Answer: Gardenias are only marginally hardy in Seattle (Kleim is hardy to about zero degrees; augusta is hardy to 20 degrees). While Kleim will show some foliage damage during the winter months if left outdoors, the roots will survive if you mulch over the top of the soil. New stems will emerge the following spring. Gardenia augustus is a little more tender but if you have a protected spot outdoors, it will probably survive. The root systems of plants in containers are at risk of freezing so if you leave your gardenias in pots outdoors, be sure to bury the pots in the ground to help insulate the roots.
Or, you can pot all of them up and plan on taking them indoors over the winter months. They'll need to be taken in when nighttime temperatures are about 50F. Place them in a bright area, water as needed and mist the leaves occasionally to counteract dry indoor air. Best wishes with your garden!
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