The barrier – which you’ll see if you look closely – is crumbled Host, also used to vampire-proof dirt in Dracula.
Bunnicula was a cool book. Apparently there was a whole series, including lots of gimmicky things like a joke book and a pop-up book, but I can only vouch for the niftiness of the original story.
Seras: There he is. Walter!
Walter: Ah, Miss Victoria. Good morning!
Seras: Good morning!
Walter: Careful — don’t get too close. There’s a barrier to vampires around the garden.
Seras: Okay. Um, why?
Walter: To keep undead rabbits from getting at the vegetables. Didn’t you ever read Bunnicula?