PAINT SHADOWS
If you want to make the composite look more realistic try painting shadows where flowers overlap. To do this, simply make a new layer above the flower layer, then hold Alt and click the line between the two to clip the new layer to the flower layer below (this means that your shadow will only show directly on top of the layer below and nowhere else). Grab a soft-edged Brush tool, set the colour to black and brush opacity to 30% then paint to add a shadow over the flower.