Coloring hydrangea blossoms blue - that's how it works!

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For the blue hydrangea flowers is responsible for a specific mineral - the alum. It is an aluminum salt (aluminum sulfate), which often contains potassium and ammonium, a nitrogen compound in addition to aluminum ions and sulfate. All ingredients are important plant nutrients, but the blue color of the flowers is exclusively caused by the aluminum ions.

Not all peasant hydrangeas can turn blue

However, the alum can do no miracles: To make the flowers of your peasant hydrangea turn blue you need a variety that is capable of doing so. Although most pale pink varieties of peasant and plate hydrangeas dominate the color change, they are not dominated by varieties with intense pink flowers such as the peasant hydrangea 'Masja'. Incidentally, the popular variety 'Endless Summer' can be dyed relatively well.

Important: An acidic humus soil

A second important requirement for blue-flowering hydrangeas is the soil reaction: only in acidic soils do the aluminum ions accumulate in the soil solution and can be absorbed by the plants. The plants show an intense shade of blue at pH values ​​below 5.0. As early as 5.5, the color turns slowly into a blue-red, and from 6.0, the shrubs have lilac-pink flowers. A low pH can be achieved by incorporating lots of foliage compost, needle control or rhododendron soil into the soil.
On sandy soils, the pH drops relatively quickly, while loamy soil shows a high buffering capacity and hardly drops below values ​​even after enrichment with acidic humus. Here is a complete soil exchange in the root zone of the plants promising - or the culture of hydrangea in the pot, because it gives you the best control over the pH of the earth. Incidentally, you can easily measure the pH of the soil with corresponding test strips from specialist retailers.

Hydrangeas blue

In pot hydrangeas - here the variety 'Endless Summer' - it is particularly easy to produce a blue flower color

Give alum with the irrigation water

If these conditions are met, the alum comes into play. It is available in the pharmacy, you can buy it in the garden shop but also as a combination product with hydrangea fertilizer. If you are using pure alum, add three grams per liter to the irrigation water and stir until it dissolves. If possible, water the plants with lime-free tap water or rainwater. If the water is too hard, the lime dissolved in it raises the pH of the earth again and the effect of the alum is correspondingly weaker. From the beginning of May until the beginning of June, water your hydrangeas four to five times with the alum solution every week. Fertilizers with "Blaumacher" should simply be used according to the packaging instructions. However, their effect is usually somewhat weaker than pouring with pure alum.

Video Board: How to Change the color of your hydrangeas like magic.

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