help plantoris plant

can anyone explain the auto-plant feature to me?
I simply want to auto plant trees so I don't have to do it manually. I cant seem to figure it our for the life of me.

I edited a fir tree and set it to auto-plant

then I tried to add it to my terrain settings and it tells me it cannot be used as auto-plant

what am I doing wrong?

[afro85]

I tried the same but you can only auto-paint the small grass available

I got it to not give the error msg but no trees