Map Generation "place trees" placing non-trees.

[BlueOysterCoaster]

Tried using the map generator, and had it place trees.  Only problem, it chose some non-trees.  First time, it littered my map with footballs.  Went back and removed the footballs from my scenery, and regenerated.  This time Suspension bridge bases all over the place.  Went back, got rid of the suspension bridge pieces.  Generated again, prison watch towers.  How do I get it to, you know, actually use just trees when I tell it to place trees?

[shadyback]

I've not had this exact issue, but when I've tried this tool in the past it simply didn't place anything (trees or other scenery items). I don't try it much, anyway though, because I usually use the scatter tool with a single tree selected.