Speedtree while no doubt fantastic as I’m sure a lot of idies and others use it, isn’t free and prob. out of range for most of us.
Having said that, and unless I missed something, you can go here ( I just found it by researching this topic to verify ) :
I have no idea, yet, as to the 'quality ’ of this clearly generous free product, but if its not workable , you can always try something like this:
THis one is newer: https://www.youtube.com/watch?v=xmQlcAcLOaU
Experimentation is needed as free or not either way, I’m not sure how much flexibility either product offers, but GL!
I"ll be needing trees , as of now myself so good luck to both of us 
Cheers
lee