The roof of the car park is home to a rare fern with tongue-shaped fronds

What may seem like a weed at first is actually the rare hart’s-tongue fern. The car-park roof appears to be the ideal habitat for this rather discerning plant.

A tiny seed – or ‘spore’, in the case of ferns – is picked up by a gust of wind and blown across campus. Its chances of surviving the journey in the concrete landscape are very slim. But this one lands in just the right place: the shady crevice of a roof above one of the entrances to the car park. Rainwater flows down the slanted roof into the crevices, allowing the spore to grow into a hart’s-tongue fern.

The species is really quite rare, says ecologist Garry Bakker from the Rotterdam Urban Ecology Unit (Bureau Stadsnatuur). The fern is a picky plant: it needs a moist, shady and calcium-rich environment. Ordinarily, you would find them on the sides of quays and poorly maintained walls. But this slanted, paved roof beneath a shady tree offers the same conditions. Bakker explains: “Think of it as a quay wall, but on its side.”

The hart’s-tongue fern is not alone: a ‘male fern’ is growing close by, and their resemblance might give the impression that they are both ordinary ferns. But close examination reveals a clear difference: the male fern has ‘pinnate’ or serrated leaves, while the hart’s-tongue fern has strap-like leaves without indentations. Male ferns are also much easier to find: they grow on almost every Dutch forest floor.

Plants that grow in the crevices between bricks and stones often face a painful fate: the weeding sickle. But Bakker hopes that a better future awaits the hart’s tongue. “It’s natural vegetation”,  he says. “Weeds is a verdict.”