• January 30, 2023

The secret to growing gardenias in cool and temperate climates

Gardenias are a fabulous plant and I began to understand their magic when my gardenia first bloomed outside my living room window. The fragrance was exquisite! But there have been times when I got desperate and was ready to pull it out of the pot. My plant was in the best possible position in my little garden but was iron deficient. I did everything I could think of to try to fix the problem. I have fed it Epsom salts, put it in a sunnier position, applied liquid chelate iron to lower the soil pH and nothing worked. So, I learned by accident that spring is the time to correct this problem, not autumn, when this problem often manifests itself.

Iron deficiency is a common problem with gardenias, especially when the weather turns colder. As soil temperatures drop, microorganisms and plant sap slow down, reducing the ability of plants to absorb 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 begins to flow again. This is the time to give him a dose of powdered iron chelate to correct this problem. Choose chelated iron (although it is more expensive) because it does not leach through the soil profile as quickly as iron sulfate.

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

So what I had to do was lower the pH and to make this sulfur is the recommended product. You can buy boxes of sulfur and that acidifies the soil. She had tried liquid iron chelate without any effect. I read the content analysis and found that there is no sulfur in liquid form, so it is not surprising that there have been no changes. Desperate, I decided to use Hydrangea Blueing Tonic (I had it in the shed) because the tonic contained sulfur plus aluminum, nitrogen, and potassium. And voila, some of my sickly looking leaves started to turn a beautiful, healthy green!

But I made another mistake, after looking into my soil textbook, I discovered that too much aluminum interferes with the uptake of phosphorus which is used for root development. So now i have followed my own advice and am using chelated iron in powder form and my gardenia is starting to look good!

Through trial and error I have also found that in the southern states of Australia, gardenias need warmer and sunnier locations than people realize. I have my plant in a large pot, which means I can move it around. In summer, it is in a northeasterly position basking in the sun until around 12:00. It then moves in winter to a northwesterly position that receives the only available sun (at noon) for about 2-3 hours. This warmer position helps this tropical plant survive the cold Melbourne winter and prevents iron deficiency.

Gardenias can be a challenge, but they are definitely worth growing because when they bloom and fill your yard with a beautiful perfume, you know they are worth all the effort.

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