Are your Azalea leaves turning yellow and you can’t work out why? The same thing happened to my Azalea plant and it took some investigation and experiments to find out the cause.
Azalea leaves can turn yellow because of:
- A lack of nitrogen in the soil
- Soil that is too alkaline (azaleas need acidic soil)
- Azalea is not receiving enough water.
- Poor soil drainage (causing root rot)
I found the reason why my azalea had yellow leaves was because of a deficit of nitrogen in the soil, cause by wood chip that was drawing up nitrogen from the soil during the process of decomposition.
This was solved by applying a general nitrogen based fertilizer and the leaves returned green and healthy in the Spring.
Keep reading for more on the causes of azalea leaves turning yellow and how to implement each solution…
Nitrogen Deficit in the Soil
One of the most common causes of azaleas and rhododendrons leaves turning yellow is a lack of nitrogen in the soil.
This can be caused by nutrient poor soils (such as sandy soils) but in my case the cause was adding mulch to the surrounding soil of the azalea.
Mulch, composed of organic materials is a really good way to maintain a good moisture balance around the roots of your azalea and add nutrients. However it is important to wait till the organic material is well rotted before applying it to the soil surrounding the plant.
If you apply garden waste such as leaf mould, grass clippings etc. without letting it decompose fully for at least a season then the mulch will continue to decompose when applied around the azalea. The same can be true of wood bark if it is applied directly to the ground without a barrier for weeds.
Organic material requires nitrogen as part of the decomposition process which it draws up from the soil. This creates a temporary nitrogen deficit in the soil during the process of the mulch decomposing around you azalea.
On a long enough time line the nitrogen returns to the soil when the mulch has been incorporated into the soil but the temporary deficit cause by the decomposition of mulch, means the roots of your azalea cannot access enough nitrogen in the soil and the leaves turn yellow as a sign of stress as nitrogen is one of the key nutrients that all plants need to grow (along with Potassium and phosphorous).
Their are two solutions to this:
- Add a nitrogen based fertilizer to the soil to address the temporary nitrogen imbalance.
- Remove the decomposing mulch and apply well rotted material.
Established Azaleas generally do not need fertilizing, unless the soil is sandy or nutrient poor. However a general plant fertilizer will address the nutrient imbalance and return nitrogen to the soil.
Azaleas with yellow leaves recover quickly once the fertilizer has been applied as nitrogen is water soluble and reach the roots relatively quickly.
You do not necessarily have to use a specialised product for azaleas or rhododendrons, but personally I used miracle-gro Azalea and Rhododendron Continuous Release Plant Food as it contained a good balance of nutrients and it was specifically formulated for the acidic soils that azaleas need to grow.
However I should stress that this solution should be done in the growing season, (Spring and Summer) and not in the Fall. Nitrogen stimulates new foliage growth that is softer and more susceptible to frost damage.
If you fertilize the plant too close to Winter then the new growth will turn black in the first frost.
If a nitrogen deficit is the cause of the azaleas yellow leaves then the addition of fertilizer can wait till the following spring as while the plant is stress it is not in any immediate danger and will respond well to some nitrogen fertilizer after winter.
Once the fertilizer is applied in the Spring the plant should quickly recover and the leaves should return to healthy green over the next week or so.
Alternatively the other solution is to remove the partially decompose mulch and replace it with well rotted mulch. Once organic material is well rotted for a year it will add nitrogen to the soil rather then cause a deficit as its is fully decomposed.
Well rotted mulch can be applied around azaleas at anytime of year as it keeps the soil moist in the Spring and Summer and insulates the roots in Winter.
Again the Azalea will recover from the soils nitrogen deficit and leaves will turn green in the Spring and Summer as the plant returns to full health however this solution will take longer to see results then the addition of fertilizer.
Soil pH is alkaline (Azaleas need acidic soil for Iron)
Another potential cause of azaleas with yellow leaves is alkaline soil. Azaleas and rhododendrons require acidic soils and will thrive in soils between pH 4-6 with pH 5.5 considered ideal (pH 7 is neutral and a value above pH 7 is alkaline).
Yellow leaves could be a sign of stress as a result of soils that are alkaline rather then acidic.
Alkaline soils make it difficult for the azalea to absorb nutrients (Iron in particular) from the soil which results in the leaves turning yellow.
An Iron deficiency (chlorosis) is specifically the cause of yellow leaves in azaleas and rhododendrons when the soil is not acidic.
Most garden soils tend to be slightly acidic because most organic material (such as fallen leaves etc.) will be acidic once it is fully decomposed and integrated into the soil by natural processes.
However some soils can be naturally chalky or limey which makes it difficult to grow a lot of common garden plants.
The first thing you will have to do is try to determine the pH of your soil. Look for clues by observing which other plants thrive in the area. Roses, Rhododendrons, other Azaleas, daffodils and camellias all grow in acidic soils.
If these plants are thriving in your garden or your neighbours garden then it is unlikely that alkaline soils are the cause of yellow azalea leaves.
If you are unsure then I would recommend that you purchase a soil gauge which measures the pH of your soil.
Test the soil in several sites around your garden to make sure as there can be significant variation in soil pH in a small area, particularly if the cause of alkaline soils is artificial (such as builders rubble under the garden or wood ash spread around as both can be highly alkaline).
Soil gauges are easy to use and will give you a reliable reading of the soil pH and best of all they are available for a great price on amazon.
If you know your soils pH then you can buy the specific plants that are suitable for your garden which will save money, time and effort in the long run.
If your soil is indeed alkaline then you can treat the soil with garden lime which is available from all good garden stores. Changing the soils pH is a gradual process and is not long term as the soil will likely revert back to its original alkalinity over time.
If at all possible I would recommend that you dig up the Azalea and transfer it to a pot or raised bed, where you have more control of the soils characteristics as alkaline garden soil is not easily amended.
The best time of year to transfer azaleas is in the late summer although they can be transplanted successfully at any point in the growing season.
For azaleas that cannot be transferred then applying iron chealtes will alleviate the iron deficiency, although this is is a temporary solution and can be quite expensive.
Only apply iron chelates to the soil if you have confirmed that the soil is either pH neutral or alkaline otherwise it may do harm or simply be ineffective. Use a product that is specifically formulated for iron deficient plants and always follow the instructions fastidiously.
Azalea does not have enough water (yellow leaves are a sign of stress)
Yellow leaves on you azalea may be a sign that your plant is stressed as it is not getting enough water. Azaleas are not a drought resistant plant and require moist soil with regular watering to stay healthy.
If the surrounding soil around your Azalea feels dry to a fingers depth then drought is likely the cause of the leaves turning yellow.
Azaleas need moist, yet well draining soil that has a high organic content.
So the Azalea could be experiencing drought because:
- Low rainfall or lack of watering.
- The soil drains too quickly and does not retain moisture (such as sandy soils).
In dry climates you will have to water your Azalea up to three times per week at the hottest time of year to avoid drought.
Ideally your azalea should be planted in soil that has been amended with plenty of organic material such as garden compost, leaf mould or well rotted manure.
These materials provide nutrients and have a great capacity to hold onto water, yet they still retain a porous structure that allows excess water to drain away, so the roots are not sat in saturated soil.
Soil amended with lots of compost will stay moist and allow the roots of your azalea to draw upon the moisture when it needs to without being in soil that is persistently saturated.
Adding a 1 inch layer of organic mulch is also a great way to conserve moisture during the hot growing season. I personally recommend using well rotted garden compost that has decomposed for at least a year.
The mulch will reduce evaporation, keep the roots nice and cool in the summer and contribute to the moist soil conditions that azaleas need to thrive.
Slow draining soils (cause root rot)
Yellow leaves on your azalea can also be as a result of root rot (Phytophthora) due to soil that drains too slowly.
Azaleas require the soil to hold onto moisture, yet let excess water drain away so that the roots are not sat in saturated ground.
This is achieved through amending the soil with plenty of organic matter (such as good compost) before planting and applying a layer of mulch in the Spring.
Heavy soils or clay soils can have microscopic particles which means that the soil is not porous enough and therefore water can become trapped or drain away very slowly.
The slow drainage creates the conditions that promote the fungal disease root rot. One of the signs of stress from azaleas with root rot is that leaves may turn yellow (along with a wilting appearance and falling leaves).
To determine whether root rot is the cause of azalea leaves turning yellow then look to see if the soil is boggy after watering or rainfall and perhaps dig with a spade near the soil to see if there is any evidence of slow drainage or compact or clay soils.
If the soil is damp but not wet and your azalea is not wilting or losing leaves, yet the leaves are still turning yellow then root rot is unlikely to be the cause, but rather alkaline soil or a lack of nitrogen in the soil.
Azaleas need well draining soil to avoid root rot. If the soil has a high clay content and therefore, slow draining then you will have to dig the azalea out of the ground and replant it in a well draining part of your garden or transplant it in a pot or raised bed if the azalea is small enough.
Replanting the azalea in fresh soil that is well draining will give the plant an opportunity to dry out and recover from root rot. If there are any roots that are visibly rotting then sip away these roots with a sterilised pair of pruners. This will remove the diseased root and stimulate other roots to grow, which gives the azalea the best chance of recovery.
If transplanting the azalea to a pot or raised bed is not an option then you will need to dig the azalea out of the ground and amend the clay soil to improve the rate of infiltration:
- Always work clay soil on a dry day, otherwise it becomes slick in the wet and will stick to your shovel and boots which is very hard work and will test your patience!
- Do not add sand to clay soil for the purposes of improving drainage but instead used gravel. Sand and clay can form a cement like texture which is incredibly difficult to deal with.
- Gravel is a much better option and it will increase the size of the pores in the soil and will not create the same difficult texture.
- You will also have to add significant amounts of compost to the hole before replanting as compost is well draining and retains the ideal soil structure.
- Adding mulch around the plant every year in the spring will also help to improve the infiltration rate of clay soil and gradually improve the soil over time.
Clay soils can be beneficial to gardeners as the retain nutrients effectively, however soils that are mostly clay without any other significant contributions from other materials will drain too slowly for most plants.
Loam soils (which contains clay about 20% clay) is consider ideal for growing azaleas with significant amounts of organic matter or silt (around 40%) and some sand or gravel (40%) as this composition will ensure high nutrients and will retain some moisture whilst still allowing for good drainage.
- Azaleas with leaves turning yellow can be as a result of a nitrogen deficit in the soil, alkaline soils, drought or slow draining soil (causing root rot).
- A nitrogen deficit in the soil caused by decomposing organic matter on the soils surface, such as partially decomposed mulch or wood bark is one of the most common causes of yellow leaves and can be treated by by feeding the azalea in the spring.
- Alkaline soils cause an iron deficiency which requires an iron formulation specifically for plants or alternatively transfer the azalea to a pot or raised bed with acidic soil so that the root can access iron in the soil.
- Yellow leaves can be as a result of drought, caused by soil that drains too quickly or a lack of water/rainfall. Amend the azaleas planting area with plenty of organic matter and add well rotted mulch to retain water. Water the plant more regularly.
- Soils that drain too slowly can results in the azalea developing the fungal disease root rot which causes yellow leaves and a wilted appearance. Lever the azalea out of the ground and cut off any rotting root and replant in an area with better drainage. Amend slow draining heavy or clay soils with gravel and compost. Apply a mulch to the compost every spring to improve the soil and increase the soils drainage.