I think the first question is: what are the on-site composting practices, if any?
Nitrogen-heavy "green" matter left in piles or clumps without a corresponding carbonaceous "brown" matter to help break it down will not be composting or decomposing, but rather rotting or putrefying. This process releases methane, which gives off odors, even subtle ones, and is also a 200x more powerful greenhouse gas than CO2.
These odors will attract the pests you mention, known in composting terms as "vectors".
If there's often clumps of weeds or vegetable trimmings "breaking down" in official or unofficial "compost piles", my suggestion would be to maintain a small pile of arborist woodchips on site that can be mixed together with the green materials as they are pulled from the community garden spaces so they immediately start to decompose rather than putrefy.
Hope this helps in some way! :)
Sean
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