Answer: Brown tips on cordyline usually indicates overly dry air. Since this is August and weather is typically hot and dry, and, since you've recently planted your cordyline and they're probably going through some transplant stress, I wouldn't worry too much about them. They will adjust and you can eventually cut off the affected leaves.
Water deeply once each week (or twice if the weather is hot) and mulch over the top of the soil with compost or other organic mulch to help slow evaporation and moderate soil temperatures. Water regularly until fall rains take over for you.
Best wishes with your new plants!
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