Necklace Fern (Asplenium flabellifolium), a native fern of southern Australia growing in colony on adjacent sections of bluestone retaining wall and balustrade in downtown Melbourne, facing the Yarra River.
I have not located previous records of Necklace Fern on urban structure. However, Melbourne’s bluestone ruin would appear to offer suitable microhabitats for this tiny fern, and its appearance at the centre of the action should not be entirely surprising. Consider the contextual description provided in Flora of Australia (vol48): ‘commonly found in rock crevices, among boulders, under overhangs, on rocky banks or in gorges, either in exposed situations or under open forest… tolerates a variety of rock types and grows from sea-level to alpine areas.’ Generalists frequently become urban volunteers, even if their range of suitable niches narrows in the face of urban stresses and management regimes.
Moisture requirements, access to shade and sensitivity to traffic would appear to be the main limiting factors to establishment on structure (along with opportunity, in the form of spores arriving at the right place). However, the species’ tiny form factor also means that this plant may be more broadly present but only rarely noticed. Certainly gutterboxes, damp buildings and even undertrafficked kerbs all appear to present possible niches for the species if it can handle this one.
Heritage conservation may pose a potent constraint to more frequent volunteerism by A. flabellifolium on public-facing structure—all the instances of this small colony were found on sections of wall that appear to have been spared repointing about a decade ago.