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How to turn a hydrangea blue

How can you turn a hydrangea blue?The blueing of a hydrangea can be done by adding aluminium sulphate during a certain period of growth. Over the last couple of years, the manner of blueing these flowers has become simpler and safer. An example is fewer application of the solid form and more use of the liquid form. The dissolution of the solid form is difficult to achieve and very often residue is left behind. Curious how to correctly dissolve aluminium sulphate? Follow our specialist’s tips. 

Liquid aluminium sulphate

Aluminium sulphate is available in the liquid form, which simplifies the blueing of the hydrangea along with more safety. The liquid form of the product has several advantages:
  • No time is lost when preparing the liquid
  • Less labour-intensive, since no bags must be lifted
  • The liquid form remains well dissolved and no precipitation (only if the storage temperature remains above 7°C)
  • No blockages 
  • Can easily be dosed by dripping systems
  • Less residue of the product on the plant, due to better solubility
  • The powders that are released from the solid product are unhealthy. The liquid product does not have this problem.

Fertilizer advice on blueing of hydrangeas

Blueing hydrangeas cannot take place without a reaction with aluminium. The pH level is crucial for this process. If the pH is too high, the plant will no longer be able to take in the aluminium, leading to damage. However, a pH that is too low is not desirable either. A pH level of 4.0-4.3 is seen as ideal for blueing hydrangeas. 

However, it should be taken into consideration that aluminium is easily taken over by phosphate. As a result, the aluminium is no longer available to the crop. This can be solved by using phosphate-free fertilizers prior to the growth period in which blueing takes place. If this isn’t done, the aluminium and phosphate will connect and precipitate, causing damage to the plant. 

Blue pot Hydrangea

The pot Hydrangea’s own pigment is called Delphinidin glycocide (delphinidin-3glykosid). Only the dark pink varieties contain enough of this pigment (DG) to be able to colour blue after the addition of aluminium sulphate. The other Hydrangea varieties will colour differently. Known discolourations after the addition of aluminium:
  • Pink: turns dark blue 
  • Light pink: turns pale blue 
  • Red: becomes violet (pelargonidine diglycocide)
Please note: The product Fertigro AS has a different effect

Blueing of the cut-hydrangea

Cut hydrangeas can be blued all year round, although the time from February to April is more suitable. If you are not satisfied with the colour of the flowers, it is possible to blue a little more. 


Product specialistsAny questions about blueing hydrangeas?

Do you have any questions about crop care, or do you want personal advice? Contact one of our product specialists, or use the form below.

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