Trees glowing in the dark

Hi guys

Have a scene where there is no light source and i have lightning striking randomly. I’m using the free unity mobile trees. I want them to be pitch black till the lightning strikes, but no matter what i try they light up in the dark ie you can see them. I tried different materails but it just screws up the trees by making them too big or blocky looking. Really dont know what to do. please help.

Any ideas?