The center is in running stitch, the inner layer is in back stitch and the outer layer is in whipped running stitch. My back stitch is not perfect - most of the time I miss going in back at the same place and it leaves a teeny weeny gap between the stitches. That's something to be careful about next time.
I like the whipped running stitch for its simplicity and the contrast effect it produces. By the way, you can whip stitch other outlining stitches too.
I also tried out the double back stitch for outlining the center of one of the flowers.The double back stitch is usually used in shadow work to produce a beautiful shadow effect.