Rockweed

Ascophyllum nodosum

Also known as Knotted Wrack, Knotted Kelp, Norwegian Kelp, Feamainn Bhuí

At a glance

Family

Fucaceae (rockweed family)

Type

Brown algae (seaweed)

Parts used

Whole frond (thallus), including the air bladders

Size

Fronds trailing 3 to 8 feet, sometimes longer in sheltered coves

Bloom

No true flowers; forms small yellowish reproductive receptacles at the frond tips in late winter and spring

Habitat

Rocky intertidal shores of the cold North Atlantic, from New England and Atlantic Canada to Ireland, Scotland, and Scandinavia

Acquisition

Forage

Traditional remedies in HerbSense

Key benefits

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Rich source of minerals

Traditionally soothing for skin

Old folk remedy for joints

Nourishing coastal tonic

Traditional uses

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For centuries, families along the cold shores of Ireland, Scotland, Brittany, and Atlantic Canada waited for the tide to fall so they could gather this leathery, bead-strung seaweed off the rocks. Most of it went straight onto the fields, since rockweed was prized as a mineral-rich fertilizer long before anyone bottled it as a supplement, and coastal farmers still swear their potatoes and cabbages grow sweeter for it. Fisherfolk and their families also worked it into home remedies, soaking tired joints and rough skin in seaweed baths drawn straight from a simmering pot of fronds, and chewing small bits of it as a mineral tonic in places where fresh vegetables were scarce through the winter. Because it holds so much natural iodine, rockweed and its seaweed cousins were also part of old coastal folk wisdom around a healthy thyroid, long before iodine was ever isolated in a laboratory.

How to grow & use it

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Traditional remedies and step-by-step preparations, a complete growing guide, and how to harvest, dry, and store it, all unlocked with HerbLens Unlimited.

✦Remedies & preparations
✦ Growing guide
✦Harvest & storage

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Pairs well with

✦Nettle
✦Dulse
✦Oatstraw

Substitutes

✦Bladderwrack
✦Kelp (Laminaria species)

Choose it when

“When the tide pulls back and leaves the rocks draped in glistening olive-gold ribbons, patient and briny, holding the whole mineral memory of the sea.”

Did you know?

Each of rockweed's paired air bladders forms once a year, so an old frond can be aged almost like a tree, ring by ring, simply by counting the bladders down its length.

Safety

Rockweed is naturally very high in iodine, so it's not recommended for anyone with a thyroid condition. Like other seaweeds it can absorb heavy metals from polluted water, so only gather it from clean, unpolluted shoreline, and it has mild blood-thinning properties worth noting if you take anticoagulant medication.