Urban nature: on the wall, vertical specialists

The true nature of the city, with all its brick, stone and concrete, makes it very different from the surrounding countryside. This is especially true of water cities that were built in drained swamps and along rivers e.g. Utrecht and Antwerp. There the urban environment provides the ideal growing conditions for plants that normally thrive on exposed mountainsides and along small streams running down rocks. As such cities provide the ideal growing conditions for plants that aren't naturally found in the Low Countries. They have grown in this urban environment for so long that they have become established in their own little niche... literally!

This Crested woodfern (Dryopteris cristatus) has colonised a crack between the stones of a retaining wall in a park (on the left). The watered lawn behind provides an ideal growing environment with a steady supply of moisture for this wetland native. A fern associated with mountainsides is the Spleenwort (Asplenium trichomanes). The Dutch name of this little fern (Steenbreekvaren) translates as stone crusher, as it was believed to be able to bring down city walls and bricked embankments. It enjoys north facing walls with loose mortar. Rock fumewort (Pseudofumaria lutea) originated in the Alps and was introduced as a garden plant into the rest of Europe. As the name suggests the plant in the poppy family enjoys to grow in places with little soil. It can quickly invade a suitable habitat, especially shady walls. It can be found growing in many parks and on many a cemetery wall. 

Another plant only found on walls is the Ivy-leaved toadflax (Cymbalaria muralis). It originated in the Alps and the mountains of central Europe. Beyond its mountainous motherland it grows exclusively on walls and in loose pavements. The Wall-rue (Asplenium ruta-muralis) doesn't look like a typical fern. This lithophyte is mostly found growing on old walls hence the German name Mauerraute and the Dutch Muurvaren (literally wall fern).The Hart's tongue fern (Phylitis scolopendrium) needs a calcareous en moist environment to thrive. It is often found on canal walls, but has recently been discovered growing wild in woodlands in the Flevopolder with its heavy chalky clay soils.