QUESTION

Our dock, the steps leading to the dock, and even parts of the boathouse have mosses and lichens growing on them. Is there an environmentally friendly way of killing these growths and preventing them from coming back?

ANSWER

Mosses and lichens are hardy—that’s why they’re all over cottage country—and the only earth-friendly way to get them off your wood is elbow grease. “A good old-fashioned scrubbing brush and plain lake water,” suggests Paul Morris, group leader for durability and protection at FP Innovations, the forest products research institute. Unfortunately, this won’t keep the mosses and lichens from returning. Rotting wood—like the kind you might find in an old dock—is “a perfect substrate for mosses,” explains Dr. Irwin Brodo, lichenologist emeritus at the Canadian Museum of Nature in Ottawa. “And the lichens are growing on the wood simply because the wood makes a good place to live.” Both lichens and mosses grow on healthy wood too, so their presence doesn’t always mean your dock or boathouse is rotting. And the growths won’t cause your wood to rot, says Morris, but they absorb moisture, making rot more likely. Also—wipeout alert—lichens lead to slippery surfaces. “Wet lichen can give way like a banana peel,” warns Brodo. (If you stepped on wet moss, you’d likely just dislodge it.)

Lichens grow at a molasses-like rate, so once they’re scraped off, you won’t need to scrub again for a few years, says Brodo. But in moist, sunny, and warm conditions, mosses grow faster, and might need removing once or twice per season, says Jennifer Doubt, chief collection manager of the Botany Section with the Museum of Nature. Doubt suggests making the wood less hospitable to mosses and lichens by keeping it drier (trim back trees, for example, to allow more sun on the area), or replacing rotted boards.

* Published in the Summer 2009 issue of Cottage Life