Gardening with Seaweed 101

Gardening with Seaweed 101

Here’s where to find and how to use ocean plants
gardening with seaweed


Farmers and gardeners have used seaweed as mulch and fertilizer for centuries, and Mainers are lucky to have a free, abundant supply dumped on their shores year-round. The stuff brims with micronutrients such as iron, copper, zinc, boron, and manganese, along with hormones that can stimulate plant growth. Kelp, in particular, contains some 60 trace minerals often lacking in garden soils, plus hormones that encourage plant growth. Seaweed improves soil aeration and texture, and all kinds of plants, from ornamentals to vegetables, and especially roses, fruit trees, berries, and herbs, benefit having it added directly to the soil or to compost. And that’s just the tip of the mat.

It’s free.

Seaweed can be collected from the shoreline, but laws prohibit the taking of growing plants attached to rocks or surfaces. Simply visit any public beach, stroll along the tide line with your collection, and take your choice of seaweeds that have washed up. (Collecting on private beaches is prohibited.) Seaweeds of a lighter texture — usually the ones with the flattest foliage — are preferable because they break down more rapidly than thicker varieties. Visit after a storm, and you may come across some kelp — long, flat plants that look like lasagna noodles on steroids, which some gardeners say are the most beneficial seaweeds for plants. Even freshwater grasses and weeds can be used, and they often dry to a hay-like texture. Follow your application of seaweed with a veneer of compost for an extra boost to soil health and appearance.

It’s rinse-free.

There’s no need to wash seaweed, because it doesn’t absorb salt  water and there isn’t enough salt on its surface to be a problem to most plants. That said, some wait to apply it directly to beds until right before a substantial rain — just to be on the safe side.

It’s pest-free.

Seaweed doesn’t contain weed seeds, insect pests or their eggs, or diseases found in land-based plants. Plus, studies suggest it can repel slugs and other pests and possibly prevent plant diseases like scab and mold.

It’s (almost) odor-free.

Seaweed that has already dried on the beach has virtually no odor. Fresh, green seaweed has only a faint one. Seaweed contains little cellulose, so it breaks down quickly in working (warm) compost and contains bioactivators that can trigger and enhance healthy compost activity. When applied directly to the surface of soil, the smell is mild, but might kick up on hot and sunny days, though only for a short time, and it’s still less offensive than some of the commercial granular or liquid organic fertilizers.