Gardenias are known for their sweet-smelling white flowers and beautiful, shiny, emerald-green leaves. But yellow leaves on gardenias are one of the most common problems and can signal poor health. Here’s what you need to know.
KEY TAKEAWAYS
- Check for pests or fungal issues first, then check soil moisture
- If watering isn’t the issue, it’s likely a nitrogen, magnesium or iron deficiency, each with a different yellowing pattern and fix.
- Avoid alkaline soil, which stops gardenias from absorbing iron properly
- Remove yellow leaves by hand rather than with tools
Why do gardenia leaves turn yellow?
Chlorosis, or the yellowing of leaves on a plant, is a common sign that something is wrong and is preventing the plant from producing enough chlorophyll for the leaves to stay green.
With gardenias, a sudden temperature change can often cause the leaves to turn yellow.
Mineral deficiencies are another major cause of chlorosis in plants, and in gardenias the most common deficiencies are magnesium, nitrogen and iron. These deficiencies can cause gardenia leaves to turn from a glossy green shade to bright yellow.
Other causes of chlorosis
Chlorosis can also be caused by poor drainage, root problems, high pH levels, pests, disease or a combination of all of these.
Another common cause of yellow leaves on gardenias is planting them in soil that is too alkaline. While people commonly try to grow gardenias as houseplants, they can be finicky as they require high humidity.
How to treat gardenias with yellow leaves
First, check your gardenia for other visual signs of distress, in case the issue is pests or a fungal infection that may require treatment.
Next, check the soil’s moisture levels. Over- or under-watering a gardenia can change the colour of its leaves.
If not, it could be a lack of nitrogen, magnesium, or iron.
According to the WA Department of Agriculture and Food, if nitrogen is deficient, the oldest leaves will appear pale and lack the lustre of healthy ones. Yellowing appears at leaf tips and will affect all the leaves. Apply nitrogen-rich fertilisers such as sulphate of ammonia or blood and bone. You can also add organic matter and manures to the soil.
Magnesium is also essential for photosynthesis and for forming proteins and chlorophyll. It moves freely within the plant and is taken from the older leaves to supplement new growth. A magnesium deficiency will affect the lower leaves on your gardenia first, yellowing them from the tip downwards. Dead spots may also appear.
To correct the deficiency, apply magnesium sulphate (Epsom salts) or dolomite (magnesium-calcium-carbonate).
A condition often referred to as lime-induced chlorosis affects acid-loving plants that cannot take up iron in alkaline soils. The newer leaves become pale green, yellow and, in severe cases, white. Veins remain green. This is a common occurrence on coastal alkaline soils. To correct the deficiency, reduce the soil pH with ammonium sulphate or agricultural sulphur, and apply iron sulphate. A foliar feed with a complete liquid fertiliser will help correct this problem.
Tip! If you are going to prune, it is recommended not to use any gardening tool to remove the yellow leaves; instead, use your hands.
How to grow healthy gardenias
Most gardenias perform best in a well-drained, humus-rich, acidic soil in a sunny or partly shaded position. Water and feed well to promote lush foliage and heavy flowering. In cool climates or during winter in Australia, move them to a heated greenhouse, as most gardenias are frost-tender.
