The reasons that your rhododendron may be going yellow are primarily linked to its growing environment and how you are treating the plant. Here are the main reasons for this problem and what to do about them. Firstly, I have found the most common reason for rhododendron leaves turning yellow is the soil not being acidic, which leads to yellowing leaves.

I recommend testing the soil ph with a soil testing kit, and you need a soil ph of below 7.0 which is considered acidic. Any higher and it’s too alkaline, and Rhododendrons do not like that. Learn more about yellowing leaves below.

Normal life cycle

Rhododendrons that are around two to three years old often have the problem of yellowing leaves. The leaves may then even fall off.

What to do

Nothing. This is part of the plant’s normal life cycle and should be embraced.

Hard water

If you live in an area with hard water like me, then your tap water has a significant amount of calcium in it. Watering your rhododendron with tap water leaches this calcium into the soil, which reduces the soil’s acidity. This affects the roots of the plant which is happiest in an acidic soil.

What to do

Collect rain water in barrels throughout the year. Water all your plants with this water for the most neutral pH hydration. If you run out of rain water in the summer, you can revert to tap water for a few months without too much of a harmful effect.

Lack of micro-nutrients in the soil

The right soil is key to growing rhododendrons successfully. These plants do their best in acidic soil, as I’ve said above. If they’re planted in alkaline soil, they can’t access the iron in the soil that they need. If the veins of the leaf are yellow, then your rhododendron isn’t getting enough iron or magnesium.

What to do

The first step in clearing up this issue is to test the pH of your soil. If you find you have alkaline soil, you can add acidic fertiliser to bring it to the acid side of the pH scale. However, you need to do this every year as the soil reverts to being alkaline after the acidic micro-nutrients are used up. If you have alkaline soil, a good work-around is to plant your rhododendrons in containers using ericaceous (acidic) compost. See the article Growing Rhododendrons in pots for information about how to do this.

If your garden soil is acidic enough for these plants, then you need to add fertiliser to bump up the amount of iron and magnesium in it. An iron-rich fertiliser adds some iron, but the best choice is an seaweed and iron mixture. Seaweed stimulates the plant growth and takes the iron along with it.

Wet soil

As I’ve said often in this series of rhododendron articles, these plants prefer moist but well drained soil. Soggy, or even water-logged, soil causes them harm, including root rot. (See the article Rhododendron pests and diseases.)

What to do

If the location in which you planted your rhododendrons doesn’t drain fast enough, you may have to move them if you can’t correct the soil problem with grit or sand. Transplant your shrub between October and March for the most success.

Planting too deeply

Most shrubs suffer in some ways if you plant them too deeply. Feel around at the bottom of your plant to see if you can feel the root ball. The top of it should be at or just below the surface level of the soil.

What to do

If you find the rhododendron root ball is quite a ways below the level of the soil, you need to move it up. Wait until October and then carefully dig up the root ball and plant, put some more soil and compost into the hole and replant everything.

You can also learn why your leaves might be turning brown here, which is a sign of leaf scorch

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