Azaleas like their soil acidic. If it is too alkaline this can lead to chlorosis, which occurs when the plant is unable to absorb the iron it needs from the ground. The high pH causes the plant to develop an iron deficiency, which will then affect its chlorophyll production, making the leaves turn yellow.
Other causes of yellowing azalea leaves are underwatering, poor soil drainage, too much sunlight, and pests.
Why are my azalea plant’s leaves turning yellow?
The soil is alkaline
Azaleas need their soil to be acidic, with a pH between 4 and 6. Their leaves will turn yellow from the stress of being in alkaline soil for too long. The alkalinity makes it difficult for azaleas to absorb iron from the soil. The resulting iron deficiency will lead to chlorosis and turn the leaves yellow due to the plant’s impaired chlorophyll production.
Most soil is acidic because the organic material that ends up on it, such as fallen leaves, turns acidic when it decomposes and is integrated into the soil.
You can remedy alkaline soil by first determining the pH of your soil. If plants like camellias, daffodils, rhododendrons, and roses are thriving in the soil, then it is probably acidic. This must mean that the yellowing leaves are caused by something other than alkaline soil.
You can buy a soil gauge to measure the pH of your soil. Make sure you test it in multiple sites to allow for any variation in soil pH. This is important especially if you suspect that the cause of the alkalinity is artificial, such as wood ash or builders rubble under the garden.
A soil gauge will give you a reliable reading and can be bought online. Once you are aware of your soil’s pH, you can then buy plants that are most suitable for your garden. This will save you money and time in the long run.
If, after checking your soil, you have determined that it is alkaline, you can treat it with garden lime. Remember that changing your soil’s pH is a gradual process and that it will not last in the long term. The soil will eventually return to its original pH.
If you want to make your job easier, you can dig up the plant and transfer it to a pot, where you will be able to control the soil’s characteristics more easily. Transplanting azaleas is best done during the late summer, but they can be transplanted successfully at any time during the growing season.
If the azalea cannot be moved, apply iron chelates to help with the iron deficiency. This solution is temporary and a bit expensive. Make sure that you only use this option when you are absolutely sure that the soil pH is neutral or alkaline. Applying iron chelate to acidic soil will be ineffective and even harmful.
Underwatering
Another reason your azalea leaves are turning yellow could be due to a lack of water. These plants are not drought resistant and need their soil to be moist with regular watering to keep healthy.
You can check whether the soil needs watering by sticking your finger into it. If the soil is dry one or two inches below the surface, the yellow leaves may be due to underwatering.
Azaleas need moist, well-draining soil with high organic content. They could experience drought due to little to no rainfall, or if the soil drains too quickly and is unable to retain moisture, such as sandy soil.
You can remedy an underwatered azalea plant by watering it up to three times a week, especially in a dry climate or during the hottest time of the year.
Plant your azalea in soil that has plenty of organic material. These components will provide nutrients and retain moisture well, but still have well-draining qualities. This allows excess water to drain away so that the roots are not constantly soaking in wet soil.
Add one inch of organic mulch to conserve moisture, especially in the summer growing season. If you can find compost that has decomposed for at least a year, that is even better. It will help reduce evaporation and keep the roots cool in the summer.
The soil is slow-draining
Another cause of yellowing azalea leaves is root rot due to slow-draining soil. As mentioned above, even though azaleas like their soil to be moist, it still needs to be able to drain excess water so that the roots are not saturated.
Heavy soils, such as clay, are not porous enough and the water becomes trapped and drains very slowly. The slow drainage makes the soil conducive to root rot from fungi. Yellowing leaves, falling leaves, and a wilted appearance are all signs of stress from root rot.
To determine whether root rot is causing the yellowing azalea leaves, check to see if the soil is boggy. Dig up the soil around the plant’s roots to see if it is slow-draining or has compacted.
You can remedy this by providing the plant with well-draining soil. If you need to, transfer the plant to a pot or a raised bed.
Replanting the azalea in new soil that is well-draining will allow it to dry out and recover from root rot. If you see rotten roots, cut them off so that newer roots are encouraged to grow.
If you cannot move the azalea, you will need to adjust the clay soil to improve its filtration.
Make sure you work on the clay soil on a dry day so it is easier to handle and does not stick to your tools. Do not use sand with clay soil; use gravel instead. Sand mixed with clay will create a cement-like consistency. Gravel is a better choice because it will increase the porosity. Adding compost will also help with drainage while maintaining the soil’s ideal structure.
Too much sunlight
Azaleas like a location where they can get some sun, but they need to be out of the afternoon sun. They are fine with morning and early afternoon sun, but when exposed regularly to harsh afternoon sun, their leaves will turn yellow.
Pests
The most common pests on azalea plants are lace bugs. These are an eighth of an inch long and have lace-like wings. They feed on the sap of the azalea leaves, leaving white, yellow or silver spots. You can get rid of these bugs by treating the plant with insecticide.
Conclusion
When azalea plants’ leaves turn yellow, it is usually a sign of stress and you need to identify the stressor before it causes further damage to your plant.
The most common causes of yellowing azalea leaves are lack of nutrients, underwatering, poor soil drainage, too much sunlight, and pests.
Azaleas are fairly simple plants to grow, and as long as you are able to provide their basic needs, they will thrive in your garden.
Image: istockphoto.com / Trygve Finkelsen