You can only assume the grass is sticker free. Not that I have ridden across grass and got a flat or anything. And IF I did, I would have meant to #1 - ride across the grass in the first place and #2 Get a big old flat from a sticker (goat head/puncture vine).
They aren't mountain bike tire tough.

Like you new avatar. You like to keep us on our toes, don't you?