Is that what the cardboard part of the hugelkultur method is for? Thats what they recommend to fill these with.
These are pressed down to the bottom of the sod level, and I might even be removing the sod in between later, so I wasnt overly concerned about it, but great to know though!
In my experience, the grass will inevitably start growing up from the bottom. This will be our fourth year with 12" raised beds and I'm having to dig up grass roots around some of the edges of three of them.