You've done a good job of minimizing the background already through proper focus, aperture selection and exposure. I would probably open this image up in your editing aplication and using a tool like the Burn brush in Photoshop, darken that one green spike which sticks up in the middle. Other than that, there's not much else to do; removing the curly thing and that fern frond going out the lower, right-hand corner would have been best done before the image was captured.