Short version: They pump CO into the holes to kill the rats.
I had to look up "tree boxes", apparently it's the name for the slightly-sunken patches of soil for plants and trees built into the sidewalk (usually between the sidewalk and the curb).
Would dumping some chunks of dry ice into rat-holes generate enough CO2 to accomplish this?
You need co. It’s leathal because it binds to your blood cells and prevents you from using oxygen. So you cash accumulate it in your blood and you don’t need immediately leathal doses
A buddy of mine used his Harley to get rid of moles in his yard using this technique.
I wonder, how it smell now, from death decaying corpses.