Gardenias are fairly hardy plants, best for zones 7 to 11. Unlike most plants, they can handle light freezes. However, in exposed locations and long cold fronts, their foliage will get damaged. If you notice any black or brown leaves this is an indication that they have gotten too cold.
When/if this begins to happen do not start to hack off dead growth, wait until the spring to assure these are truly dead parts of the shrub. If your gardenia gets too cold and does suffer damage, it will recover in spring or within 1 to 2 years.