Foliage and lightmap issues

I added some foliage to my scene, but it caused some problems with lightmaps.
There are some weird black spots, which I suppose are caused by the foliage. How can I prevent it? I tried disabling “Casts shadow” option but there was no improvement.
When there is no grass on the surface of the ground there aren’t any problems and the surface is perfectly shaded.
Hope someone helps :slight_smile: