Answer: Stop feeding your gardenias and don't fertilize again until after new growth begins in spring. For future reference, you should stop fertilizing gardenias in late summer to slow new growth. Anything that develops in late fall may not have time to harden off before winter weather arrives, and is subject to winter dieback.
You didn't include your zip code so I don't know what your USDA growing zone is, but August Beauty is hardy to about 10F. If your temperatures get lower than that, you'll need to move the plants to a protected area so they can survive over the winter months, or build a temporary shelter for them so they receive some winter protection. In the spring, prune back any winter damage and fertilize. Your plants should rebound when the weather warms.
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