Answer: This is a very small space to plant into. The root competition from additional plants will not do the tree any good.
It would be best to mulch over the area with an organic mulch about three inches thick. Keep it several inches away from the bark of the tree. The mulch will break down slowly over time and help feed the soil.
Also consider a small fence to keep dogs and foot traffic off the tree's immediate root area.
If you really want to plant something, you could do a sun loving groundcover such as a low growing sedum, creeping thyme, or similar undemanding plant.
I'm sorry I can't be more encouraging.
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