On the island of Laeso in Denmark, a revival of a centuries-old tradition is taking place—one that could have profound implications for sustainable building materials. Henning Johansen, a passionate craftsman, is breathing new life into the lost art of eelgrass thatching, drawing attention to an extraordinary natural material with untapped potential for eco-friendly construction.
On a warm August afternoon, Johansen twisted strands of eelgrass into ropes, a skill that dates back generations. He carried these ropes to a nearby thatched hut, methodically laying them on the roof until the structure began to take shape. Eelgrass, a plant that washes up in vast quantities along Denmark’s coastline, hardens over time, forming air pockets that help it retain its shape, repel rain, and resist both fire and rot. This process ensures that the roofs, made from eelgrass, will protect the historic buildings of Laeso for generations to come—another 350 years, in fact.
There are currently 36 homes on Laeso with these traditional eelgrass roofs. Their preservation, thanks in part to Johansen’s advocacy, has earned the island a spot on the UNESCO World Heritage Site shortlist. The use of eelgrass for roofing dates back centuries, and Johansen’s efforts to revive it are helping to preserve not only the cultural heritage of Laeso but also to highlight eelgrass as a sustainable, bio-based building material.
Eelgrass is not just an ordinary plant—its environmental benefits are significant. The endangered plant has the remarkable ability to remove carbon dioxide from the atmosphere up to 35 times faster than rainforests. According to a 2023 Environmental Protection Agency report, eelgrass can sequester up to ten times more carbon than an equivalent area of forest. This “blue carbon” storage is crucial for combating climate change, stabilizing marine ecosystems, and reducing ocean acidification.
Beyond its carbon-capturing abilities, eelgrass plays a vital role in marine ecosystems. Growing on the shallow seafloor, it absorbs marine minerals, filters water, and provides a habitat for sea creatures, helping to stabilize sediment and prevent coastal erosion. Even after it’s uprooted by wind and dried, eelgrass continues to store carbon, making it an excellent, environmentally friendly alternative to materials like cement, steel, and aluminum—whose production is responsible for a staggering 37 percent of global greenhouse gas emissions, according to the United Nations.
Johansen is careful not to harvest eelgrass from the ocean floor. In Denmark, it’s illegal to disturb living eelgrass meadows, and harvesting is restricted to strands that wash ashore after storms. This approach ensures that the wild eelgrass population remains undisturbed and the plant’s ecological role is preserved.
Despite the potential of eelgrass as a building material, its use faces significant challenges. The plant has experienced a dramatic decline in recent years, due to pollution, climate change, and invasive species. For example, invasive green crabs have uprooted eelgrass in many areas, and disease wiped out 90 percent of the plant in Europe and the U.S. during the 1930s. Globally, eelgrass has experienced a 29 percent decline since the 1700s, and if current trends continue, up to 97 percent of the world’s eelgrass could be lost by the end of the century.
Restoration projects are underway in various parts of the world to reverse the damage. Efforts in the Chesapeake Bay and the Gulf of Maine, where eelgrass populations have decreased by 60 percent since 2005, are helping to repopulate eelgrass meadows. These efforts could help secure eelgrass’s role as a sustainable building material in the future, if farming techniques are developed to grow it outside of the wild population.
Richard Lilley, the founding director of Project Seagrass, acknowledges that while there is potential for commercial eelgrass farming, ecological sustainability must be the priority. He explains that growing eelgrass for building materials is a complex and still nascent endeavour. However, if successful, it could aid in both restoration efforts and the development of a bio-based alternative to traditional building materials.
The idea of farming eelgrass for construction purposes has gained traction among environmentalists. Similar to how oysters have been farmed to restore reefs, eelgrass could be cultivated to help restore meadows while also serving as a valuable resource for sustainable building practices. Coastal scientist Bowdoin Lusk sees potential parallels between oyster farming and eelgrass farming, suggesting that, like oysters, eelgrass farming could both restore natural habitats and provide an eco-friendly alternative to conventional materials.
For now, it is agreed that eelgrass wrack—the seaweed that washes up on beaches—can be used in moderation, but its ecological importance must be respected. As Dorte Krause-Jensen, a marine ecologist at Aarhus University in Denmark, points out, the wrack should not be overly exploited. However, if the wrack is already being removed for beach clearing, it makes sense to repurpose it.
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Johansen’s return to Laeso in 2010 marked a turning point for the revival of eelgrass thatching. When he first arrived, the roofs of Laeso’s cottages were in dire need of restoration, and the community had resorted to using straw as a substitute. But Johansen saw the value in the traditional eelgrass method and decided to revive it. With help from local residents, he has since restored 25 of the 36 eelgrass-roofed homes on the island, earning recognition for his work from the European Commission.
Johansen’s stepdaughter, Kirsten Lynge, is carrying on his legacy in her own way. As a co-founder of Copenhagen’s Sould, Lynge has designed innovative eelgrass products, including acoustic panels and insulation, for commercial and residential projects. She is also exploring the possibility of bringing eelgrass products to the U.S. market.
In Denmark, the use of bio-based building materials is gaining momentum. The country has introduced laws to reduce carbon emissions from new buildings, and companies like Normann Copenhagen and Getama are using eelgrass to create furniture and mattresses. Lynge’s work with eelgrass is part of this larger movement to create more sustainable building materials.
As the world grapples with climate change, eelgrass may hold the key to reducing carbon emissions while providing a durable, eco-friendly alternative to traditional construction materials. If restoration efforts succeed and farming techniques are developed, eelgrass could be a game-changer in sustainable building. The revival of this ancient material could pave the way for a greener, more sustainable future.
