If you're feeling masochistic and have some time on your hands, try the accurender plant editor.
It can be hard to find, hence this tip.
Open one of the standard plants and go to Properties-Edit, click in the Plant Name box and a drop down menu appears for the plant library showing thumbnails and season appearance. Right click on a thumbnail, go - New, Use current plant as template. You are now in the plant editor, and pretty much on your own from here. You can add/edit tree structure, branches and leaves. Dig deeper for geometry, branching and settings.
AFAIK there are no help files / chewts dealing with any of this within revit help. The accurender site is also pretty well mute about it too.
I believe there is also some way to change the the leaf brightness in these settings, ( a known rendering problem ), but I havn't been able to do that and rely on Photoshop for such tweakings. I suspect its necessary to download additional accurender materials for plants into another library, to change the leaf colours.