The gift of the sea

By Lynette L. Walther | Nov 02, 2012
Photo by: Lynette L. Walther Spread directly in ornamental or vegetable beds, the benefits of seaweed are many.

Shrouded in myth and mystery, from ghastly tales of murderous, clutching tentacles that grab sailors and shoreside bathers to that delicate Italian Burano lace that legend claims was patterned after the aquatic plant, seaweed long ago captured our attention and imagination. It has also proved to be a storehouse for a host of products, from thickening agents for a variety of food items like ice cream and instant puddings, to a component in products such as toothpaste, glass, soaps and dyes. Of importance to the gardener is the fact that seaweed also has had a long history of use in agriculture, and that we are lucky to be close to a real hotbed of the stuff.

Fall storms are notorious for churning up the sea, and ultimately depositing a mother lode of seaweed on our shores, ripe for the picking. Not only is the supply being delivered almost to our doorstep, but this is a great time of year to apply seaweed to the soil as we trim up and clean out gardens to prepare them for the winter.

“The northern New England coast is one of the more productive seaweed growing areas in the world. Its climate, over 3,000 miles of rocky habitat, abundance of nutrient-rich waters during much of the year, and large tidal flow make it an ideal habitat for seaweeds,” states a 1994 report by S. White and M. Keleshian for the University of Maine/University of New Hampshire Sea Grant Marine Advisory Program.

The report defines seaweed: Seaweeds are large algae (macroalgae) that grow in a saltwater or marine environment. Seaweeds are plants, although they lack true stems, roots, and leaves. However, they possess a blade that is leaflike, a stipe that is stemlike, and a holdfast that resembles a root. Like land plants, seaweeds contain photosynthetic pigments (similar to chlorophyll) and use sunlight to produce food and oxygen from carbon dioxide and water. (This field guide to economically important seaweeds of northern New England is your go-to spot for details on types of seaweeds found here and more —

“Seaweed provides trace elements and organic matter to the soil,” University of Maine Extension Educator Mark Hutchinson in the Knox-Lincoln counties office explains." Seaweed can be collected from the shoreline, but laws prohibit the taking of attached and growing seaweed without permits. Seaweed can be used as a mulch around plants or incorporated in the soil. It can also be added to your compost pile.”

All manner of garden plants, from ornamentals (roses love seaweed) to vegetables, especially fruit trees, berries and herbs benefit from the addition of seaweed either directly to the soil or in compost. Being low in cellulose, seaweed breaks down quickly.

The first question that arises whenever seaweed in the garden is discussed, is whether it is necessary to wash it before applying it to the garden. The answer is no. Seaweed does not absorb the salt from the sea, and there isn’t enough on the surface of the seaweed to be a problem for most plants. That said, however I usually wait to apply it directly to beds until right before a substantial rain is predicted — just to be on the safe side.

And then the next question that often surfaces when the subject is seaweed, is about the possibility of odor. Of course when seaweed is added to working compost piles there is no odor. When applied directly to the surface of soil in ornamental beds or vegetable gardens the smell is only minor, but can sometimes be detected on hot and sunny days though only for a short period. Unless you have piled up a mountain of seaweed with a fork lift or are expecting the Queen for tea, I’d say you don’t need to worry about odor. In fact it is much less offensive than some of the commercial granular fertilizers.

Now, for the good stuff. To begin with, seaweed does not contain weed seeds, nor does it contain or spread diseases as are found in land-based plants. Seaweed does not harbor insect pests or insect eggs. And get this: Seaweed can actually repel slugs and insects, possibly prevent disease too. According to one fact sheet from Maine Organic Farmers and Growers Association: “Some scientists believe that seaweed has developed antitoxins to fend off bacteria and viruses in the ocean. In the gardens, these antitoxins interrupt the reproductive cycles of some insects and appear to repel others. Seaweed also reduces fungi when applied to plants or soil. In tests at the University of Maryland, seaweed meal reduced soil nematodes in turf grass plots. Clemson University studies showed fewer aphids and flea beetles on foliar threatened plants, and other studies showed resistance to spider mites and scab. In Clemson studies, fruits and vegetables treated with seaweed didn't grow mold and thus had a longer shelf life.”

Another great reason to use seaweed in your garden is because it enriches the soil in ways no other addition can with a good measure of micro-nutrients (such as iron, copper, zinc, boron and manganese) readily absorbed by plants. Brimming with trace minerals straight from the sea, seaweed can be considered a broad spectrum fertilizer. As a bonus, scientists have found that seaweed contains hormones that can stimulate plant growth. One report stated that: Plants in seaweed-amended soil grow faster and larger than plants in soil with a comparable amount of conventional fertilizer. And when worked into the soil, seaweed can improve aeration and soil texture.

Of course everyone wants to know how to get some of this miracle stuff. Good news, it’s free for the taking. Simply visit any public beach, collection bags in hand, stroll along the tide line and take your choice. There are more than 250 species of seaweeds found in the Gulf of Maine/Bay of Fundy region, and many of them are perfect candidates for your garden improvement schemes. I prefer those seaweeds of a lighter texture, essentially the ones with the flattest foliage. They seem to break down more rapidly than the thicker ones. Visit a beach after a good storm, and if you are lucky you’ll come across those big long flat seaweeds that look like lasagna noodles on steroids — kelp. Grown in deeper water, kelp is reputed to be the most beneficial (at about 20 percent potassium chloride, sodium carbonate, boron, iodine and other trace elements), though it often takes longer to break down than other seaweeds. Follow your application of seaweed with a veneer of compost for looks and good measure and your beds and will be ready to leap into action next spring.

Even freshwater grasses and “weeds” can be used in gardens and they often dry to an almost hay-like texture. It is one way to put some of those freshwater invasive weeds to good use.

Using seaweed in the garden is a long-standing tradition that dates back to at least 1681 in this country, though much longer in European countries where farmers in England, France and Ireland’s Aran Islands created “lazy beds,” a layered concoction of sand and seaweed on top of a foundation of rocks where they grew potatoes. Often rights to seaweed beds were coveted and occasionally fought over, though free-floating seaweed was and still is available to anyone who wants to collect it. As I researched seaweed, I found that the deeper I dove into the topic of seaweed, the more fascinating it became.

Some of the earliest reports also tell of using seaweed as a fall mulch, seaweed being less attractive to mice than is hay, bringing us back to where we started. There’s no time like the present to take advantage of this gift from the sea.

Lynette L. Walther is the recipient of the Garden Writers Association’s Silver Award of Achievement for 2012. She is a member of the Garden Writers Association, and she gardens in Camden. Got questions, or comments? Visit her blog, and join in the conversation at: or “friend her” on Facebook.

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