a beach, consider Neptune’s Harvest Fish and Seaweed Fertilizer, recommended by Brooklyn gardener Marie Viljoen in 10 Secrets for Growing an Urban Balcony Garden.

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Dawn Richardson
Dawn Richardsonabout a year ago

You don't have to make this that complicated. Grab a couple plastic supermarket bags every time you head to the beach. Fill them with seaweed as you are leaving for the day. Dump around tomatoes or in compost pile. The end.

Lynn C.
Lynn C.1 years ago

Thanks! This article is a keeper.

Colin Hope
Colin Hope1 years ago


Andrew Pawley
Past Member 1 years ago

Real problems have been caused by over harvesting. Seaweed is great stuff but I wonder whether we may not be taking too much in many places.

Anne F.
Anne F.1 years ago

Appreciate the suggestions for care while harvesting.

Nimue P.
Nimue Pendragon2 years ago


John S.
Past Member 2 years ago

What happens if everyone harvest 1/3 of the seaweed?

Dmitry Nikiforov
Dmitry Nikiforov2 years ago

Thanks for sharing.

Caili W.
Caili W.2 years ago

Neat ideas. And, I don't live far from the beach.

Edward Wilkes
Past Member 2 years ago

Good to know. Thanks for sharing.