Our winter weather tends to fall into one or other of two broad categories: damp and mild, and, dry and cold. So far this winter, the damp and mild option has tended to dominate and one the downsides of that is that the external walls of homes and businesses are becoming coloured as they support growths of various different life forms.
The colours vary from green to pink to black and the life forms that cause them fall into three main groups: mosses, moulds and algae.
Mosses are the easiest to identify as they are bright green, usually appear to be leafy and have tiny root-like structures. Mosses don't have woody tissues to hold them up so individual plants often gain support by leaning against their neighbours resulting in the familiar cushions with the tallest members of the group in the centre and the smallest around the periphery.
Mosses need a constant supply of water or dampness to survive and elevated temperatures promote growth so damp and mild weather suits them resulting in the present eruptions on walls and in lawns.
Moulds and algae are more difficult to identify. None is leafy and none has roots. They stick themselves to damp surfaces and very many of them are of a size that is beyond the capabilities of our unaided eyes. While a microscope is needed to see how their bodies are structured, we can see that they are present by their colours. In general, moulds tend to be black or grey and their surface tends to feel dry to the touch. Algae tend to be green or pink and to feel damp and slippery.
Algae are remarkable plants in that they can grow in very challenging environments like on the vertical surface of glass in a greenhouse. Individual plants stick themselves to the glass. They absorb water and minerals from rainfall and/or damp air and use these and sunlight to make their own food, grow, reproduce and spread.
Most people are more interested in how to get rid of them than in the details of their lives. The twofold time-honoured methods are physical and chemical. Physical methods involve scrubbing by hand if the area is not too large or power washing to blast the offending organisms off the surface they are sticking to.
Chemical methods involve washing with bleach or any of the host of proprietary products available in any good hardware store of garden centre.