You could try the hue/sat palette click the colour picker tool and click on the colour and drag down, trouble is it's very similar to the flower colour so it may affect that as well. Theres also the local ajustmet brush, set it to saturation (minus) and carfully bruch over the background flower.
EDIT. Sorry for some reason I thought you were using lightroom.
For photoshop I'd select the area you want to ajust then on the hue/saturation palette simply lower the saturatuon a bit