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The Secret of Growing Gardenias in Cool-Temperate Climates

The Secret of Growing Gardenias in Cool-Temperate Climates

Gardenias are a fabulous plant and I began to understand their magic when for the very first time my gardenia flowered outside my lounge room window. The fragrance was exquisite! But there have been times when I despaired and was ready to rip it out of the pot. My plant was in the best position possible in my small garden but was exhibiting an iron deficiency. I had done everything I could think of to try and fix the problem. I had feed it Epsom salts, put it in a sunnier position, applied liquid chelate of iron to low the soil pH and nothing worked. Then, I learnt by accident that spring is the time to correct this problem, not in autumn when this problem often exhibits itself.

Iron deficiency is a common problem with gardenias, especially as the weather gets colder. As the soil temperature drops, the micro-organisms and the sap of the plant slows down, reducing the plants ability to take-up nutrients. The result is yellow leaves with green veins. There is nothing you can do until spring, when the soil temperature warms up and the sap starts flowing again. This is the time to give it a dose of powdered iron of chelate to correct this problem. Choose iron of chelate (even though it is more expensive) because it does not leach through the soil profile as quickly as sulphate of iron.

Gardenias are fussy about their soil pH. pH is the number of hydrogen ions in the soils. It is classified as acidic (between 2 – 6), neutral (6-7) and alkaline (between 7 – 10). They like an acid soil of 4.5-6 and I learnt (accidentally again, by reading the directions of the pH kit) in a pot they like a potting mix of 4.5 to 5.5. If the soil pH is alkaline, it will show the symptoms of iron deficiency. Iron availability decreases as the pH become more alkaline even though the iron is still present in the soil. A pH of 6.5 is the cut off point, so this means there is no iron available. Iron is essential to produce chlorophyll (the green pigment in the leaf) otherwise the photosynthesis process cannot take place and there is no growth. This is easily fixed by apply sulphur as it lowers the pH. I decided to test the pH and it revealed the pH was 6.5.

So what I needed to do was lower the pH and to do this sulphur is the recommended product. You can buy small boxes of sulphur and that acidifies the soil. I had tried liquid chelate of iron with no effect. I read the contents analysis and discovered there is no sulphur in the liquid form, so no wonder there was no change. In desperation, I decided to use Hydrangea Blueing Tonic (I had it in the shed) because the tonic contained sulphur plus aluminium, nitrogen and potassium. And hey presto some of my sick looking leaves started turning a lovely healthy green!

But I made another mistake, after searching through my soils textbook I discovered that too much aluminum interferes with the uptake of phosphorus which is used for root development. So now I have taken my own advice and am using chelate of iron in the powdered form and my gardenia is beginning to look good!

By trial and error I have also realised that in the southern states of Australia, gardenias need warmer and sunnier spots than people realise. I have my plant in a large pot, which means I am able to move it about. In summer, it is in a north easterly position catching the sun until about 12.00. Then it is moved in winter to a north westerly position which receives the only sun available (at mid-day) for about 2-3 hours. This warmer position helps this tropical plant survive the cold Melbourne winter and prevent iron deficiency.

Gardenias can be challenging, but definitely worth growing because when they flower and fill your courtyard with beautiful perfume, you realise they are worth all the effort.