High exposure areas like super-strong reflection of sundisc always tend to create sharp edges even in Blurred areas, as the highlight area is many times brighter, not just clamped white, so it creates sharp fringe, like on this photo:
To increase clarity of DoF, you can decrease PT samples, that means less samples will go to the secondary effects (reflection, refraction, translucency, GI and such), and more of them will go into primary effects (antialiasing, DoF and Motion Blur). So try set PT samples to somewhere around 2 to 4.
Also, people often create a bit more reflective materials than they really are. So make sure your grass does not have too high reflectivity, or too high glossiness. Grass is not as reflective as it may seem, unless it is wet after rain ;)