Gardenias are sub-tropical evergreen flowering shrubs that bloom in the summer and produce red berry-like fruits in the fall. In keeping with their tropical origins, gardenias are very sensitive to frost and freezing temperatures. They can be grown outdoors in climates where ambient temperatures and winds remain above 35 or 40 degrees F with little damage but they must be overwintered indoors in cooler climates. Newly planted gardenias are still acclimating to their new environs and should be treated with extra care to ease stress and prevent shock.
Maintain bright direct sunlight or morning sun and afternoon shade for your gardenias through the winter to keep them healthy. Whether growing them outdoors or overwintered indoors never put the evergreen plants in a darkened location. Place indoor plants in a south facing window for best winter light.
Maintain even soil moisture but ensure that the roots are not sitting in muddy soil or in an area where water is pooled on top of the soil.
Mulch around the base of outdoor gardenia shrubs with several inches of organic material such as shredded bark or compost to help keep the soil at a consistent temperature. Spread mulch from the main stem out to a foot or so beyond the shrub drip line. Mulch will hold moisture in, keep weeds at bay and increase soil fertility over time.
Refrain from fertilizing the shrub from early summer onward to prevent new growth that may be damaged by winter temperatures.
Refrain from pruning which can stress the plant and create entry points for disease over the winter.
Maintain cool ambient temperatures for your gardenia when overwintering it indoors. Provide temperatures of roughly 55 degrees F at night and 65 degrees F during the day.