In the Pacific Northwest - cedar deck: Every year clean...packaged mildew solution or diluted bleach. Protect your plants. Scrub with a stiff bristle push broom. Rinse. Let it dry. Touch up/re-apply deck finish as needed. Then, every 2 years, mildew treatment then strip the old finish - that stiff bristle push broom is the ticket. No pressure washing - raises the grain. Refinish. And every 7, 9 years (climate dependent) mildew treatment and strip - then mechanically sand (ok, 9 years) and re-stain/refinish. Yes, a cedar deck is work...and beautiful. I had a cedar deck for 25 years in a temperate rain forest, replacing three boards due to mildew damage. No maintenance, no deck!