Thoughts often turn to gardens as February and March roll out their lengthening days and seed catalogs arrive with their visions of vegetable bounty and flowery beauty. By March and April, shelves and window ledges are home to tender seedlings that, with nurture and a bit of luck, grow into sturdy little starter plants. In May and June, the soil is tilled, seeds are sown, and indoor starts are transplanted to their new home under sun and open sky. As summer unspools, we faithfully water, pull weeds, battle bugs, and curse either the too-drenching rains or the dusty drought. When all goes well, we’re finally rewarded with lush green foliage, fresh vegetables, and butterflies fluttering among flowers. As summer eventually gives way to fall, the last of the crops are gathered and garden beds laid to rest, maybe with a portion set aside for garlic and other overwintered crops.
Through this all, seaweed in the garden can be friend and helper. The same attributes making sea vegetables such nutritious food for humans – an abundance of minerals, bioactive compounds, and other properties – makes them also nutritious for soil and plants. Seaweed in the garden is easy to use as compost and as a mulch, fertilizer, growth stimulant, soil amendment, nourishing tea, and foliar spray. If you’re lucky enough to live near the coast it can also be free and abundant, but even gardeners living miles from any seacoast can use seaweed.
Seaweed - What’s in a Name?
First, let’s dispense with the matter of macroalgae vs seaweed vs kelp vs sea vegetables. Many people tend to use the common term seaweed to refer to every type of macroalgae species. Biologically speaking, though, macroalgae are very diverse and are divided into three broad groups based on color: brown, red, and green. Although macroalgae are often referred to as plants, they actually are not. Red and green macroalgae are only distantly related to plants and brown macroalgae aren’t related to plants at all. Kelp is also a broadly used term, usually for brown seaweed, but some people use it even more broadly to refer to all seaweed. Sea vegetables is the preferred name for edible seaweeds. Almost every seaweed species is edible, but a few are so tasty and widely consumed that they merit the name sea vegetables.
In this article we use the familiar term seaweed to refer to all macroalgae, even sea vegetables. All seaweed species are good for the garden, but in North America and Europe, the most commonly used garden kelp is rockweed Ascophyllum nodosum.
Rockweed is abundant on the Maine coast and harvested for agriculture
Horticultural Seaweed Properties
Seaweed has multiple properties that make it ideal for horticulture. The first of these is mineral content. When seaweed is dried and then burned, the remaining ash can be 40% or more of the original dry mass. Seaweed ash contains a plethora of minerals needed by both plants and humans, including calcium, phosphorous, potassium, magnesium, manganese, iron, zinc, selenium, sulfur, molybdenum, vanadium, and iodine.
Soils can be deficient in one or more of these minerals and many of them aren’t included in regular fertilizers. Some are essential for plants, and all can potentially be taken up by plants to produce more nutritious crops. In that regard, iodine may be of special interest to inland growers. Inland soils are often iodine deficient, meaning crops grown on those soils also are. This is one of the reasons why we need to get iodine from sources such as iodized salt. Seaweed in the garden adds iodine to the food we grow, as well as many other trace ocean minerals that might otherwise be lacking.
Seaweed also contains small amounts of the fertilizer elements nitrogen, phosphorous, and potassium (N-P-K). Depending on species, dried seaweed is about 1-2% nitrogen, 0.2 to 1.5% phosphorous, and 2.8 to 10% potassium. This means seaweed can be used as a low-strength, slow-release fertilizer, or to add nitrogen to compost. It also makes an interesting complement to bone meal, which has a typical N-P-K rating of 3-15-0. Where seaweed is low in phosphorus, bone meal is relatively phosphorous rich, while seaweed helps make up for the lack of potassium in bone meal. Both contain small amounts of nitrogen, and added together they form more than the sum of their parts.
The bioactive compounds found in seaweed, and which are in some cases only found in seaweed, have especially intriguing properties. They include phytohormones such as auxins, cytokines, and gibberellins; growth regulators such as abscisic acid, salicylic acid, polyamines, and ethylene; and antioxidants including flavonoids, phenolics, tocopherols, ascorbic acid, terpenoids, and carotenoids. These bioactive compounds are known as biostimulants because they stimulate natural plant processes to enhance or benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stresses, and crop quality.
Seaweed plant biostimulants have been shown in hundreds of studies to improve root development, synchronize fruit sets, accelerate flowering, increase yields, and improve drought and stress tolerance in plants or crops such as marigolds, soybeans, broccoli, chili peppers, watermelons, shallots, corn, tomatoes, and carrots. In Maine, commercially produced seaweed biostimulants increased potato yields by as much as 3,000 extra potatoes per acre with no additional fertilizer or pesticides.
Seaweed biostimulants are often extracted from rockweed, which has a very long history of being used for agriculture. North American Kelp and Ocean Organics, two Maine companies founded in the 1970’s, make and sell rockweed biostimulants and extracts for golf courses, agriculture, greenhouses, home gardens, and even for animal health. Another Maine company, Source micronutrients, specializes in rockweed-based animal nutritional supplements. If you till your garden the old-fashioned way with horses or oxen, you should consider adding some to their feed!
How to Use Seaweed in the Garden
It’s easy to use seaweed in the garden because it’s so versatile, and it’s good for houseplants too. It can be used sparingly or in large amounts, on individual plants or on acres of plants. The only real limitations are availability and cost.
- Seaweed compost. Seaweed breaks down rapidly and is an ideal composting material due to its nitrogen, mineral, and carbohydrate (mostly carbon) content, and also because it has no weed seeds. Seaweed is best mixed with “fluffy” materials like straw, leaves, or sawdust because it tends to get soggy and dense when wet. A fall formula is to mix three parts shredded leaves with one part of shredded seaweed. Dried seaweed can be shredded like any other dry material but it’s probably not advisable to try shredding wet seaweed.
- Seaweed sheet compost. Sheet composting is where a layer of organic matter is placed over the soil to naturally decompose. Seaweed is ideal for this approach because it degrades so easily. Sheet composting can be combined with green manure, which is when a fast-growing cover crop like vetch, oats, clover, buckwheat, millet, or rye is tilled while still young and green into the soil. Instead of tilling the cover crop, smother it with a thick blanket of seaweed. After 3-4 weeks (or over winter), the remaining organic matter should be easy to work into the soil.
- Seaweed mulch. Seaweed makes an ideal decomposable mulch. It’s easier to apply when wet but it can also be applied dry. Dried seaweed should be spread as a thick 2-4” mulch because it breaks down within a few weeks, but be sure to leave open soil at the base of plant stems to prevent rot. An added advantage of seaweed mulch is that it can become ‘spiky’ as it dries. This, in combination with natural seaweed salts, acts as a slug deterrent. Seaweed is not good for mulching paths and walkways, however, because it’s slippery when wet.
- Seaweed soil amendment. A tried-and-true technique, and one that’s been used for hundreds of years, is to till seaweed directly into the soil in the fall or before fallowing. This improves clay or hard pack soils by loosening the soil and increasing aeration, and it improves sandy soils by adding organic matter and increasing water retention. Seaweed adds nitrogen, carbon, and minerals to all soils, and seaweed carbohydrates are food for beneficial soil bacteria.
- Seaweed Plant Tea. Seaweed tea is good for houseplants or for prized individual garden plants or perennials. To make it, fill a container 1/3 to 2/3 with seaweed, add water until full, and let it steep outdoors for 2-4 weeks or longer (no need to boil). Caution – seaweed tea can be odoriferous! The resulting tea is decanted and used diluted or straight as liquid fertilizer or diluted as foliar spray. Diluted seaweed tea is said to be great for seedling root growth due its cytokinin content. A typical dilution ratio is about 1 part tea to 10 parts or less water until it is a light brown color. You can make your own seaweed tea even if you live miles from the coast because it only takes a pound or two of dried seaweed, which of course can be ordered from Maine Coast Sea Vegetables!
- Seaweed fertilizer. Dried seaweed is a low nitrogen – low potassium – moderate phosphorous natural fertilizer. Many garden stores and the internet sell pre-made seaweed fertilizer if you can’t access free seaweed from the beach and don’t want to order raw dried seaweed to make your own. The chances are good that the seaweed used to make these products is rockweed sourced from Maine. Coast of Maine fertilizer is a trusted brand that can be ordered on-line, and Gulf of Maine fertilizer is another reputable brand. Seaweed is a slow-release fertilizer because its nutrients are released over time as it breaks down. Applying it along with bone meal, as mentioned earlier, could be especially beneficial.
- Seaweed foliar spray. Liquid seaweed extracts for fertilizer or foliar spray are available under such brand names as Neptune’s Harvest, Bloom City CleanKelp, Eco Gardener Seaweed Extract, and Kelp-It™ Concentrated Liquid Seaweed Extract. Powdered seaweed extracts are also available. Seaweed extracts are made by fermentation, water-based extraction, or hydrolysis (acid or alkaline). Seaweed tea also contains extracts, though steeping in water isn’t as efficient as commercial processes at extracting, concentrating, and preserving nutrients and plant biostimulants. Whether liquid or powder, be sure to follow manufacturer instructions when preparing a foliar spray from extracts. It’s best to apply foliar spray in the morning or late afternoon when the sun isn’t strong, and it doesn’t have to be very often: once every 2 to 4 weeks is good. Leaves should be thoroughly drenched, top and undersides.
Where to get Seaweed for the Garden
It’s well worth driving 1-2 hours to collect beach-cast seaweed. However, heavily populated areas, anywhere close to industrial or municipal outfalls, or the mouths of large or potentially polluted rivers should be avoided because seaweed absorbs heavy metals and other toxins from seawater. It’s best to get landowner permission before traipsing across private property to gather seaweed, and in some cases in Maine, the intertidal zone is also considered private property (although citizens for justice are working to overturn this anachronism!). Keep in mind that it’s illegal to collect seaweed (or anything else) from National Parks, whereas policies for state and municipal parks or nature preserves may vary.
Large storms often break kelp beds apart and wash them ashore, but beach cast seaweed can usually be found no matter the weather. Beach cast seaweed tends to get deposited along the high tide “wrack line,” where other organic matter and debris is also found. The wrack line serves ecologically important functions and some people consider it the most interesting part of the beach. Don’t be greedy when collecting seaweed from the wrack line; leave some behind as nutrients and shelter for coastal critters.
Live seaweed can be harvested for personal use without a license in most states. Live seaweed is free of sand and other debris and it may contain more vital nutrients than partially dried beach cast seaweed. However, it’s also heavier, being almost 90% water. Many websites urge people not to harvest live seaweed, but if done with care it can be sustainable. Live seaweed beds should be harvested in a sparing, patchwork fashion to avoid denuding areas. The seaweed should be cut well above the holdfast (rootlike structure at base of plant) so that it can regenerate. Never pull or yank live seaweed from where it’s attached.
Rockweed hand harvesters
If you can’t get to the beach, then dried seaweed can be purchased in bulk for delivery, including rockweed from Maine Coast Sea Vegetables. Admittedly, this is not the most economical approach because our sea vegetables are food grade and Certified Organic, but on the other hand, they are also top quality and tested for impurities. A little bit goes a long way for making seaweed tea, and if you do go this route you can experiment with various applications and proportions. However, it’s probably more cost effective to purchase pre-made seaweed fertilizer or extract. Maine Coast Sea Vegetables does have some “compost grade” dried seaweed for employees and friends. Just another perk of working for this great company!
Five pounds of dried rockweed
Is Seaweed in the Garden Safe?
Gardeners sometimes express concern about the potential for seaweed to contain harmful levels of heavy metals or other contaminants. Almost every seaweed species absorbs mineral elements such as arsenic and cadmium from seawater. These heavy metals are present in seawater from both natural geological processes and human activity. However, seaweed collected from clean waters poses no health risk to humans, animals, plants, or soil, as evidenced by hundreds of years of safe use around the world. As noted earlier, though, certain areas where seaweed grows may have elevated levels of harmful elements and chemicals, and should therefore be avoided when collecting it for garden use. See our Product Testing Page and our FAQs for more on this topic.
Salt content is another concern. Anyone who has enjoyed sea vegetables knows they have a rather salty flavor. If you haven’t eaten them before, now might be a good time because they can help restore electrolytes lost after a day of hot summer toil in the garden! Sugar kelp, Alaria, and dulse are all customer favorites. The salty flavor of sea vegetables is somewhat misleading, though, because it’s not entirely sodium. Some of it comes from potassium salts, which are good for plants and animals alike. The renowned umami flavor of sea vegetables, which comes mostly from the amino acid glutamate, can also add to the perception of saltiness. Dried seaweed does contain about 10-40mg of sodium per gram. At the higher range this works out to be about 1TBS of salt per pound of dry seaweed (less when wet because of water weight).
Assortment of tasty sea vegetables
In rainy climates there is no danger of soils accumulating salt from seaweed use. In Maine, for example, people have used seaweed in the garden for decades without seeing signs of salt accumulation. Signs of salty soil include yellowing leaves and stunted growth. For those wishing to exercise an abundance of caution, most of the seaweed salt can be rinsed off with a garden hose. Spread the seaweed on a hard surface with drainage, thoroughly soak, and use it wet or after drying under the sun for a while.
Sulfur is a seaweed element that doesn’t always get the attention it deserves. Sulfur is the 3rd most abundant mineral in the human body, and it’s needed for healthy skin, hair, and nails. It’s found in life-essential organic molecules such as the amino acids methionine and cysteine, and a variety of sulfur compounds are being studied for health benefits. Seaweed is often rich in sulfur, and alliaceous crops (alliums such as onions, garlic, and leeks) and cruciferous vegetables (such as broccoli, cabbage, turnips, and collard greens) also contain sulfur compounds.
When you use seaweed in the garden you help ensure your soil and plants have enough sulfur. Soils high in clay or organic matter are said to benefit the most from sulfur addition, but sandy soils can be deficient as well. Sulfur is often used to make alkaline soils more acidic because it gets converted into sulfuric acid by soil bacteria. Nutrients become more available to plant roots when soil is neutral to slightly acidic, with a pH of 6.5 to 7.0. It’s unlikely that soils can be made too acidic from seaweed, but when in doubt it’s always good to get a soil test.
Which plants benefit the most from seaweed in the garden?
Most plants benefit from seaweed, whether indoors or out. However, there are some situations where it can be especially helpful. Here are just a few examples.
- Young seedlings. Seaweed tea helps promote seedling root growth. Apply once per week diluted in water.
- Garlic. Seaweed is a good mulch once garlic has sprouted. Remove any straw mulch used to overwinter fall-planted garlic and replace with a thick layer of seaweed to suppress weeds and enhance flavor. Garlic is an allium that benefits from the sulfur compounds found in seaweed.
- Stressed plants. Seaweed foliar spray or liquid fertilizer can help recover plants stressed from transplant or drought.
- Broccoli and other brassicas. These crops take up sulfur, which they convert into health-giving compounds. Seaweed can be used in all its forms with these crops to add sulfur.
- Potatoes. Seaweed can be used to mulch potato plants and bury their stems to produce more tubers. This can permit closer spacing because less mounded soil is needed, and the seaweed adds trace ocean minerals and iodine for healthier spuds. Check out this YouTube video for one way to do it.
- Roses. The American Rose Society recommends foliar application once every three weeks or so for more vigorous and disease resistant roses. They do warn about leaf burn, or phototoxicity, which can be avoided by applying the spray early in the day and not when the temperature exceeds 75 degrees.
- Tomatoes. Seaweed extracts have been shown in multiple studies to improve tomato yields, leaf production and photosynthesis, ripening, and stress resistance. Seaweed can be applied as a foliar spray or in dried meal form as a slow-release fertilizer. However, seaweed should not be used as a sole fertilizer because it is low strength. Its’ value lies in the minerals and biostimulants it contains and it’s best used with tomatoes as a fertilizer adjunct.
These are just some of the plants that benefit from seaweed in the garden. The only plants said not to benefit are succulents. If you live near the coast than you will be part of a very long tradition when you gather this free and abundant resource for garden use. If you don’t live near the coast you may have to search for a local nursery or farm supply store that offers seaweed fertilizer or extracts, or order it online. Either way, you’re sure to be happy with the results when you use seaweed in the garden!
