For the grass to restore, the blend mode needs to be modulate (it multiplies the values), position (0.0) and the color something like this: 0.8, 0.8, 0.8. That way every time you call this function it will multiply the color values by 0.8 lowering them. Call that function for example once per second with a delay node and viola! Your grass is restoring =) you can make the color 0.9 to make the process slower.
