I would say its not been photoshopped, if it has its been done very well. That is not to say its not been faked...but I dont think any trickery was done in post....its not impossible but it would have taken someone very skilled and taken some time to do.
If I was 'shopping this, just to make life easier I would reuse, some of the mini sunflowers round the edge, maybe flipping them/rotating/warping them to hide the fact i had done this.
Each mini flower looks different so to shop it, he must have loads of sunflower photo's rather than a few.
If he had taken stock photo's for the mini sunflowers, there petals would be normal...IE not pushed forward like the one in the pic, again to fake he/she must have manually pushed forward the petals on a load of different sunflowers in order to shop them in...alternatively he could have warped/puppet warped/drawn the petal pushing forward...but this would have been loads of work..
In summary no real way to tell if photoshoped, if its done well, but given the amount of work i would think not.. still never say never