Hawaiian Tropic Moss

There are three words that you are unlikely to see together in the same sentence ever again. What is my excuse here? There is a well-known relationship between sunshine and moss or lichen growth, but there are subtler, more interesting ones as well. I have lost count of the number of people who have confidently told me that moss only grows on the north side of trees and buildings. I have dealt with that partial myth elsewhere, but here I want to explore a rarely-noted trend.

In England and large parts of northern Europe mosses and lichens will often grow on the south side of trees and buildings, but they will nearly always display differences to nearby brethren growing on the north side. The crucial thing to remember is that we are dealing with trends and patterns when observing nature, avoiding rules wherever possible. The navigator who tries to live by the rule that moss only grows on the north side is likely to find themselves lost and bedding down on the stuff before too long.

The two pictures above show roofs on either side of East Street, Chichester. There is lush green moss growing on the north-facing roof, but if you look closely at the second photo you will see a brownish ochre growth running along the top of the south-facing, very sunny, roof tiles. This colour difference is a pattern that is quite consistent and satisfying to spot. The way I like to remember it is the ‘Hawaiian Tropic Moss Trick’, if you find what looks like normal moss that has has gone south with a bagful of lotion and lain on a bed in the sun for too long then the chances are it is south-facing. Beware though, this works well on buildings, but on trees the colour patterns change again. It will be their turn next week…

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