Hi ,
Thank you for your input and time. Unfortunately, erasing the foliage manually doesn’t seem to work very well because it comes back if it’s ever necessary to re-run the simulation.
As this is the intended behaviour, can I change this bug report into a feature request? 
Perhaps the user could be given the option of disabling spawning on static meshes (and BSPs) at the same time as restricting the foliage to particular landscape layers.
By the way, I’m finding the procedural foliage tool really helpful, so please don’t think this is a general complaint. Even with the need to create blocking volumes, it will save me a lot of time, and probably give better results too.
Pete