Hard to say what might be causing it. Could you try opening a plant that works and copy it’s material then paste it into the plant that becomes black and use that material temporarily just to see if it’s still happening? I’d imagine it’s something related to the material but it’s hard to say without access to the model. If you don’t mind I could take a look at it if you upload the Speedtree file and it’s dependencies somewhere and send me the link in PM. Just let me know. 