grass spawning under floor

[Robert Hanrahan]

What stops the grass from growing through my base?

I built a 6x6 base mounted 1 level up on 4 Scaffolds in the corners. (was surprised that there was no structural integrity)

The area under my platform is open. Do I need to place something under there( like maybe clay platforms?) I suppose I could move all my plank and log storage under there but there is not 32 of them.

 

Thanks in advance