There are approximately 3,000 plant species in the British flora. Some of these plants occur in very specific and very restricted habitats: for example, the purple mountain saxifrage (Saxifraga oppositifolia) grows in moist rock faces on Welsh and Scottish mountains. By contrast, other plants may be able to survive in a wide range of situations. The humble dandelion, for example, is a common weed of lawns. But dandelions are also present in wetlands, in mountainous areas, on pavements or in gardens.
Our cities, though heavily urbanised and concreted, offer a wide diversity of habitats for spontaneous flora. Take this picture of East London. On this 1 km² area, you will find: river banks with brackish water, freshwater ponds, amenity grassland (sports grounds), a woodland, private gardens, brownfield (industrial land) with sand and gravel, roof gardens, street trees, old terraced houses with brick walls, modern housing estates with landscaped gardens. This diversity of habitats can result in a richer flora than for a comparable area in the countryside.
So just how many species are there in our cities? I don’t have exact data for the UK, though I know that a new Atlas of the Flora of London is currently in preparation. The Flora of the London Area published in 1983 lists over 2,000 species, although a significant number of these were fleeting records, seen perhaps once or twice in the area, rather than well-established plants. By comparison, the flora of the Paris area is thought to comprise, as of 2019, 637 plant species.
Of course, cities that are closer to the sea, or at altitude may be home to a very different range of species.
Nevertheless, some spontaneous plants have been extremely successful in colonising a wide range of habitats. Here are a few species that you are likely to encounter on pavements, in many parts of the UK!