You can not. It works just like the ore spawn location in the 4-Corners which I have a lot of experience with. That terrain is not tillable soil, which is what is required for grass starter.Now, I haven't tried this so I don't know if you can place them there, but you probably can.
Flooring is your best option if you insist.Is there a way to remove ore respawn in the Hill-top farm? I'm using this place for my animals and I don't want to clear it all the time. Thanks!