I haven't tried this myself yet, but have heard it works pretty well: screw a cuphook onto the tip of a wooden broom handle, use the hook to scrape the junk out from between the boards, then just sweep it up.
I would clean the cracks,as best as I could,with a garden hose and a stiff bristled brush-an old broom works well-You don't want cracks between the deck boards.[expansion/contraction pulls the nails loose]
How about pulling up the boards and installing them with wider gaps? That would be the proper fix. I did remove a bird feeder on my deck once because it loaded up the gaps too much. BTW, too much high pressure washing will roughen the wood and make it splinter more easily.
You might need to buy or rent an actual power washer to get them clean. Also, there is a product called "OxiOut Algae and Mold Remover" that can be applied with a sprayer or a brush. I know that Improvements catalog carries it for $17.99 for a 64oz bottle of powdered concentrate that makes 10 gallons of cleaner, but you can probablly find something similar elsewhere. Also, you can use a bleach solution to clean it (DO NOT mix with the commercial cleaner) and wash it off with the hose or you can actually use a bleach water solution in the pressure washer.
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