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January 12, 2017

How to Colour Your Pink Hydrangea Blue: Practical Tips

Colour your own Pink Hydrangea Blue

Every garden has a Hydrangea. Many garden lovers ask themselves how they get a blue colour in the flowers of their Hydrangea. In this article you read about the key factors for the blue discoloration. And practical tips to colour your own hydrangea blue. A reverse discoloration is of course also possible.

Three basic requirements for blue hydrangeas

To get the flowers of a hydrangea a blue discoloration you need to take three requirements into account:

  1. Requirement 1: Species- & Variety Selection
  2. Requirement 2: Acidic soil (so no chalk)
  3. Requirement 3: Aluminum salt (alum)

Requirement 1: Species- & Variety Selection

Not every species of Hydrangea can get blue discoloration in their flowers

  • Only cultivars of the species Hydrangea macrophylla en some varieties of Hydrangea serrata can get blue discoloration.
  • Hydrangeas with white or cream coloured flowers, such as the well known  Annabelle (Hydrangea arborescens) and oakleaf hydrangea (Hydrangea quercifolia), will never turn blue (or pink).  At most they can get a pinkisch hue during autumn .
[mks_icon icon=”fa-hand-o-right” color=”#666666″ type=”fa”]Tip: Buy a hydrangea when it is in bloom, in the summer. When the plant has pink or blue flowers, then you are sure to get the plant potentially blue flowers.

How intense blue a hydrangea can be is different for each race. A light pink hydrangeas can discolour to a pale shade of blue; a dark red or dark pink hydrangea to dark blue Hydrangea or even deep purple.

Blue hydrangeas that you dry on the plant get a purple red colour in autumn. This is caused by the colder nights.

[masterslider id=”9″] Picture: Cultivars that belong to Hydrangea arborescens, Hydrangea paniculata, Hydrangea aspera and Hydrangea quercifolia you CANNOT colour blue or pink.

Requirement 2: Acidic soil (so no chalk)

Without acidic soil your pink hydrangea will never turn blue! Only in acidic soil (low pH) can a hydrangea absorb aluminium (and iron), which is necessary for the blue discoloration. Some practical tips for what you can do and what cannot do:


  • Rhododendron ground
    Plant your hydrangea in acidic soil (pH level 4,5 tot 5,5). Use for example Rhododendron-soil (with peat). The more acidic your soil is, the more blue discoloration your hydrangea gets.
  • Acidifying materials
    After planting you can gradually lower the acidity in the soil by sprinkling acidifying materials with your hydrangeas.  For example peats, citrus peals and rhubarb leaves.
  • Sparling fertilize 
    Fertilize your hydrangeas sparingly,  and only in during April, May and June. By not fertilizing the plant from July onwards, the ‘hungry’ plant will easier absorb the alumin September .


  • Sprinkle chalk
    Do not give your hydrangea chalk, because it will turn pink. Chalk increases the pH levels and combines the aluminium  and iron, en bindt aluminium en ijzer, whereby the plant can no longer absorb the  aluminium and iron.
  • Plant combinations with met chalk loving plants
    Chalk loving plants benefit from a yearly dose of chalk. Combine a hydrangea therefore not with plants that you give chalk to on a yearly bases.
    Chalk loving plants are: Aucuba, Clematis, Ceanothus, Dianthus (carnation), Geranium, Hedera, Helleborus, Lavandula (lavender), Papaver (poppy), Rosmarinus, Syringa (lilac ), Viburnum and Wisteria.
  • Tap water
    Tap water contains more chalk than rain water. Use therefore preferably rainwater.
  • Concrete
    Concrete emenates chalk. Avoid planting a  blue hydrangea next to a concrete foundation or a concrete tub.
[masterslider id=”10″]

Requirement 3: Aluminum salt (alum)

Sprinkle twice a year potash alum (aluminum sulfate) with a pink plant in bloom:

  • 1x in May-June
  • 1x in September/ Start October.

How much alum?

The required dosage depends on the plant size. Give a mature plant (7 years and older) with pink flowers at least 500 grams of potassium alum at a time; in young plants 150 grams is sufficient.

When you add to little alum to the flowers they either become less blue or they remain pink. If you add to much the flowers may burn.

Repeat the adding of alum every year. When the hydrangea has reached the correct blue colour, you can lower the alum dosage by half or omit it entirely.

How to dosage alum?

You can add alum in two ways to your hydrangea

  1. Sprinkle alum
    Sprinkle the alum around the plant on the ground.  The best spot is where the young roots are: in a circle around the plant, evenly far from the plant base as the shoots of the plant. For a quicker result you can rake in the grains of alum and water the plant twice. The rain does the rest.
  2. Pour alum
    Desolve the alum in warm water and pour it on to the plant (not on the leaves).
[mks_icon icon=”fa-hand-o-right” color=”#666666″ type=”fa”]Tip: Give your hydrangea first a watering can of clean (rain)water, to prevent root burning by the door het aluminum sulfate.

[mks_icon icon=”fa-hand-o-right” color=”#666666″ type=”fa”]Tip: Do not let alum lying on the leaves, as this can lead to leaf burning.

How quickly turns a pink hydrangea blue?

Please note that a hydrangea does not change color in a day. Blue staining is a slow process. It takes 1 or 2 years before reaching the desired result. Gardening requires lots of patience, but the reward is much greater.

Are there different kinds of alum?

Alum is aluminum sulfate, a salt, and is available in different versions:

  • Aluminium Sulfate[Al2(SO4)3]
  • Aluminum potassium sulfate dodecahydrate [AlK(SO4)2·12H2O]

Aluminium Sulfate desolves less easy in water than aluminium potassium sulfate dodecahydrate (=potash alum). Hence, in the latter variant, the aluminum is more quickly available to the plant; first-mentioned variant has a longer duration of action as an advantage.

[mks_col] [mks_one_half]Pink Hydrangea macrophylla Aesha.[/mks_one_half] [mks_one_half] Blue Hydrangea macrophylla 'Aesha'.[/mks_one_half] [/mks_col]

Picture: Blue staining or pink coloration of Hydrangea macrophylla Ayesha. Left the pink variety, the right blue colored variant, a year later.


Blue hydrangea flowers that remain on the plant, color in autumn often pink red under the influence of cold (night) temperatures. The next summer the plant will display blue flowers.

[mks_col] [mks_one_half]
Picture : Seasonality. Left a blue stained hydrangea in June.
Picture : Seasonality. Left a blue stained hydrangea in June.
[/mks_one_half] [mks_one_half]Picture : Seasonality. On the right the same plant in October; the dried flowers on the plant are red.[/mks_one_half] [/mks_col]

Picture : Seasonality. Left a blue stained hydrangea in June. Right the same plant in October; the dried flowers on the plant are red.

[mks_icon icon=”fa-hand-o-right” color=”#666666″ type=”fa”]Tip: You can use Hydrangea flowers at home in a floral arrangement. They are also suitable as a dried flower.


It’s a myth that hydrangeas  would color blue by spreading rusty nails near the plant. In theory, rusty nails could help, but only if the plant were growing in acid soil  and the plant has no iron available. In practice, iron deficiency usually isn’t the limiting factor to coloring hydrangeas blue.

Personal Advice

We are happy to provide you with advice when buying a hydrangea or its care. Please feel free to email us!




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