Try using cheap biological washing powder.we use the big own brand ones from either Tesco or SuperValu. Shake it well all over the yard and dampenit with a small amount of water and scrub it in well then power wash it off. Repeat as necessary and give it a final rinse with cheap plain bleach and sweep away
Lasts up to 2 weeks in summer and abut a week in winter. Just have to keep on top of it then
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