Green scum on your patio umbrella could be caused by mildew, mold, algae or a combination of all three. This scum can discolor the umbrella as well as cause a nasty, unwanted odor. Household cleaners may temporarily remove the problem, but the green film is likely to return to the umbrella. A homemade cleaner that includes a common household liquid eliminates the scum and prevents its return to the patio umbrella.
Mix 2 cups of undiluted white vinegar, 4 cups of hot water and 2 tbsp. of non-bleach dish soap inside of the bucket or sink.
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Open the patio umbrella fully.
Saturate the bristles of the brush thoroughly in the cleaning solution.
Scrub areas of the patio umbrella covered in the green scum. According to Reader's Digest, scrubbing other areas of the umbrella will prevent such growths. Use a clean brush (one not used to remove green scum) to scrub the rest of the umbrella.
Repeat steps 3 and 4 until you've finished cleaning the umbrella. Leave the umbrella opened in a sunny portion of your yard or patio until it has fully dried.