I want to plant a defined area of wildflowers, but as it is almost mid-June, I wanted to plant in a raised bed to eliminate the need to till the soil of the existing lawn. I was thinking of putting a barrier down (over the grass) and then the black dirt/amended soil, to a depth of about 6", but I don't know if the barrier will prohibit root growth to sufficient depth.
Planting Wildflowers in a Raised Bed
Certified GKH Gardening Expert
If you used the right kind of barrier, this will not be a problem. Consider using cardboard or thick layers of newspaper as your barrier. This way, the barriers would kill the grass but would decompose over time and would no longer pose as an issue to the roots of the growing wildflowers.