A mosquito net, well that could do the trick, but it has to be really fineley woven, cause pollen are real small.
I allready saw a guy use a cotton sack to isolate a branch, but he had to remove it regulary so the branch doesent die off (asphixiation, CO2 production during night time, lack of O2 ...).
Maybe others have better ideas, which are very welcome as I'm also interested in this stuff!
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Greetz,
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Oli