When you are doing a render that does not have many faces in front of ZTransparency planes, this is not an issue at all, but when you're talking trees or even city scapes, you have a serious issue on your hands.

The reason I don't use RayTrace for transparency that is not refracted is that:
1) ZTransparency is faster
2) ZTransparency does not darken it's background or act funny with procedural textures behind it

Be that as it may, it does create that funny outline, but it doesn't have to! Monday, GOD showed me something very cool, while working on the Esther project, and that is that if you use Full Sample, under anti-aliasing, you rid yourself of that horrible outline.

There is a catch though: It means that Blender will composite for every anti-aliasing sample. That means, at the lowest possible setting, your compositing will be run 5 times.

There is good news though! You can keyframe the Full Sample option to only use it when applicable in your animation. If you choose to follow this route, I strongly urge you to not only apply the speed ups in the previous post, but also to use OpenCL, Buffer Groups and Low compositing quality in your compositing to make things as fast as possible. Just by applying those techniques, GOD brought my render down from 5:10 (m:ss) to 3:17 (m:ss). About 1:15 (m:ss) of that is the 5x compositing and we're talking FullHD here. That is not only fast, it is a huge increase in speed - almost half again!

I urge you, check out the post and apply it to your renders if you do animation. Like I told a friend this morning - animation is great, but it still has to be practical. Those render times need to be as low as possible without compromising excellence in your work.

Have a great one and GOD bless!!