Last updated on March 8th, 2022
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Rhododendrons offer spectacular flowers but they are among a particular group of plants that require acidic soil in order to thrive. While most plants can tolerate a wide range of soil types, Rhododendrons cannot. They will not tolerate alkaline soil and if they are exposed to it their leaves will turn yellow, their growth will be stunted, and eventually, they will stop growing entirely and look very sickly.
If Rhododendrons are grown in the correct acidic soil or you are growing them in pots, and you notice that they are still suffering from yellow leaves, it could be because of hard tap water, however, if you water them regularly it could be suffering from a lack of sufficient nutrients. But rest assured if you have yellow leaves on Rhododendron plants, no matter the reason, there are solutions out there to help.
We also looked at other Rhododendrons problems, from dropping buds to common pests.
Causes of Yellow Leaves on Rhododendron Plants
Natural Yellowing
If your Rhododendrons are two or three years old and it is just before winter when the leaves are starting to turn yellow and fall off, this is a regular part of the life cycle and not something to be worried about.
Wet Soil
If your Rhododendron leaves are turning yellow, especially around the veins, you need to check first and foremost for proper drainage. Rhododendrons do not survive well in wet soil and this can result in yellow discolouration. So water your plant and take a look at how fast the water soaks into your soil. If you have bad drainage you may need to transplant your shrub (between October and March) when dormant into a better position. The best way to check if you have bad drainage is to dig a small hole and fill it with water, leave for 24 hours, and then see if there is still water in the hole. If you do have wet soil you can see our top recommendations of the best plants for wet soil here.
Soil pH – Acidic Soil is a must for Rhododendrons
It is important to test the acidity of the soil using a home pH test kit. If your soil comes out alkaline, you may need to relocate your Rhododendron because it will suffer from a mineral deficiency. If it is neutral or far into the acidic levels then the soil is fine, so it could be a result of hard water, especially if your Rhododendrons are in pots.
Planting to Deeply
When you plant your Rhododendrons the root ball should be at the surface level of the soil. If you feel around and you can’t actually feel the root ball you may have planted it far too deeply, at which point you will need to replant it to the correct depth but this is best done when the plant is dormant from October onwards. The planting depth can have a negative impact on the colour of your leaves and most shrubs do suffer if planted too deeply.
Proper Soil and Care
Iron Deficiency
Rhododendrons need acidic soil. If the soil conditions in which they are grown are alkaline they will be unable to access certain nutrients, such as iron, that they need. Rhododendrons require iron in order to survive and if they don’t get enough it turns their leaves to a yellow shade, especially along the veins, which is a big giveaway. The iron needs to be absorbed easily and quickly, so adding a fertiliser that is heavy in iron is insufficient for these types of plants.
Iron deficiencies are best cured by adding a seaweed and iron mixture to help restore the original foliage. Seaweed stimulates plant growth and the iron added to these products helps to give the Rhododendrons the nutrients they need in the manner that they need it.
Use Slow Release Fertilisers
You should be using Rhododendron fertiliser on an annual basis. This is a type of organic fertiliser that breaks down slowly, releasing the appropriate amount of nutrients at the correct time.
As is the case with most plants, it’s imperative that you keep the soil moist in order to help expedite growth for your plants. After flowering, you can give your plants additional fertilisers and it will help encourage vigorous growth and flowers in the following season.
Having seaweed and sequestered iron additives mixed with regular fertiliser can help your Rhododendron foliage if you already have neutral or acidic soil. It’s effective, especially in pots, where the cause of the yellow leaves is hard water. Adding the essential nutrients once the supply has been exhausted will correct just about any issue your Rhododendron is having which is the good news.
Overall once you’ve ruled out any potential influencers of the yellow on your leaves and you’ve figured out the main issue, it is quite easy to rectify the situation using the measures listed above. Issues with drainage can be tackled by transplantation, as can any issues with improper soil. You could also choose to improve the drainage by adding lots of organic matter and grit into the soil. Problems with nutrient absorption or hard water can be properly tackled by way of seaweed and iron mixtures as well as regular Rhododendron fertilisers.
