Flood them with lots of very bright light, even full sun, all year long. Above all, keep the humidity as high as you can. Trays of wet pebbles, hand-held misters and humidifiers are in order. If your walls are not turning green, it is probably too dry. Yellowing leaves can be caused by an iron deficiency, over- or underwatering, or having the pH too high.
Gardenias are subject to scale, red spider mites, mealybugs, nematodes, and a canker disease.
And even under the best of circumstances, some gardenias appear to be suicidal. New Trees in Town Q. New York City has been planting small, conical shrublike oak trees on the streets of Staten Island. Many of the 15,000 trees planted each year by contractors for the Parks and Recreation Department are pin oaks, red oaks, swamp white oaks, sawtooth oaks and English oaks. These trees have, in addition to their beauty, the qualities city trees need to survive. They hold up better under drought conditions than many other trees.