In July 2024, thousands of botanists gathered in Madrid for the world’s largest plant science conference, the International Botanical Congress. At the end of the meeting, attendees came together to issue an urgent statement. This document, titled “The Madrid Declaration,” proposes ten strategic actions to address plant life degradation caused by human activities.

From tackling inequalities in the plant sciences to harnessing nature-based solutions, the scientists’ appeal aims to nurture the relationship between plants, people, and the planet. The declaration emphasizes how plants can help us solve pressing environmental, social, and economic challenges—especially the climate crisis.

One plant’s connection to our climate dates back roughly 50 million years to a time when Earth was too warm for polar ice caps. Azolla filiculoides, also known as “water fern” or “mosquito fern,” is one of the world’s tiniest fern species. At just one or two centimeters across, each plant is the size of a fingernail and has leaves as small as gnats. But, as Zoë Schlanger suggests in her 2024 book about plant intelligence, The Light Eaters, “it is unwise to mistake size for complexity.”

Azolla filiculoides is one of the fastest-growing plants on the planet. The fern carpets fresh water in scalloped layers that can double in size in as little as two days. Its remarkable ability to multiply, however, is both a strength and a shortcoming. The same plant has been praised as an ancient climate hero and condemned as a modern invasive species.

Fifty million years ago, Azolla filiculoides played a pivotal role in cooling the planet. Back then, the Arctic Ocean was a vast freshwater lake separated from other oceans by land bridges. Fueled by abundant nitrogen and carbon dioxide, Azolla filiculoides spread across large parts of the Arctic Ocean and crept onto surrounding continents. Over an 800,000-year period known as the “Azolla event,” the species caused temperatures to plummet by pulling an estimated 10 trillion tons of carbon dioxide from the atmosphere. That’s more than 250 times the amount of last year’s global energy-related CO₂ emissions.

Today, Azolla filiculoides is known for the same growth habits in smaller water bodies including ponds and slow-flowing rivers and canals. The plant’s common name “mosquito fern” derives from its tendency to form mats that are so wide and thick that they prevent mosquitoes from reaching the water to lay their eggs. As Henry Knute Svenson explained in 1944, “the entire surface of quiet ponds may be so covered by the tiny branching fronds as to exclude mosquitoes from the surface.” These blooms block out light and reduce oxygen levels in waters underneath, threatening the survival of native aquatic flora and fauna.

The ease with which Azolla filiculoides is transported by boats, birds, and amphibians has allowed the fern to expand its habitat range across the globe. The plant native to warm temperate and tropical regions of the Americas has since been introduced to Europe, North and sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean, and Hawaiʻi.

Azolla filiculoides is now considered one of the most invasive plants in the United Kingdom. It was brought to Great Britain in the late nineteenth century as an ornamental plant, and, in the years that followed, it spread rapidly across the country’s ponds, canals, and lakes.

“By impeding navigation, water flow and angling, causing fish kills and threatening wetland nature reserves, thick floating mats of A. filiculoides have become serious weed problems,” writes biologist Rachel Janes. The plant has also led to the deaths of livestock that mistook fern-covered water for solid ground.

South Africa faced a similar challenge when Azolla filiculoides was introduced in 1948 and infested more than 300 rivers and water reservoirs by 1990. To combat this worsening problem, the country released the frond-feeding weevil (Stenoplemus rufinasus), a beetle no longer than two millimeters, as a biological control agent in 1997. Frond-feeding weevil larvae are voracious eaters, capable of devouring several water ferns per day without harming native plants. Within a year, the weevil eradicated Azolla filiculoides from most sites, and today, the fern no longer poses a threat to South Africa’s aquatic ecosystems.

Frond-feeding weevils are present in the United Kingdom, but climatic conditions reduce their efficacy. Compared with South Africa, the nation’s cooler climate limits weevil development, dispersal, and survival through winter. In the mid-2000s, the Centre for Agriculture and Bioscience International began supplementing the United Kingdom’s naturalized frond-feeding weevil population by rearing and collecting the species in controlled ponds. Each summer, they raise thousands of weevils and ship them to water resource managers fighting Azolla filiculoides infestations.

Other methods of preventing the fern’s uncontrolled spread include restricting the sale of the plant in garden centers and other retail outlets, raising public awareness of its aggressive tactics, and monitoring the distribution of the species.

Agricultural Applications

With careful and constant management in agricultural settings, Azolla filiculoides can be more of an asset than an annoyance. The plant’s nitrogen-fixing capabilities and resistance to insects have huge implications for the health and resilience of current food systems.

All living things need nitrogen to produce biological building blocks like proteins. Nitrogen is the most abundant gas in our atmosphere, but most organisms cannot absorb the element in its gaseous state. It must first be converted into a “fixed” form such as ammonia.

Because the natural supply of fixed nitrogen is limited, farmers often inject their soils with nitrogen fertilizers to boost crop yields. But what supports a farm can threaten other ecosystems. The excess nitrogen that is not utilized by growing plants pollutes land, water, and air, and exacerbates climate change.

“What we need are ways to increase food production on existing farmland while reducing nitrogen pollution,” write scientists Doug Gurian-Sherman and Noel Gurwick. Azolla filiculoides presents an opportunity to do just that. The species has evolved special cavities inside its leaves that harbor nitrogen-fixing cyanobacteria. These microorganisms act as a “nitrogen fertilizer factory,” converting atmospheric nitrogen into a usable form both for the fern and other neighboring plants.

Farmers in Asia have recognized the agricultural value of Azolla filiculoides for more than 1,000 years. Growing the fern in rice paddies and grinding it into the soil increases rice production without the need for artificial nitrogen fertilizers. The earliest known written record of this practice is in The Art of Feeding the People, a Chinese book from 540 CE.

Azolla filiculoides could also help farmers transition away from the use of pesticides that are linked to pollution and human health problems. Compared to flowering plants, ferns are rarely attacked by insects. The gene responsible for this natural insect resistance was identified only recently, when scientists sequenced the genome of Azolla filiculoides. This discovery marks a major step toward the development of a fern-protein-based pesticide, which could protect crops without the harmful consequences of modern industrial pesticide use.

Looking Forward

In the face of increasingly frequent and severe weather events, warming oceans, and rising sea levels, researchers are investigating whether Azolla filiculoides could counteract escalating greenhouse gas emissions and global temperatures. The fern’s nitrogen-fixing capabilities and natural insect resistance also offer a gateway to more sustainable agriculture.

Nevertheless, the risk that Azolla filiculoides overgrowth poses to aquatic life cannot be ignored. Whether the fern lives up to its potential as a climate solution and natural fertilizer will all come down to the question of management. The United Kingdom’s ongoing control efforts, South Africa’s successful management of the species, and the ways in which farmers in Asia have worked with rather than against the fern for more than a millennium will serve as invaluable case studies moving forward.

“The Madrid Declaration” calls for “collaborative and transdisciplinary approaches to plant research, including local and Indigenous knowledge, the arts, humanities, and diverse scientific approaches.” The Dumbarton Oaks Plant Humanities Initiative is working toward this goal by advancing the understanding of plant-human relationships through the methods and perspectives of both humanities scholarship and science.

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