Applying seaweed in the garden. Credit: Courtesy of Noah David Smith

Keeping your garden healthy throughout the winter months is a challenge for gardeners in Maine. But there might be a solution waiting along the coastline. Seaweed, experts say, can add organic matter and some nutrients to your soil and protect it over the winter.

“It’s a great source of organic matter for your garden and compost pile, and an excuse to visit a beach in the fall,” said Liz Stanley, horticulture community education assistant at the University of Maine Cooperative Extension.

As it breaks down, seaweed adds minerals in the soil, which is especially valuable in the relatively rocky soils of Maine.

“We have enough rainfall to grow trees, but that also means we have enough rainfall to leach minerals from the soil,” said Larch Hanson, co-owner of Maine Seaweed, LLC, in Steuben. “Using seaweed, we can replace those minerals.”

Autumn is the best time of year to gather seaweed on the beach if you want to use it in your garden.

“This time of year, migrating shorebirds eating arthropods from the seaweed have flown off to their wintering grounds, so from a sharing-the-habitat perspective this is a good time [to collect seaweed for the garden],” said Severine von Tscharner Fleming, an organic farmer, activist with The Greenhorns and owner of Smithereen Farm in Pembroke. “Putting it into the garden now, it can sit and freeze all winter and break down gently.”

How to use seaweed in the garden

One of the best ways to take advantage of the mineral properties of seaweed in the garden is to apply it on top of raised garden beds after they are cleared and prepared for the winter.

“Basically, it decomposes over the winter, and you till it in the spring,” said Ryan Smith, a gardener in Rockland who said he has been gardening with seaweed for 20 years. “Seaweed has 16 or 17 different essential plant macro and micronutrients, [including] selenium manganese and iodine [which are] depleted in our soil and can get replenished through the seaweed. It’s offering a little bit of nitrogen [which will] increase your soil’s water holding capacity.”

Smith said that allowing seaweed to degrade over the winter is more effective than using as mulch during the growing season.

“When it’s used fresh as a mulch, it can be too high in sodium,” Smith said. “The residue from the sea salt and the concentrated salt can burn the plant if used as a fresh mulch.”

Still, when used in moderation, seaweed can be an effective mulch for certain plants. It can be used in the fall in perennial beds or around fruit trees, though it can be messy to handle come spring. For overwintering crops like garlic, seaweed can be used to cover bare soil before winter.

“I mulch garlic with [seaweed] because it is a better insulator,” Hanson said. “Seaweed doesn’t have air spaces in it as much as straw.”

Seaweed can be saved for the growing season, too. Hanson recommended placing it in the furrow when seeding, or underneath transplants. Leafy greens especially like seaweed close to their roots, in his experience.

“It makes your plants glossy and happy,” Fleming said. “It’s helpful for transplant shock.”

Be aware that it’s crispy when dry, and dangerously slippery when re-hydrated, so Stanley advised keeping it clear of garden paths. You may also want to be cognizant of the smell, too, if you are saving it for the growing season.

“If spread soon after collected, it has the pleasant odor of the ocean,” Stanley said. “If left in a pile for a long time, it will decompose anaerobically and smell pretty awful.”

After the winter thaw, the seaweed on raised garden beds can be tilled or added to an active compost pile. Layered with carbon sources like leaves, straw and wood shavings — aeration is important for rubbery, thick seaweed — it can create a well-balanced carbon to nitrogen ratio to feed microbes and soil organisms.

“Seaweed has benefits for a compost pile, being extremely bioactive, that is, it starts decomposing immediately, and it really heats up the pile,” Hanson said.

How to gather seaweed for gardening

Harvesting seaweed to use for gardening. Credit: Courtesy of Rachel Darke

In Maine, most of the seaweed that washes up on the shore is rockweed with a few strips of kelp now and then, though the mix ultimately depends on ocean depths, currents and storms.

When gathering seaweed from the shoreline, avoid industrialized coastal areas, and make sure you have permission to access the shore.

“Just like ‘shopping local’, look for some if it’s nearby and convenient to do so. Start with a map that shows public access and ask fellow gardeners,” Stanley said. “Always get permission to gather seaweed on private property. Some town and city parks require permission, too.”

Timing also matters. Fleming said that the trick is to figure out which way the wind is blowing and the tide is right to drop a nice fat, slimy line of seaweed on the shore. Smith also advised checking out south-facing coves for rockweed.

“Usually what happens in the fall is you get these hurricanes, big southerly blows, a lot of the rockweed breaks off of its holdfast or stem and then what happens? It gets pushed in all these south facing coves,” Smith said. “You go to these south facing coves after an astronomical tide or after a hurricane comes through.

Fleming said that seaweed is easier to move when it’s still wet because dry seaweed is “awkward and pokey,” though some seaweed foragers prefer to move dry seaweed to avoid the excess water weight.

Regardless, only take seaweed that has washed up around the high tide line. Never take plants that are attached and still growing on rocks or the ocean floor.

Harvesting seaweed to use for gardening. Credit: Courtesy of Rachel Darke

“That’s one of the considerations is how much removal of seaweed is acceptable from an ecological standpoint,” Fleming said. “It’s a critical understory of the marine ecosystem.”

Bring a manure fork, as well as buckets, garbage bags or tarps for easy hauling. An open pick-up truck is the ideal vehicle for hauling seaweed, but if you do not have one available, be ready for sand — and sand fleas — in your vehicle.

“I simply line my trunk with the largest tarp, and use a smaller one for gathering, using the ‘body bag and drag’ method,” Stanley said.

Concerns about gardening and seaweed

For the most part, seaweed is a sustainable way to feed your garden. However, farmers and gardeners that live along the coast have long been concerned about the overharvesting of seaweed, mostly aimed at corporations collecting seaweed, like Acadian Seaplants.

In 2019, the Maine Supreme Court ruled that the intertidal zone is private property belonging to the upland owner. Still, that doesn’t alone protect the shoreline, so anyone harvesting needs to be cognizant of best practices for sustainable harvesting. Make sure you are following good foraging practices if you plan to use seaweed in your garden.

“It also has its ecological purpose on the beach, so always leave a lot more than you take away,” Stanley said.

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