I'm stuck on the Burn It Down mission. I've killed all the guards, blown up 3 sprinklers, burned 9 drug piles and 3 fields. I can't seem to burn the last field??

The field I'm talking about is the one below the central house, with two rising walls in front of it. If you're looking out from the house there is a big gate at the end of the road to the left. I think it had two red fire barrels in it, but I blew them up too. I've dropped 6 molotovs on it and hurled fire all over it but can only burn a small patch in the corner. Is there a hidden sprinkler for this (though without any guards alive I'm not sure if they activate?)? I've looked at walkthroughs and people just shoot the barrels but it didn't work for me!essay writer