There's a theory called the urban cliff hypothesis that suggests our cities are surprisingly similar habitats to rock faces, that our homes still resemble caves, and that the plants and animals that thrive in our harsh cityscapes evolved from those natural places: species such as rock doves (pigeons), goat willows, buddleia and red valerian.
Nowhere is more like a rocky edge or clifftop than a window ledge, lofty balcony or rooftop.
Plants that can cope with barren rock ecosystems, that are used to shallow soils and droughts, and can withstand high winds and harsh rains, are useful here.
The low-growing, rosette-forming houseleek (sempervivum species), for example, has thick, fleshy leaves to withstand drought and such shallow roots that it needs the barest minimum of soil.
It thrives on neglect, so much so that you can grow it in air-ventilation bricks where, with time, it will cover the whole surface with its rosettes and star-shaped flowers.
Put a mixture of 50% grit and compost in the air holes and nudge in a single rosette.
Keep an eye on it while it roots, but once it’s established leave it somewhere it can get wet.
Another classic is sea thrift (Armeria maritima ), with its sweet pink pompoms that flower all summer long across the British Isles on sea cliffs and rocky edges.
It is tiny — not getting much above 15cm even in flower — and doesn’t have a deep root system, meaning it can happily live in a small window box about 15-20cm deep.
There are a number of varieties worth looking out for the deep hot pink of A. maritima ‘Düsseldorfer’, the pure white A. maritima ‘Alba’ or the slightly taller, A. pseud armeria ‘Ballerina’.
On a north-facing ledge try ferns such as maidenhair spleenwort, Asplenium trichomanes, which grows to only 20cm high, but loves damp shade and will thrive in a stone trough.
If conditions are a little drier, try hart’s tongue fern, A . scolopendrium, which will grow up to 90cm; it will need watering to establish it but once its roots are happy, it is surprisingly tolerant to dry shade.
The cultivar A. scolopendrium ‘Angustifolia’ has crimped edges and A. scolopendrium ‘Cristatum’ highly crested ones, if you want something a little different.
The pure white form of the native herb robert, Geranium robertianum ‘Celtic White’, would look lovely among these ferns: it is surprisingly good at tough spots and will bring bees with it. It is not easy to get hold of as a plant, but is simple enough from seed and well worth hunting for.