It seems there is this old feature request but nothing much
http://sourceforge.net/p/sweethome3d/fe ... uests/139/

Designing isn't the main feature of Sweet Home 3D until now, and designing trees is complicate.

is there a way to just use a 2D plan w a texture to represent it ?

You can do that with a PNG image using transparency, that you'll import as a texture and apply on a thin box.