I am in Central Florida, the last year or so my Gardenia has had yellowing leaves, low yield of flowers, The bush is 5 years old, lots of buds coming out now but I still have the yellowing and I see what appears to be dirt or dark sand sprinkled around the inner parts of the new growth leaves.
I tried Epsom Salt treatment 3 and 6 months ago, did not see any result.
Gardenias like their soil acid like azaleas, roses and blueberries. You can acidify your soil by sprinkling Ironite according to directions and watering it in. The black "grains" might be aphids which like the youngest most tender new growth. Insecticidal soap will take care of these but you will have to apply more than once.
You can blast the aphids off with a strong stream of water. They come and go on Gardenias, no big deal on an otherwise healthy plant, which yours looks like, just a little chlorosis. An all-purpose fertilizer would probably be the most helpful thing. Looks like a lot of competition from the ground cover.
Is the Gardenia getting more shade that it used to? Could that black dirt appearance be mildew? I have the yellow leaf problem to a lesser degree with my Gardenias, they are in the shade and need more sun. Often they will also get a black mildew on the leaves. Gardenias don't like to be moved I have read, but my neighbor moved three of hers so she could have a tree cut down and they have done so much better in the sunnier location.
Gardenias are generally found outdoors in southern regions and are admired for their glossy green foliage and fragrant blossoms. Gardenia requirements are very exact, and to insure your plants continued bloom, these needs must be met.
- Full sun, with some shading in the summer (in the hottest areas)
- An acid soil with a pH between 5 and 6
- Keep the soil moist, but never soggy
- Humidity, or daily misting
- Heavy feeding
- Cool night temperatures
I agree ...feed it and get rid of the aphids and check to see if you have tiny white flies on the shrub...if so you will need to address them.also. ...if you see no improvement in the foliage after a couple weeks, then you can examine other options. . Gardenias are heavy feeders and require regular feeding during the growing/blooming season. Here in zone 9a I have 2 gardenias in Morning sun/bright shade, 1 in full bright shade/dappled light and 1 in full sun...they all perform about equally as to growth and blooming.