Scientists Discover World’s Largest Plant: It Covers 80 Square Miles and Sprouted From a Single Seed 4,500 Years Ago

A single plant stretching for miles, carpeting the seabed, now holds the title of the largest living organism on Earth. Meet Posidonia australis, a remarkable seagrass found off the coast of Western Australia.

We have already posted about giant plants like the world’s largest flowers, the world’s tallest bamboo, the world’s largest pine cone, or the tree with human-sized leaves, but this one probably surpasses them all. Meet the largest plant on earth.

Unlike Pando, a massive clonal colony of the quaking aspen tree, which is widely considered to be the world’s largest living organism by mass, Posidonia australis, also known as fibre-ball weed or ribbon weed, thrives underwater. Scientists discovered that an individual P. australis colony in Shark Bay, Western Australia, covers a staggering 80 square miles (200 km2), an area larger than Washington D.C. They summarized their findings in a study published in Proceedings of the Royal Society B.

“It’s the largest known example of a clone in any environment on Earth,” co-author Elizabeth Sinclair from the University of Western Australia told New Scientist. It is “arguably the world’s largest living organism,” wrote Kate Golembiewski for the New York Times. 

This beats the previous contenders by a large margin: a colony of the Armillaria ostoyae fungus in Malheur National Forest, Oregon that extends 3.5 square miles (9.1 km2), and the aspen tree colony just mentioned, which covers an area of 0.15 square miles (0.5 km2).

The vastness of this seagrass meadow isn’t the result of seeds spreading far and wide. Well, not anymore. Although the current scientific understanding is that the Posidonia australis colony in Shark Bay did sprout from a single seed approximately 4,500 years ago, it has since perfected the art of clonal reproduction. It doesn’t produce flowers or seeds anymore; instead, it generates new shoots that spread horizontally from its root system. Over millennia, this process has allowed the single plant to clone itself repeatedly, creating the sprawling underwater kingdom we see today.

The scientists reached this conclusion by analyzing the genetic makeup of P. australis samples collected across the vast Shark Bay meadow. They found remarkably little genetic variation, indicating that the entire colony likely originated from a single source.

The results “blew us away: it was all one plant,” the authors write in The Conversation. “One single plant has expanded over a stretch of 180 km [112 miles].” The authors suggest that seagrass likely emerged in Shark Bay a few thousand years after it was flooded approximately 8500 years ago, spreading to adjacent newly submerged areas.

The plant’s immense size aside, its genetics are also noteworthy. Unlike most seagrasses, which inherit half of each parent’s genome, the Shark Bay seagrass carries the complete genome of both parents, a condition termed polyploidy. Moreover, it seems to be a hybrid of two species. This form of clonal propagation has resulted in even older organisms, such as a turtlegrass estimated to be approximately 6000 years old, The Smithsonian notes.

“Polyploid plants often reside in places with extreme environmental conditions, are often sterile, but can continue to grow if left undisturbed, and this giant seagrass has done just that,” Sinclair said in a statement. “Even without successful flowering and seed production, it appears to be really resilient, experiencing a wide range of temperatures and salinities plus extreme high light conditions, which together would typically be highly stressful for most plants.”

Despite a 2010-2011 heatwave in Western Australia damaging Shark Bay’s seagrass meadows, including ribbon weed, the plants are already showing signs of recovery. This surprised researchers given the seagrass’s lack of sexual reproduction, which typically aids adaptation to changing conditions. They speculate that the seagrass’s remarkable adaptation to its local environment, near the edge of its species’ range, may contribute to its resilience. The study suggests that species reproducing through cloning may adapt more swiftly in such environments compared to sexually reproducing species, which adapt more slowly.

Posidonia australis isn’t just a remarkable natural wonder; it’s also an ecological powerhouse. These underwater meadows provide vital habitat and nursery grounds for countless marine creatures, from tiny seahorses to majestic dugongs. Its dense network of roots also helps to stabilize the seabed and prevent coastal erosion. Additionally, P. australis plays a crucial role in filtering water and storing carbon dioxide, contributing to a healthy and balanced marine environment.

The plant has been traditionally harvested by locals for use as a fiber source to make clothing and baskets, and for medicinal purposes and in cultural ceremonies.

Despite its resilience, Posidonia australis faces threats from climate change, pollution, and human activities. Rising sea temperatures, increased sedimentation, and damage from boat anchors can all harm these delicate ecosystems. As we strive to protect our oceans, understanding and conserving these underwater giants is crucial.