Question: when did seattle grace and mercy west merge

title: Invasion (Grey's Anatomy)
text: `` Invasion '' is the fifth episode of the 6th season of the American television medical drama Grey 's Anatomy , and the show 's 107th episode overall . It was written by Mark Wilding and directed by Tony Phelan . The episode was originally broadcast on the American Broadcasting Company ( ABC ) in the United States on October 15 , 2009 . In the episode , the physicians of Seattle Grace Hospital must learn to co-operate with new Mercy West residents , who do not act in a kind manner to them . Other storylines include Dr. Callie Torres ( Sara Ramirez ) 's father returning to the hospital to condemn her homosexuality , and Dr. Izzie Stevens ( Katherine Heigl ) putting her career into jeopardy after administering the wrong treatment to a patient . 

Answer: answer: `` Invasion '' is the fifth episode of the 6th season of the American television medical drama Grey 's Anatomy , in the episode , the physicians of Seattle Grace Hospital must learn to co-operate with new Mercy West residents.
answer: In the `` Invasion '' episode, which is the fifth episode of the 6th season of the American television medical drama Grey 's Anatomy, the physicians of Seattle Grace Hospital must learn to co-operate with new Mercy West residents.