Actually, we don't leave everything - ivy, bramble and bamboo are taken to the recycling centre ... a tiny piece of any of those will just start growing. I try and get weeds before they flower but other than those considerations, I was carting bagfuls of green waste to the recycling centre which just seems silly now. There's lots of leaf waste and right now most of my weeds are tiny tomato plants (thousands)!
I guess community gardens have to be more draconian when there's many gardeners using the same resources and not everyone may have the same understanding. I've had to teach my apprentice gardener to pick out the bramble and ivy (and not fling pieces behind him all over the garden) - "work tidy, work safely"I keep saying (he'll start teasing me soon). He's done a fab job, though, and he's dead pleased with himself!