Cant figure out the name of the movie?!

it came out in theaters awhile ago but i cant remember the name!. Having a blonde momment!. anyways its about where trees and stuff are making people act weird which makes them eventually kill themselves at the begining they are in a park sitting on a bench and a girl stabs herself in the neck does anyone know what its called it would really help meWww@Enter-QA@Com

---

---