Custom trees on custom grass

This is driving me crazy!  In my mod I am creating a new dimension; it has nether-like gen, and the only biome in it is a custom biome I created.  The custom biome's ground block is a custom grass block I made - currently almost identical to grass, except its just not actually grass and it has a different color.  Since it seems like custom trees can only spawn on normal grass blocks, and not user defined blocks, I opted to make a few custom structures and put them in to function as trees.  Currently I have them restricted to only my custom dimension for spawning, but I did try restricting them to both the dimension and biome, and tried restricting them to only the custom biome.  When I restrict to block type [my custom grass], they do not spawn at all - I tried increasing the spawn rate very high and I still did not see them.  When I leave no block type restriction, they spawn floating in the air or going into the ceiling, not at all the kind of spawn I want.  I did try to put the offset to -1 when it was still restricted to only my custom grass block and I saw one tree, but it was lodged in the side of a cliff two blocks down.  Any ideas for this?