You can create leaves using transparent textures with only the leaves,branches showing.. This is the method I used to create realistic trees in my map CTF_Wild_Snipe.
Use a Face brush and then copy it and rotate it so you get a cross so that it becomes viewable from all angles.
Here is what It looks like in game..
Of course you can get much different look by using different textures..
I got mine from RedFactionView Custom Trees download.
Well the least you can do is show us a picture to help explain what you want.. I cant help you anymore right now because I am leaving the computer for the day.. Cya