He wasn't even originally in the script. He just showed up one day dressed like a tree yelling "I am Groot!" over and over. Everyone was just too afraid to tell him to leave so they made it work with a little "Hollywood Magic".
I heard he's actually planted knee deep in James Gunn's backyard. In the last few F&F movies, Dom Torretto is actually played by Sean Gunn in a baldcap and touched up in post.