Answer: Your yellow golden Sienna is Cassia splendida 'Golden'. It isn't a good idea to raise the soil level over tree roots - they can suffocate if you change the grade. Also, if you put soil against the trunk of the tree it will hold too much moisture against the bark and eventually cause rot. I'd move the tree rather than changing the soil grade. Since you've already build a planter box, you might want to use what you have as an inner wall and extend the planter out by a couple of feet by building an outer wall. Then fill the area between the two walls with soil and plant in there. Your tree will be much happier!
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