Botanists stunned by tropical fern flourishing in Znojmo’s underground tunnels
Venus hair fern is usually found in tropical and subtropical regions, growing on palm trees and in rainforests. But—much to the surprise of botanists—it has also taken root in the 14th-century underground tunnels of Znojmo, a town in South Moravia. Despite its unnatural surroundings, the plant is not only surviving—it’s thriving.
The Znojmo underground, one of Central Europe’s largest labyrinths, stretches up to 25 kilometres. Though its original purpose remains unclear, today, it attracts tourists eager to explore the winding corridors—and now, to see the unexpected colony of venus hair fern.
Also known as maidenhair fern, the plant is named for its delicate, long leaves that waft in the breeze. It typically thrives on rocks near waterfalls, requiring constant moisture. Due to global warming, it has slowly spread to southern Europe and the Balkans, but it would normally struggle to survive in Central Europe, due to its relatively harsh winters.
About five years ago, botanists in Znojmo were surprised to discover the plant flourishing in the town’s historic tunnels. How did it get there? Botanist Radomír Němec from the South Moravian Museum in Znojmo offers two theories:
“The more likely explanation is that venus hair was once a popular houseplant, thriving in cool interiors of town houses before insulation was common. The plant spreads via spores, much like fungi. Spores from one of these ferns, grown in the centre of Znojmo, could have travelled through ventilation, landed on damp rock, and germinated—eventually forming a whole population.”
A less probable theory, he says, is that the fern’s spores, which are surprisingly mobile, may have traveled on the wind from a tropical country and been washed underground by rain.
Biologists first spotted venus hair in the Znojmo tunnels five years ago. Since then, it has spread significantly. Němec explains:
“It grows on wet rock that is constantly supplied with water, both from the surface and through condensation on the cold stone. The fern is relatively undemanding, but it does need nutrients, which it absorbs from Znojmo’s granite bedrock. That’s enough to sustain it.”
In its natural habitat, the plant thrives on dappled sunlight filtering through treetops. But of course, the underground is naturally dark, so the plant shouldn’t be able to grow there at all. However, as Mr. Němec explains, it has found an unusual source of light:
“Visitor tours are regularly organized in the Znojmo underground, so artificial lighting provides just enough for the fern to thrive. From the mother plant we found five years ago, it has spread into nearby corridors. Now, we’re seeing it as far as 50 meters away, and we expect it to spread even further.”
While Mr. Němec jokes that researchers may one day need machetes to cut through the fern-filled corridors, he reassures that venus hair is unlikely to spread beyond the underground or pose a threat to local species.
