I think I've mentioned before that I've been experimenting with extension tubes when shooting some of my flower images. One thing about using extension tubes is that there is less light coming in to the camera compared to using the same lens without any extension tubes. (Stick with me, I don't speak "technical" when I talk about shooting pictures!)
Shooting with the aperture wide open results in the very shallow depth of field you see here. (The alternative is to use flash which isn't something I like to do when shooting flowers in a natural setting.) Yarrow is a perfect candidate for shooting with extension tubes because the cluster of tiny blossoms is nearly all in the same plane, making it easy to get a nice sharp image of the flowers with everything else "fuzzed" into the background bokeh.