Answer: Winter pruning typically doesn't harm a gardenia (as shown by the winter dieback yours suffered a few years back). However, if it was a mild winter and the gardenia was severely pruned - say more than a third of it was taken off - then it could be showing signs of stress. This would be because it wasn't dormant when it was pruned and the pruning was extensive. If the root system is healthy it will recover later this spring. You can help it become full and green with careful tip cutting (just an inch or two from the tip of each stem), by feeding and by watering as usual.
Best wishes with your garden.
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