Bernard Wilkinson of Oamaru asks :-

I have a newish garage with colour steel roofing. On the south side a lichen is starting to establish. the 8 to 20 cm splotches are roughly circular and are whitish. What is it and can it me removed?

Jennifer Bannister, an independent botanist, and Allison Knight, an independent lichenologist, at Otago University, responded.

This lichen is probably Xanthoparmelia scabrosa. It grows naturally on rocks throughout New Zealand, and has adapted to grow on man-made surfaces such as tile, steel, concrete, glass and bitumen. It has even been found growing on old cars. Another lichen often found on roofs is the orange Xanthoria parietina.

The body of these lichens is a thallus made up of fungal hyphae enclosing algal cells that can photosynthesize, thus providing the fungus with carbohydrates. This association between fungus and alga is a symbiotic relationship, which enables both partners to colonise habitats that neither could survive alone.

All lichens need sunlight and moisture to survive. X. scabrosa tends to grow best on the shady side of roofs and foothpaths, where moisture lingers. Lichens grow much more slowly than plants. The thallus of X. scabrosa growing on glass has been found to expand by 16 mm per year. Where the growth rate of a lichen is known, it can be used for dating past events such as the retreat of glaciers, the age of a landslide or even the age of stone sculptures on Easter Island.

Lichens are sensitive monitors of air pollution, such as sulphur dioxide produced by burning coal and oil. When pollution is reduced, lichens will recolonise inner city ‘lichen deserts’.

Lichens reproduce by releasing many small, light spores or asexual fragments that can be blown by wind, which explains how they disperse so widely. They can be very firmly attached to surfaces by their fungal hyphae. To get rid of your lichen you need to kill the fungal partner by using a fungicide or a solution of bleach. This will leave an ugly dead lichen which will need brushing away. It might be easier to leave the lichen as it will not penetrate or harm the Colour Steel roof.