Answer: Unless the roots are escaping through the drainage holes of the pot, it probably isn't necessary to repot your purple fountain grass. Wait until December when you plan to plant it in the ground. Sounds like fertilizing is right in line with the needs of your ornamental grass and watering may be adequate - plants in pots tend to dry out faster than the same plants in the ground so pick the pot up every few days; if it is light, it needs water; if it feels heavy, it can go a few more days without water.
Best wishes with your landscape!
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