Moss, mould or lichen — which one have you got?
Why it matters
Because it decides how long you will be waiting. The treatment is the same; the timeframe is not.
Moss
Green, soft, springy, grows in clumps. Lifts off in a mat when you scrape it. Loves shade and damp — south-facing roofs, the shaded side of a path, under trees.
Treated: days to a couple of weeks.
Green mould and algae
A flat green or grey-green film rather than a clump. Makes concrete slippery. Turns up on paths, decks and north walls that stay damp.
Treated: days to a couple of weeks.
Lichen
Flat, crusty, circular patches, often grey-green or pale, sometimes with a raised edge. Welded to the surface rather than sitting on it. Common on roofs, concrete tiles and stone.
Treated: months, and up to six for a bad case.
If you can scratch it off with a fingernail, it is moss. If it feels like part of the surface, it is lichen.
Black streaking on a roof
Usually algae, often running in streaks down the slope from flashings or ridges. Clears on the shorter timeframe.
The practical upshot
Most sites have a mix. The path transforms in a fortnight and the roof takes until spring. That is normal, and it is the single most useful thing to know before you start, because it stops you respraying something that is already working.
Frequently asked questions
What is the difference between moss and lichen?
Moss is green, soft and sits on the surface. Lichen is flat, crusty and bonded to it. Moss clears in weeks; lichen takes months.
What is the black stuff on my roof?
Usually algae, typically streaking down from ridges and flashings. It clears on the faster timeframe.
Does the same product treat all of them?
Yes. One concentrate handles moss, mould, algae and lichen. Only the timeframe changes.
Why is the moss gone but the lichen still there?
Because they die at different speeds. Moss goes in weeks, lichen takes months. Nothing has gone wrong.
Not sure what you are looking at? Send a photo to ben@purplestrike.co.nz.
See also how long treatment takes to work.
