How can mould be removed from rough cedar without using bleach?
Don’t worry about how bleach will affect your wood, because bleach isn’t recommended for removing mould anyway. Never mind the unpleasant and also unhealthy fumes, bleach is just not very effective. With a small area of mould (according to the Canada Mortgage and Housing Corporation, a “small area” means less than three patches, each patch less than one metre square), it’s best to just use good old-fashioned soap, water, and a brush. Sponge off any soapy residue with fresh water and then let the cedar dry. Larger areas of mould may require a more thorough cleaning: Use a vacuum with a high efficiency particulate air (HEPA) filter to first remove the bulk of the spores to help prevent the mould from flaking off and spreading when you scrub. For health reasons, with large infestations of mould, scientists say you should even wear protective gear: goggles, a dust mask with a N95 rating or better, rubber gloves, and long sleeves.
But back to the pretty cedar walls: gently cleaning with soap is okay, but harshly scrubbing walls will damage the stain and colour. Once the wood is clean and dry, touch up the area with a matching stain, if necessary.
Many stains contain fungicides, which—bonus!—will help keep mould at bay. Make sure the walls stay dry and the humidity in the cottage stays low. If mould does return, you may have a dampness problem—a wet crawl space, leaky roof, or significant humidity—and should get professional help.