Yellow leaves on hydrangeas: how to treat an iron deficiency


I studied agricultural sciences and have always preferred spending my free time outdoors. Apart for my enthusiasm for gardening and agriculture, I love taking photos and rarely leave home without my camera. Whether it is landscapes, blossoms or wildlife, I can usually find a perfect shot that captures the beauty of nature.

Favourite fruit: strawberries, blueberries, plums
Favourite vegetables: radishes, tomatoes, pumpkin

Yellow leaves on hydrangeas are quite a common occurrence and are an indication that the plant has a nutrient deficiency. Read on to find out how to identify and remedy the issue.

Iron deficiency on hydrangea leaves
If your hydrangea has an iron deficiency, the leaves turn yellow, but the leaf veins usually remain green [Photo: Alena Brozova/]

If the leaves of your hydrangea turn yellow, it is likely down to a lack of chlorophyll production caused by a shortage of iron. This is known as chlorosis. Iron is one of the central components in the production of chlorophyll, which in turn is fundamental to photosynthesis. Photosynthesis, as we all know, is essential for plant survival, so chlorosis needs to be treated quickly.

While chlorosis can be caused by other nutrient deficiencies, it is most commonly an iron deficiency. In the following section you will learn how to recognise whether your plant has an iron deficiency or something else. We also give instructions on how to remedy an iron deficiency, as well as tips on how to prevent chlorosis from occurring in the first place.

Tip: Iron deficiency is particularly common in hydrangeas and other bog plants that love acidic soils.

Recognising chlorosis in hydrangeas

The symptoms of chlorosis look the same whether the plant is in a container or in a bed. The young leaves turn yellow while the thick leaf veins remain green at first.

Distinguishing an iron deficiency from similar nutrient deficiency symptoms:

  • Nitrogen deficiency starts on the oldest leaves, causing them to turn completely yellow with no green leaf veins.
  • The same applies to a sulphur deficiency, though this only occurs very rarely.
  • Magnesium deficiency also first appears on the older leaves, but the leaf veins remain green.
Yellow leaves on hydrangeas
A magnesium deficiency appears first on old leaves and should not be confused with an iron deficiency [Photo: izzzy71/]

Preventing iron deficiency-chlorosis on hydrangeas

While there is usually enough iron in natural soil, hydrangeas often struggle to absorb enough of it. This occurs when the soil pH is too high. Hydrangeas are adapted to soil with low pH values – around 4 to 5.5. Unlike other plants, they depend on being able to absorb iron directly from the soil as a 2-valent form (Fe 2+). However, divalent iron is found almost exclusively in acidic soils in which iron has been oxidised to this level. In calcareous soils, hydrangeas are sure to develop a so-called “relative iron deficiency” – iron is present but cannot be absorbed by the plant.

Hydrangea in the woods
A hydrangea’s natural habitat is moist and acidic [Photo: yspbqh14/]

Tip: It is also interesting to note that the pH of the soil affects the colour of the plants’ flowers. For example, blue hydrangeas need a pH value of 4 to 4.5 to be able to absorb enough aluminium for blue flowers to develop. For more information, read our article on blue coloured hydrangeas.

The best way to prevent iron deficiency-chlorosis in hydrangeas is to plant them in suitable ericaceous or acidic soil. When planting in a bed, mix in some peat or reduced-peat ericaceous compost and check the pH value once a year. This is necessary because the compost mixture around the plants will influence the pH value of the soil in the long run and the pH value may rise again.

In order to determine the pH value of your soil, do a pH test – you can buy testing kits from a variety of different suppliers either online or in some garden centres and hardware stores. If the measured pH value is too high, it can be lowered with ericaceous compost or lime-free peat. It is best to plant potted hydrangeas in an ericaceous (acidic) soil from the outset.

Planting hydrangea in flowerbed
Using acidic, iron-rich soil when planting prevents deficiencies [Photo: Mariia Boiko/]

How to treat chlorosis in hydrangeas

The first step in remedying chlorosis is to check and adjust the pH of your soil. In very rare cases, however, there may be an “absolute” iron deficiency, where iron simply is not present. This happens more often with potted plants than bedded plants because of the limited supply of iron in the potting compost. If the pH value of the soil is low and the hydrangea still shows the iron deficiency symptoms described above, then you need to fertilise with iron. This can be done either with fertiliser granules, liquid fertiliser or foliar fertilisation:

  • Granular fertiliser with iron: A specially adapted granulated fertiliser is well suited as a remedy for iron deficiency – it can also help prevent it in the first place. Our Plantura Hydrangea Food, for example, contains enough iron to rule out the risk of an absolute iron deficiency from the outset. By slightly acidifying the soil with iron sulphate, this also prevents a relative iron deficiency. Our Plantura Hydrangea Food is made from 100 % animal-free materials, as well as mineral ingredients that are also used in organic farming.
Hydrangea Food, 1.5kg
Hydrangea Food, 1.5kg
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  • For beautiful hydrangeas with lush blooms in pots & flower beds
  • Prevents common deficiency symptoms & supports healthy plant growth
  • Long-lasting fertiliser that is free from animal products - child & pet friendly

  • Liquid fertiliser with iron: A good liquid fertiliser containing iron with so-called chelating agents is faster and more effective than the granule method. Plants produce chelates at their roots in order to convert iron into a usable form. As hydrangeas are unable to do this themselves, they require a special fertiliser. Our Plantura Liquid Citrus Food contains iron, other trace elements and suitable chelating agents that work quickly and efficiently to remedy deficiencies.
  • Iron foliar fertilisation: To a certain extent, plants can also absorb nutrients through their leaves. In doing so, the trace element reaches the plant parts exactly where it is needed. However, iron foliar fertilisation is usually unnecessary for private users. It is only used on a commercial scale, where a prolonged iron deficiency would mean significant yield losses. The special fertilisers for this are often expensive and need to be applied in very precise doses to avoid damage to the leaves. Foliar fertilisation is only effective for a short period and must be applied frequently or supplemented by normal iron fertilisation.
Potted hydrangea
Hydrangeas in pots need a regular supply of nutrients from an appropriate fertiliser [Photo: Janemf1/]