Question
I live in tropical North Queensland Australia and my Gardenias (Magnifica)
certain leaves are turning  yellow, and I do not have as many buds on them that I
would like.
I await your advice.
Rini

Answer
Start watering your Gardenias with Vinegar spiked H2O.  You want to get the pH of the Water down to 5.0 to 5.5 at the MOST.  And replace the Soil if you can with a high Peat Moss/Sulphur blend.

The plants are chlorotic.  But not from lack of Iron.  The pH of the Soil is too high for these plants to absorb the Iron already in the Soil.  Lowering it is critical and urgent.

Gardenias need a rest period, particularly after the harrowing experience of a force-feeding/growth hormones program to get them to market quickly covered with flowers.  That's where the profits are.  Give it a break.  You'll be glad you waited.  Follow up with Full Sun, mild temperatures, and Acidic Soil.  Also understand that healthy fragrant plants yield MORE fragrance than the same plants that are under-the-weather.

When professional greenhouse growers take on the Gardenia, they MUST know what makes them grow and what makes them flower.  Blooms after all are what makes Gardenias fly off the shelf.  You can use what they know to become a better Gardenia Gardener.

Let's review then one report titled 'FLOWER INITIATION AND DEVELOPMENT IN GARDENIA PLANTS AS AFFECTED BY PHOTOPERIOD AND NIGHT TEMPERATURE.'

www.actahort.org/books/515/515_11.htm

Scientists E. P. Makridou and A. S. Economou exposed Gardenias just like yours to different daylengths: 8, 12 and 16 hour days.  They did this for 4 weeks.  Then they took ALL the Gardenias and exposed them to 16 hours of daylight for 4 more weeks.  Which Gardenias do you suppose had the most flowers?

The ones that began with 8 hours of light?

The ones with 12 hours of light?

Or the ones with 16 hours of light?

Turns out that the Gardenias that had the shorter days, and then switched to longer days, had the most flowers of all.  The researchers concluded that 'short photoperiods promote flower bud initiation in Gardenia plants.'

But that's not all.

They did another experiment.  This time, they exposed ALL their Gardenias to 8-hour days for 4 weeks and then split them up into three groups.  These pots were now grown under different daylenghts -- 8, 12 and 16 hours long.

Guess the results for this experiment.  Did changes in day length affect flowering?

Answer:  The ones that had long days for a full 8 weeks -- days that were 16 hours long -- flowered quickest.

Then they did ANOTHER test.  They wanted to know if night temperatures would have any effect on Gardenias.  So they grew them ALL with 8 hours of daylight and set the thermostat at 75 degrees F; when the lights went out, they lowered the temperature to while the night temperatures were kept at 55, 65 and 75 degrees F.  And they did this for 4 weeks.  Then they CHANGED the night temperature.  ALL Gardenias were now spending their nights in a 65-degree F room.  They did this for 4 weeks.  And they tried several other combinations of temperatures as well, all with the same 8-hour day length.

Results:  Gardenias flowered earliest when exposed to days and nights of 75 degrees F.  All the Gardenias flowered.  There was no other effect observed on the number of flowers.  

So if you want to use this research, give your Gardenia only 8 hours of daylight for 4 weeks.  Then make the day longer.  Give it 16 hours of daylight.

These Gardenias will have constant temperatures of 75 degrees F the whole time.

And you will have Gardenias coming out of your ears.

THE LONG ISLAND GARDENER