I cant make my tree features spawn on grass only

I have a custom dimension thats like this big cave dimension(nether type with giant mushrooms and redwood trees(all features)) and although the giant mushrooms spawn normally (grass only), redwoods spawn in water and ontop of the mushrooms. I cant find a way to stop that. ANY help with be much appreciated, even if you arent 100% it will work, help because i need this to work asap