I've got a few trees to spawn in the Nether, but they won't use netherrack as the base block. I've spawned them on the upper bedrock layer as well as on magma blocks, but they just don't seem to take to the netherrack itself. I tried turning off any of the mods I may have that alter the Nether, but that didn't change anything.

I surprised you get anything at all spawning in the nether. Are you seeing trees spawning on bedrock/magma with just treetweaker alone or only with other mods?

Yeah. Literally with just Crafttweaker, TreeTweaker and nothing else installed, it seems fine. Just need to target biome "minecraft:Hell" and whitelist dimension -1.

I didn't know it wasn't expected to be used in only dimensions other than the nether? o.O

EDIT: It doesn't seem to like soul sand either. Though, it's fine with Nether Bricks for some reason.

Wow totally forgot to set the target biome in my tests. Glad someone around here knows how to use this mod :P I've added some stuff to better pick blocks in the nether so you can have nether trees next update.

Sweet, thanks man! :)