Which mosses are commonly found on sidewalks and other concrete structures?

Where were mosses when concrete was invented by the early Romans?  Were their roads covered with green?  Well, we don't know, but in moist climates moss are common residents on concrete.  Most of these species originally came to our cities from areas with outcrops of limestone. Concrete, which is made of cement containing limestone and clay, combined with aggregates such as crushed stone, sand, and gravel, provides a calcareous substrate attractive to some mosses.

But only certain moss species favor concrete. Which ones grow on concrete sidewalks in the Pacific Northwest? 

We identified all mosses found at each of five central sites in the city of Corvallis in western Oregon, latitude 44° 34' N, longitude 123° 15' W, elevation about 61 m. (200 ft.), and annual rainfall 101 cm (40 in.).  Study sites included two residential sidewalks, a small apartment complex (walkways and steps between buildings), sidewalks at the downtown post office, and the plaza between Central Park and the Corvallis Arts Center.  Representative mosses were taken to Oregon State University laboratories for identification to species.  Voucher specimens are stored in the OSU Herbarium.

For results of our study, click here.

The sites were chosen for their relatively luxuriant growth of moss.  They varied in percent shade, pedestrian traffic, and associated vegetation.  Except for accumulation of soil in spots, all substrates were concrete, with varying sizes of aggregates (largest at the apartment complex), and all were solid, with one exception.  At the Corvallis Arts Center, curvy concrete tiles form an attractive, patterned surface with green moss growing in the spaces in between.  The moss serves a useful purpose on this sort of walkway or plaza, as it naturally creates a clean, cool, and aesthetic filler and soil holder between the tiles.  (Photo: Barbula vinealis forms a reddish green carpet on a little used sidewalk. When moist the carpet turns a deep green)


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