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Last updated on July 12th, 2019
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 proper acidic soil or you are growing them in pots, they might still suffer from yellow leaves because of hard tap water if you water then regularly or 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.
Causes of yellow leaves on Rhododendron plants
If your rhododendrons are two or three years old and just before winter the leaves are starting to fall off and turn yellow, that is a regular part of the life cycle and not something to be worried about.
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. Any way to check if you have bad drainage is to dig a small hole and fill 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 – acid soil is a must for Rhododendrons
After that, it’s important to test the acidity using a home pH test kit. If your soil comes out alkaline, you will need to change your rhododendron because of a mineral deficiency. If it is neutral or far into the acidic level than the soil is fine but 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 soil surface level. 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 at the right level but this is best done when the plant is dormant from October onwards. 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
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 when 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 are a big give away. The iron has to be absorbed easily and quickly so adding fertiliser 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 which is a type of organic fertiliser that breaks down slowly, releasing the proper amount of nutrients at the proper times.
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 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 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.