How to use seaweed to put your garden to bed for the winter

It’s free, it’s abundant on Maine shorelines, it has many uses around a homestead, it grows year round and it washes right ashore.
It’s rockweed, perhaps the most common type of seaweed used as fertilizer, animal feed and even a human food supplement.
Rockweed harvesting is a contentious subject in Maine, perhaps partly because its many unique benefits are so desirable. If you can get it responsibly, the material has numerous benefits for your soil and livestock.
The commercial harvest of the kelp species has caused disputes for years over land access and sustainability. At the same time, commercial harvests have roughly tripled since 2001, totalling nearly 20 million pounds in 2023.
As it stands today, you can harvest 50 pounds a day without a license if you’re going to use it at home, or collect dead rockweed that washes ashore. Don’t eat the dead stuff, but it’ll still work in your garden.
It adds nitrogen, sulfur, phosphorus and potassium, along with magnesium and more trace minerals, that may not be provided by other common fertilizers. Iodine in particular is rarely found in soil, which is why people eat iodized salt or other sources to get enough of it.
Seaweed also contains compounds called biostimulants, which studies have found help many crops develop better roots, flower early, weather environmental stress and ultimately produce more.
Winter harvests can be spread on garden beds to decompose over the cold months, enriching the soil as it decomposes. Compost or powdered fertilizers might wash away, but the seaweed is more stable and can be tilled under in the spring. A thick layer will help moderate the soil temperature and prevent moisture from evaporating away.
It can also be put down as mulch in spring, where its rougher structure is reportedly a good barrier to snails and slugs.
You can till it into garden beds, mix it with less dense materials like straw or leaves for composting, steep it in water for a few weeks to make a fertilizer “tea” or a spray to apply directly on plant leaves. Traditionally, it was even burned into an ash of concentrated minerals.
Seaweeds also contain iodine and can provide protein for livestock. A study made headlines several years ago for finding it also reduces the amount of methane ruminant animals produce. The seaweed may collect heavy metals, though, so you might not want to feed it in large concentrations or collect it from areas with contamination problems.
To collect it on private land, you need permission from the landowner even in the intertidal zone. A lawsuit that could overturn that 2019 legal decision is being argued in Maine’s Supreme Judicial Court.
Some conserved land might have other restrictions too, so check before you go.
If harvesting living seaweed, it needs to be cut at least 16 inches above its hold-fast, or connection to the surface it’s growing on. Environmentalists recommend collecting only the dead rockweed that’s washed ashore.