Lighting issues with billboard sprites in 3d

So I am currently using billboarded sprites in my 3d environment for grass detail on a flat piece of terrain. But I run into this very ugly issue when my light starts to wane (I plan on having day/night cycles):

I know why it happens, since the sprites are using "Sprites/Diffuse" and they are lit depending on what way they are facing, but how can I make them match the light level of the terrain (aka a face pointing upwards). My options seem to be no lighting at all, or this. How would I work around this?