Answer: In general is is better to leave ample room for the tree's roots to become established without competition from other plants; dogwoods are also rather shallow rooted and particularly resent digging around their roots so I would suggest you proceed with some caution. For this reason I would suggest a simple planting of a groundcover or even leaving the area mulched. If however you are determined to plant around it, why not simply adapt one of the rectangular designs you like. Do this by pulling the plants for the "back" of the bed into the middle of the circular area and putting the plants for the "front" of the rectangle around the perimeter of the circle.
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