Answer: Since you've been giving your plant such good care I suspect that it is only now showing some signs of transplant stress. The browning foliage is probably a reaction to the change in exposure to sunshine that it is getting now that you have planted it as opposed to what it was getting in the garden center. I'd prune off the affected stems and leaves. It should replace the pruned off areas with healthy new growth. When that happens you'll know your plant has established itself and will recover from the transplant stress.
Best wishes with your garden!
Q&A Library Searching Tips