Staring at the walls

'Life after concrete' urban botany walk, Tues 2 June

In two hours we recorded over 100 plant species in the overlooked gaps, cracks, pavements and walls around the very central parts of Exeter, around the quay and Catacombs park. Or as Jeremy, BSBI recorder and our walk leader, pointed out, these urban survivors more accurately live 'with concrete' than after it.

City wall behind Cricklepit St

Spray-dodging weeds might be another view of them, but all have interesting tales to tell - how they can support other biodiversity, or reveal insights into Exeter's history.

Within a few steps, we found common mallow, shining crane's-bill, lesser trefoil, spotted medick, wall speedwell and common sow thistle, mixed with cosmopolitan Mexican fleabane, purple toadflax and wall bellflower. Some species are more accepted and tolerated as wall plants than others. For example, Jeremy reported that Ivy-leaved toadflax Cymbalaria muralis is Exeter's most widespread urban wall plant, found in every one of the city's survey squares. Exeter's city wall would have been suitably ready and prepared, having been standing for over a millennium and a half, for the arrival of this plant to Britain in 1640.

The building material itself, 290-208 million year old Permian sandstone, is slightly older. Romans laid down the purply red, often pock-marked, basalt blocks of 'trap'; later Saxon and Mediaeval builders made use of the orange-red Heavitree breccia stone.The type of mortar is the key, according to a favourite book for the occasion, Arnold Darlington's 1981 Ecology of Walls (among Amazon's top 625,020). High pH suits certain plants, but once Portland cement became standard in the 1850s, there was no longer a substrate which allowed access to the gradual slow flow of moisture and nutrients. The result is Ivy-leaved toadflax can be a handy indicator that a wall is probably well over 50 years old. 

Ivy-leaved toadflax

Ivy-leaved toadflax is a valued bee plant: I've seen Red mason bee Osmia bicornis following the honey guides on the pale and purple petals to enter the flower tube. After fertilisation, the stalks turn inward and wedge the ridged seeds into gaps in the wall. Sometimes one can trace back the pattern of historical plant colonisation; sometimes ants carry the seeds away first. Lying in wait for the ants might be the common jumping spider Salticus scenicus, which we saw on the walk.

Jumping spider seen on the wall

Pellitory-of-the-wall Parietina judaica is another plant species particularly prevalent in the south west. A non-stinging relative of the common stinging nettle, it can support caterpillars of the Red admiral butterfly, and the Nettle tap moth Anthophila fabriciana.  A rare, but possibly colonising and spreading moth species Cosmopterix pulchrimella is also worth looking out for: the leaf-mining caterpillars feed within the Pellitory-of-the-wall leaves. This moth was first discovered in the UK in Dorset in 2001, and has since been found in Cornwall in 2004, and in the Channel Islands.

Pellitory-of-the-wall

A final example is Red valerian Centranthus ruber, a long-flowering, nectar-rich wall specialist.

Introduced to Britain in the 16th Century, it is a food plant of two moth species, Large ranunculus and Angle shades, though the nectar is attractive to a wide range of species. I've seen Hummingbird hawk-moth feeding at Red valerian alongside Exeter walls this year; at night the plant can be smothered in moths and other insects.

That's enough individual examples. The wider point was that there are numerous such small and random patches of wildness, trying to bloom across the cityscape: footpath verges, triangles of roadside visibility splay, small greens with a tree in the middle. If someone were to repeat our walk along Cricklepit St now in late July, they'd find many of these plants have been chopped down and strimmed away: no more bees, spiders or Hummingbird hawk-moths. It would need only a small adjustment in attitude and management - and it should be a cost saving - to cut less often, and allow a few clovers and trefoils to flower in the lawn for the bees and butterflies, instead of everything everywhere having to be mown flat to a green desert.