heyas; i havent messed with this, so i am guessing, here. the glow mainly applies around edges of an object, and in your alpha planes, the edges (of the plane) are hidden. maybe if you tried using a bit map material with glow on it, and mix it with a plain air shader, it might work better? hmmmm... dunno. i would guess that the preview is somehow mis-calculating the material size. probably a bug e-on should investigate. meanwhile, you can always do a quick preview render of your scene (or a patch render of the particular area) to see where your stuff really is.