Outdoors, gardenias thrive in areas where, during the growing season, days are warm (60°F to 80°F) and nights are cool (55°F to 60°F). Plant them in sun or light shade in sandy, slightly acid soil mixed with peat moss. Mulch them in summer; feed twice a year with an acid fertilizer.
As houseplants, gardenias like fairly high humidity and bright light, but not strong direct sun; mist them during dry months or keep them on a tray of damp pebbles. Keep the soil uniformly moist and feed them with an acid fertilizer every 2 or 3 weeks from March through September.
For successful flowering, these conditions must be kept constant and temperatures should range from 70°F to 75°F in the day, 60°F to 65°F at night. Gardenias bloom in summer, but you can encourage winter flowers by pinching off summer buds. When the container becomes crowded with roots, or roots grow out of the bottom, repot in an acid potting mixture.