It's been quite a while since we've featured a lichen on the blog. This one is Parmelia sulcata, sometimes known as "Hammered Shield Lichen" or "Waxpaper Lichen" because of its appearance. Dobson's Lichens field guide says that it's "grey to glaucous white" in colour, but it always looks distinctly bluish-green to me. The brownish tips to the lobes, which are ridged and overlapping, and the network of faint, white lines along which the soralia develop are distinctive. (Soralia are the structures that hold the soredia: bundles of fungal and algal cells which break off to form new lichen colonies.) And it really does look like a bit of hammered metal. The underside is dark-brown to black and covered with a mat of rhizines, which are the fungal filaments that help to attach the lichen to its substrate.
There are quite a few colonies sprinkled around the garden, mostly on the Japanese Cherry tree and on the slowly-rotting wooden garden bench. This lichen, which is a great example of a foliose species (the name given to lichens that are flattened and leaf-like), is common species throughout much of the world. It grows abundantly on trees and rocks across most of Britain and Ireland, particularly in the lowlands. People in various parts of the world have used it to make dyes (it produces a reddish-brown colour) and medicines.
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