I have been playing with inserting plant populations such as some free Xfrog ones. But the plants look fake, almost cartoony compared to the rest of the scene. I'm not sure if it is the lighting being wrong or the coloring of the objects. I can get decent realism otherwise but when I insert plant objects they just don't look real. Is there a good tutorial out there?
Do you have a sample image to show us what's going on?
Try playing with translucency of leafs shaders (at last at 1) and reflectivity (eg. 0.1 to 0.4). You can also tint the reflections a little into green or yellow.
I found Tangled-Universe's advice useful in this message: http://forums.planetside.co.uk/index.php?topic=6019.msg64098#msg64098