Gardenias are beautiful, shrubby, evergreen plants well-loved for their creamy, fragrant blooms. They are temperamental in their cultural requirements indoors. Gardenias respond best to cool temperatures, high humidity, constant soil moisture and bright to full but filtered sunlight.
Gardenias need bright light but keep out of direct summer sunrays. Best placed in bright filtered light protected with a sheer curtain between the bright sunlight and the plant especially in summertime.
Gardenias do not have a well-defined rest period but grow less actively during winter in areas of shorter days (northeast). Water plants moderately during summer, giving enough at each watering making the potting mixture thoroughly moist (tepid water given during the day not at night) letting the top half inch of the mixture to dry out between waterings (check with your handy fingers). In winter months (October-February) allow the top 1-2 inches of the potting mixture to dry out completely before watering again. In areas of less reduction of daylight, regular watering regimen can be followed ongoing. Use slightly warm water, preferably lime free.
Cool temperatures are preferred, 65° to 70°F during the day and below 65°F at night. High temperatures can result in leaf and flower bud drop. The key to bringing gardenias into flower is to provide a steady temperature of 62 - 63°F. during the period flower buds are forming. Sudden drop or rise in temperature can cause buds to drop. When plants are not forming flower buds, normal room temps of 60 - 75°F are preferable. Ongoing humidity is also required when flower buds are forming.
Provide extra humidity with daily misting (use tepid water) along with a pebble tray kept filled with water placed under the plant's pot. Setting the plant on a tray of wet pebbles helps provide ongoing humidity (refill pebble tray with fresh water as needed). Humidity is also important deterring spider mites that thrive under dry conditions common indoors, causing leaf yellowing. Discoloration on flowers can happen from misting, so only mist the foliage.
During the growing season (March through September), fertilize gardenias every two weeks with a dilute fertilizer made for acid-loving plants.
Repotting is only necessary when the roots have nearly filled the pot (as indicated by root appearance at the topsoil surface or outside the bottom drainage hole). Repot into a regular peat based potting mixture so a lime-free mixture. Repotting is best practiced during spring as the plant begins growth disturbing the roots as little as possible when repotting up to next size pot so not too large a pot causing overly wet soil for too long.
Propagate gardenias in early spring with 3" stem cuttings taken just below a leaf node. Dip the end of the stem cutting into a rooting hormone if available, plant into small pots of peat based potting mixture, and enclose into a plastic bag for added humidity at a temperature of 60 - 65°F, placed in a bright filtered window by a translucent curtain for protection from direct sunlight. Transplant the rooted cuttings (4 - 6 weeks later) into 3" pots with the preferred growing mix as described above. Water cutting(s) moderately and feed once a month with a dilute acid fertilizer.
To encourage continuous blooming, cut off the faded flowers just below the leaf node, maintain a cool (62 - 63°F) night temperature, and feed with a dilute acid fertilizer with iron every two weeks during the growing season. In addition, keep the humidity level high when flower buds are forming. To increase humidity levels place plant pot on a larger tray of pebbles kept wet, also mist spray with room temperature water. Try not to wet any flowers blooming which may cause discoloration. Prune/shape the plants in early spring to encourage branching and compact growth.
Monitor and treat insect and mite pests as soon as possible. Pests such as spider mites are arachnids, eight legged creatures related to spiders. They are extremely small and are difficult to see without a hand lens but can be detected by the damage they cause (white or yellow speckling on the upper surface of leaves eventually distorted leaves are apparent). Healthy plants kept reasonably moist (see watering how to) are less sensitive to mite infection. Control mites with a strong water spray. Be sure to spray the leaves top and undersides, repeat as necessary (check weekly for a month or so).
Avoid leaf and flower bud drop by keeping night temperatures cool, providing high humidity and giving sufficient bright filtered light.
Some early-spring pruning is usually necessary to keep the shrub low and bushy. You can nip out any long growing tips on young plants. Cut out half or even two-thirds of the old wood of mature shrubby plant. Do not cut out flowering buds. After flowers die, stems can be cut back. Cuts should be made immediately above points where growth-producing buds point outward rather than toward the center of the plant so new growth will not overcrowd the center of the plant.
Pruning a very old overgrown potted plant may need drastic reduction in size. Accomplish this by cutting the plant by one third of the branches down to the trunk. Prune remaining branches lightly to shape the plant. Make a cut at a 45-degree angle close to a side branch or bud. Repeat every two years. Any dead wood should also be removed. After that process prune lightly to keep the shape.
A handheld pruner should be suitable unless the branches are very thick in which case you can use a lopper. It's best to sterilize the blades of the pruner with rubbing alcohol first. The best time to prune is immediately after flowering.
The Complete Houseplant Survival Manual by 
The Propagation Handbook by 
Not Another Jungle by 
The Unexpected Houseplant by 
