Terrain and Glow Effect = Glowing Trees

I've got a scene with a terrain and a number of trees. I'm also using the Glow effect on my camera.

When at a distance (shown as a billboard) the trees are fine. But when you get close and the 3D model appears, they glow brightly because of the Alpha values in their shaders.

Is there a way to either turn down the Alpha amount on the various colours used in the optimised materials (changing the alpha of the base material doesn't seem to have an effect - the optimised version always has 100% alpha), or is there a way to alter the basic tree shader so it has a COLORMASK RGB statement to prevent it writing Alpha values to the frame buffer?