maybe you could spread a few human fingers around the base of the plants...or just some evil looking shit, like candles in a circle and a dead chicken...or better yet, half a dead chicken...not be confused with a half dead chicken

. paranoia can be your friend, and ALOT of ppl are superstitious, if you do leave a little sign, write it in blood on a rusted old piece of metal and quote some evil sounding scripture..don't forget a pentagram, and maybe a bloody condom. that's just off the top of my head though...maybe you can do better
I mean seriously..if you came across some shit like that in the woods would you go back there? how bout...after dark?