Ascophyllum nodosum is in the family of brown seaweeds often grouped together as kelps.  Its color ranges from dark green to yellow green or golden, and it’s mostly found in the mid to low intertidal zones.  A. nodosum attaches tenaciously to rocks and ledges with the help of a natural glue secreted by the discus shaped holdfast.  The fronds develop as a fleshy tubular or flattened primary thallus (shoot or stem) growing from the holdfast. As the primary stem grows it develops tuft-like secondary stems identical to the primary one but with limited growth.  The holdfast also forms secondary thalli that do not assume full proportions unless the primary thallus is lost. 

The stems all have large air bladders located at intervals along their length. The air bladders appear as egg shaped bulges along the stem, and they help float the seaweed vertically towards sunlight. Each stem forms one new air bladder per season and it’s said that one can estimate the plants’ age by counting the air bladders along the main stem from bottom to top (don’t include the side shoots, and the first 2-4 years growth may lack air bladders). The distance between bladders gives a measure of each season’s growth. The stems also bear numerous round or oval spore bearing receptacles attached by short stalks.  These are often yellowish in color.  The receptacles start developing in summer following each spring reproductive season and reach their largest size and maturity the following March – June.

Ascophyllum nodosum is often exposed at low tide in the mid intertidal zone, and fronds found there are shorter than specimens growing fully submerged in the lower intertidal.  It usually has a stringy, whip-like appearance. The entire plant can reach up to 6 meters in length (almost 20 feet) in deeper water but is more often 30 to 150 cm (12 to 60 inches).  It tends to grow in thick, tangled beds that completely cover the rocks with a mat at low tide, making footing treacherous for beachcombers.

Bladderwrack (Fucus vesiculosus) resembles A. nodosum and is often associated with it.  They can be easily distinguished from one another by the air bladders. A. nodosum has single bladders and the fronds lack a midrib, whereas F. vesiculosus bladders are generally in pairs along either side of a mid-rib.

Scientific & Common Names

The term Ascophyllum comes from the Greek word for skin or sack and from the Latin word phylum, which is in turn derived from the ancient Greek word phûlon, meaning tribe or race. The species name nodosum comes from the Greek work nōdus, meaning nodes or joints. Because Ascophyllum nodosum is so common, widely distributed, and of such long importance for coastal dwellers, it has quite a few common names. 

Many of these include the word wrack, which comes from the Old English word wræc, meaning persecution or misery.  This word evolved into meaning something wrecked or destroyed, and from there it was just a short jump to referring to cast ashore seaweeds as wrack.  Thus, in English A. nodosum may be known as knotted wrack, knobbed wrack, egg wrack, or simply as wrack. A. nodosum is especially abundant in Norway, which is why it’s sometimes called Norwegian kelp or just kelp. In the UK it’s known as yellow tang or sea whistle, while Canadians know it as bottle kelp. It’s often simply referred to as asco, an abbreviation of the Latin name. Here in Maine, we usually call it rockweed. 

Life History & Ecology

Ascophyllum nodosum is a cold-water species native to the Atlantic Ocean and recently invasive to the Pacific coast of North America, with close relatives in other oceans.  It’s usually the dominant and most abundant seaweed along sheltered, intertidal, rocky shorelines, but it’s rarely found along wave exposed shorelines.  Rockweed tolerates low salinity water and is often found far up estuaries. The optimal temperature for rockweed growth is about 15°C (59°F) but it tolerates temperatures less than 0°C and as warm as 25°C (77°F).  In the north Atlantic it grows up to the polar ice sheet, and as far south as Northern Portugal in the eastern Atlantic, or Long Island NY in the western Atlantic. 

Rockweed has only a single, sexual generation.  The plants are dioecious, meaning male and female reproductive receptacles are found on separate plants.  Immature male receptacles are greenish yellow and female ones darker green.  When mature (March – June in Maine), the male receptacles are almost bright yellow and contain many small, orange antherozoids within a gel-like matrix. Female receptacles are more of an olive yellow coloration and contain green oospheres.  Reproduction occurs in Maine during ‘spring tides’, usually in May and June, but exact timing depends upon the high tide water temperature reaching a critical threshold of about 7°C.  The receptacles release their antherozoids or oospheres into the water, and after fertilization the zygotes settle and attach as germlings onto the bottom. Whether these become established or not largely depends on whether they settle within dense, established stands, or in more open areas where they’re vulnerable to wave action and grazing by gastropods.  Rockweed is slow to colonize exposed shores, but once established it’s resistant to loss.

Rockweed bed at low tide. Photo by Sarah Redmond

Rockweed is a perennial and a slow but steady grower. Germlings may be less than ½ cm after a full year and just 1.5 cm (about ½ inch) after two years.  Rockweed doesn’t sexually mature until it’s about 5 years old. The plants grow faster as they get larger, and individual fronds may live 15 years or more before eventually succumbing to their own weight and breaking loose from the holdfast. New fronds can then grow from the holdfast, which itself may survive for many decades.

Rockweed plays an important ecological role and the thick beds can shelter a variety of marine fish and invertebrate species. Waterfowl such as eider ducks often use rockweed beds as feeding and nursery grounds. The detached fronds contribute considerable amounts of organic matter into bays and estuaries, much of which gets recycled into the detrital food web, and contributing to ecosystem productivity.  The fronds regularly shed epidermal cells as a way to remove epiphytic overgrowth; this epidermal shedding also contributes to the detrital food web. Although germlings and small plants are susceptible to grazing by a variety of herbivorous snails, larger plants are very resistant and are not a preferred food for most species.  Even sea urchins, the dominant ocean herbivore, tend to avoid rockweed.  A notable exception is that episodic outbreaks of northern lacuna snail, Lacuna vincta, have been known to sometimes devastate rockweed beds.  Fortunately, they only graze upon the fronds and the plants can regenerate from the holdfasts.  Strong winter storms and ice scour can also damage or remove established rockweed beds.

History of Use

Rockweed is frequently cast ashore in large quantities, where it can be easily gathered at low tide for fertilizer (never eat cast-ashore seaweed!).  Rockweed, along with other seaweeds, has been used as a convenient crop fertilizer or animal feed supplement for centuries.  Cast-ashore rockweed was piled into horse drawn wagons and then spread onto fields in the fall, where it slowly released nitrogen and minerals such as magnesium into the soil over the winter.  In the spring it was tilled into the soil as organic matter.  Many coastal communities still have ‘ware roads’ once used to transport ‘seaware’ and other goods from the shore to fields and villages.  Unfortunately, in our modern times ocean plastic pollution often contaminates cast-ashore rockweed, making it less desirable for use as fertilizer.  Today, rockweed is commercially harvested as a live crop, which is kept clean and mostly free of plastic by waves and tidal currents.

The first industrial use of rockweed and other brown seaweeds began in the 1600’s when it was discovered that soda (sodium carbonate) and potash could be extracted from the ash of burnt seaweed.  Soda and potash were important raw materials in the glass and soap industries respectively, and were also widely used to bleach linen. Potash was also important in gunpowder manufacturing. The word kelp was originally used to describe the ash of burnt seaweed, not the live plants, and the industry was known as the ‘kelp industry’.  It was only in later years that the word kelp became applied as a generic term for the brown seaweed species used in the kelp industry.

In the early 1800’s the kelp industry declined with the discovery of large mineral deposits of potash and the invention of new processes to extract soda directly from seawater.  However, within two decades a new industry arose to extract iodine from brown seaweeds.  Although deep water seaweeds were preferred over rockweed because they contained more iodine, rockweed was often included in the process.  This industry was short lived, though, because large mineral deposits of iodine were discovered in Chile in 1840.

In the early 1900’s another new kelp industry emerged to extract alginates from brown seaweeds.  Alginates were discovered in the mid 1800’s by the chemist E.C. Stanford, who had previously developed the process of extracting iodine from seaweed. Alginates are a type of seaweed polysaccharide derived from alginic acid, with phycocolloid properties. Phycocolloids have water retention, gelling, emulsifying, and stabilizing properties that make them a tremendously useful biopolymer in a wide range of food and industrial applications. Rockweed was and remains an important source of alginates, though today many other brown seaweeds are also harvested to meet the demand.  The global alginate market is estimated to be about $400 million annually, with demand expected to grow as new applications are invented.

Rockweed and other brown seaweeds have enjoyed a fertilizer renaissance in recent decades in a more refined form as plant biostimulants.  Biostimulants contain substances and/or microorganisms that, when applied to plants, stimulate natural processes to enhance nutrient uptake, nutrient efficiency, stress tolerance, and crop quality.  Ascophyllum nodosum is the most widely used and studied biostimulant seaweed species, and a large share of the rockweed harvested in Maine and Canada goes to this application.  Rockweed biostimulants are manufactured using proprietary processes and can be applied to a wide range of crops, including turf on golf courses, lawns, and parks.  This helps reduce the use of pesticides and non-organic fertilizers, and Ascophyllum biostimulants have become an important and accepted tool in organic farming. The micronutrients found in rockweed also make it a valuable feed supplement that improves the coat and health of domesticated animals such as cattle, horses, dogs, and cats. These micronutrients are good for humans too.

Culinary Attributes

Fresh or dried whole leaf rockweed is not especially palatable, being rather leathery and tough, and it’s rarely eaten in this form. Due to its less culinary nature, there are not many recipes using whole leaf rockweed, but people do often add it to mineralize dishes such as a pot of rice or soup, and it can also be steeped as a tea. The fibrous plant is removed before consuming. Maine Coast Sea Vegetables offers edible grade whole leaf dried rockweed.   

When rockweed is milled into granules or powder it becomes highly palatable, lending nutrition and a minerally saltiness and umami to foods.  We include rockweed as an ingredient in our Kelp Granules Sea Seasonings and in our Sea Salt with Sea Vegetables Sea SeasoningsRockweed granules or powder can also be purchased from us in bulk. Rockweed powder can be added to green drinks, teas, or supplements for people, plants and animals.  The granules can be sprinkled onto salads or other foods as a crunchy and highly nutritious topping.

Nutritional & Medicinal Attributes

Like all edible seaweeds, rockweed has high mineral content, with just a small 5g serving of granules or powder (about 1½ tsp) providing about 6% RDI of calcium and iron, 10% RDI magnesium, 4% RDI manganese, and 3% RDI zinc, along with other minerals.  Rockweed is also rich in iodine, and the same 5g serving contains about 3,500µg of iodine, or more than 23 times the RDI of 150µg.  Rockweed is included as one of four species in our Seaweed Support Iodine Formula in convenient capsule form. It is also one of the three kelps in our Icelandic Kelp Powder blend.

The therapeutic potential of including rockweed in the daily diet is especially intriguing.  Rockweed has been heavily researched for containing a large number of bioactive compounds with therapeutic or health promoting properties.  Many of these are sulfated polysaccharides such as alginic acid, fucoidans, laminarins, and ascophyllan, which was first extracted from Ascophyllum.  Alginic acid (or alginate) was investigated in the 1960’s and 70’s for its ability to bind heavy metals and radioactive isotopes in the intestine.  Since alginate is an indigestible carbohydrate, it’s excreted as dietary fiber along with anything bound within it. 

Fucoidans have been found to reduce the intensity of the inflammatory response when consumed and to also promote wound healing.  This has led to the recommendation that people suffering from tissue injury or wound trauma drink a brown seaweed broth to promote healing.  Fucoidan has also been shown to possess anticoagulant and antithrombotic (prevention of blood clots) activities.  Laminarin has been studied for inhibiting HIV and for enhancing macrophage immune response, giving it antitumor and wound-healing properties. Ascophyllan has a similar structure to fucoidan but differs in having a molecular backbone comprised of uronic acid, which research indicates may give it unique bioactive properties that can inhibit cancer cell growth.  Ascophyllan has also been found to stimulate the activity of immune cells in a number of studies.

Some people purchase rockweed leaf to make seaweed baths.  Seaweed spas, or thalassotherapy, were once popular in Victorian England, and although bladderwrack was considered the luxury treatment, its close cousin rockweed is also good in a hot water bath.  It’s best to enclose the fronds within a mesh bag to keep them from clogging the drain.  We describe thalassotherapy in more detail on our FAQ page and in this Seaweed Bath Hydrotherapy blog article.

Wild Harvest & Processing

At the peak of the European kelp industry in the 1700’s entire villages worked together to harvest rockweed and other brown seaweeds during spring tides. Teams of men would work their way down the beach with the outgoing tide, cutting the seaweed and throwing it behind them as they went.  When the tide turned and started coming back in, they worked their way back up the beach, tying off large bundles of harvested seaweed and then using the rising tide to pull it ashore.  Villagers spent several weeks at this while living in makeshift huts, often harvesting from remote islands as well as the mainland.  The harvested kelp was burned near shore in large stone lined pits and trenches, usually tended by women and children.  The burnt chunks of kelp were shipped away for processing to extract soda ash, potash, and iodine.  The practice of burning kelp for mineral extraction is said to have continued in Ireland until about 1948, and the remnants of kelp kilns can still be found there on some remote coastlines.

The rise of the alginate industry in the early to mid-1900’s saw the rise of a more managed and scientific approach towards harvesting.  In the 1940’s it was recognized that leaving the holdfast and part of the stem behind, as was already commonly done, allowed the rockweed to regenerate.  It was also understood that harvested areas should be left alone for 3-5 years to allow luxurious regrowth.  This was the start of intentional sustainable rockweed harvesting, and the experience of the Irish company Arramara Teoranta speaks to its success.  The company was originally sponsored by the Irish government in 1947 under the name Alginate Industries but is now privately owned.  Arramara Teoranta has sustainably cut from the same shores on an industrial scale since it was established over 70 years ago, with no loss of Ascophyllum beds.  The fronds are hand cut with a small knife, taking care to leave at least 25cm (about 10 inches) behind.  Harvesting is done mostly by local residents who know, after many years of experience, how often a particular bed can be harvested (usually 3 to 5 years).

The practice of leaving some of the plant behind and harvesting beds on a cycle (or only partially harvesting them) is now done everywhere rockweed is harvested, whether it be Europe, Canada, or Maine.  In Canada, where rockweed is the most commercially important seaweed, management of the resource is under joint federal/provincial authority.  Area based management includes harvest limits, protected areas, minimum cutting heights, and gear restrictions.  Use of mechanical harvesters, which peaked from 1986-1994, has since been phased out and all cutting is now done by hand using a long cutting rake, usually just below the water surface.

Maine Coast Sea Vegetables sources our rockweed from Iceland (granules and powder) and Maine (whole leaf).  In Iceland, the rockweed is harvested from certified organic grounds on the edge of the Arctic Circle, in the pristine Breiðafjörður bay.  It’s harvested between low and high tides with a sickle bar mower; much like harvesting hay, only on water.  Each bed or area is mapped and managed for sustainability and annual harvesting is rotated between beds every 3-5 years.  The freshly harvested rockweed is sent on the day of harvest to geothermal drying facilities.  Iceland is renown as the land of ice and fire, and its volcanic and geothermal resources have long been used for everything from natural hot water baths to heating homes, baking bread, and drying seaweed.  The seaweed is dried at carefully controlled temperatures to retain nutrients and avoid cooking it.

Harvesting in Maine may be done by hand using rakes or knives, or by machine, using mechanical harvesters.  In either case, most harvesters follow guidelines jointly established by the Maine Seaweed Council and the Maine Department of Marine Resources to ensure long term resource sustainability.  As in other regions where rockweed is harvested, guidelines include rotating harvest beds, harvesting no more than 17% of a particular bed per year (or less than 50% every 3 years), and leaving a portion of the plant behind for regrowth (16” in Maine, more than most other regions).  In addition, many harvesters (and all those who MCSV purchases from) follow an additional set of harvesting and processing guidelines for Organic Certification.

Maine rockweed mechanical harvester

Rockweed harvesting is an example of human natural resource extraction that hasn’t yet led to resource collapse, and there’s no strong evidence that it causes significant environmental harm.  For the most part this has been true of rockweed harvesting for over three centuries.  However, there have been exceptions in Europe and Canada. During the period 1986-1994, the Canadian rockweed industry went through a period of over-exploitation with the introduction of ‘Norwegian suction cutters’, and intense competition among three large companies for a limited resource base.  Some areas were overharvested and the government had to intervene by tightening restrictions and management plans.  Although mechanical harvesting itself didn’t cause the over-exploitation, today all harvesting in Canada is done by hand and it’s generally regarded as a well-managed and sustainable fishery.

Despite a long record of sustainability in most regions, rockweed harvesting remains controversial with some scientists and members of the general public.  Some are motivated out of genuine concern, while others may simply be irritated by the sight and sound of rockweed harvesting occurring along their shoreline property.  The Canadian experience certainly offers a cautionary tale and there’s no doubt that rockweed harvesting affects the ecosystem, whether by reducing a source of primary productivity or by altering the shape of the plants into a ‘bushier’ form as they regrow after cutting.  However, some claims made by opponents are questionable, such as the claim that rockweed harvesting adversely affects juvenile lobster habitat.  This claim is belied by the fact that in Maine, lobster landings increased to historical levels throughout 2000 to 2019, even as rockweed landings also increased.

We believe the industry has long operated in a sustainable fashion, and we know with certainty that harvesters are deeply invested in the long-term health of the resource and the ecosystem it helps support.  We do not view human activity as a malevolent force outside of nature; rather we see humans as a part of and operating within nature, sometimes for good, sometimes for bad, but more often in ways that defy judgment.  Rockweed offers many benefits for human health and sustainable agriculture, and as long as we continue to harvest wisely, we believe it can do so for many years to come.


Rockweed is sometimes used in aquaculture as a feed binder or ingredient, but rockweed itself is not currently farmed and it’s not considered a particularly promising culture candidate.  Some attempts have been made at research scale.  An experimental multi-tier land-based system trialed in 1984 showed it could be done but the delivered cost was $1,620 per dry ton vs $140 per dry ton for wild harvested rockweed (1984 dollars).  Cost-effective sea-based rope or net culture methods haven’t been developed for rockweed as they have for other species and may not even be possible because the shape and function of the discoid holdfast isn’t particularly amenable to rope culture methods used for other seaweeds. Ultimately, the production scale required to supplant or even compete with wild harvested rockweed would be so large as to itself create a new host of issues and user conflicts in the marine environment.