Which fertilizer is suitable for blue hydrangeas? How to dye hydrangeas blue and keep the color Our tips for the correct handling of blue-colored hydrangeas. Farm hydrangeas ( Hydrangea macrophylla ) are special in terms of their flower color because this does not only depend on the hydrangea variety, but also the soil conditions.
Garden hydrangeas and plate hydrangeas of the species H. macrophylla, which are grouped under the name of farmer’s hydrangeas, bloom in pink, red, purple, white, or blue. But blue and purple hydrangeas quickly lose their extravagant color and turn red or pink – we explain how you can avoid this. In the following, we show the conditions under which hydrangeas bloom blue and what the soil conditions have to do with it. You will also find instructions on how to color your hydrangeas blue yourself.
What makes pink hydrangeas blue?
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The reason for the change in color of the hydrangea flowers of pink and red varieties lies in the flower color of the plant. The red or blue dye is called delphinidin and belongs to the anthocyanin group. This color molecule changes its structure with the help of metal ions (aluminum, iron) and forms so-called chelate complexes. These give the hydrangea flowers an intense blue color.
To color a hydrangea blue, the flower must ideally be pink first. If their color is too red, the combination with blue delphinidin tends to result in a purple hue. Since white hydrangeas do not contain any delphinidin at all, they cannot be colored blue.
Requirements for dyeing hydrangeas blue:
Hydrangeas need acidic soil. This is characterized by a pH value below 6. Hydrangeas preferably have a pH value around 4.5, they are ideally adapted to these acidic soil conditions. If the pH value is too high (> 4.5), the iron and aluminum ions that are important for the blue color are no longer readily available in the soil and their uptake by the hydrangea roots is severely restricted.
Another basic requirement for successful blue dyeing is that the plant to be colored is a hydrangea of the right species. In addition to white varieties of the species H. macrophylla, other hydrangea species such as velvet hydrangeas ( Hydrangea Aspera ssp . Sargentiana ), climbing hydrangeas ( Hydrangea petiolaris ) and panicle hydrangeas ( Hydrangea parniculata ) cannot be colored blue. Only farmer hydrangeas with violet and pink flower colors can produce this shade.
Furthermore, the hydrangea must be healthy and sufficiently supplied with nutrients and water. To ensure a needs-based supply of nutrients, hydrangeas should be fertilized annually with a suitable fertilizer such as our Plantura hydrangea fertilizer.
Here’s how to test the pH for blue hydrangeas
An acidic soil (soil pH <5) is one of the basic requirements for blue hydrangea flowers. How to Test Your Soil’s pH:
- Put about 100 g of your soil in a mason jar.
- Add 100 ml (distilled) water and mix everything thoroughly.
- After 10 minutes, you can dip a pH test strip into the solution and read the result on a color scale (you can get the pH paper in hardware stores or pharmacies, for example).
- However, the result obtained in this way is not exact: the actual pH value is about 0.5 points lower than indicated.
- You can obtain a more precise and faster measurement result with a special measuring device, a so-called soil pH meter.
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If your pH value is above 5 according to the measurement, you should acidify it as described below in the article.
The lower the pH, the more noticeable the blue color of the hydrangeas:
- Weakly acidic soil (pH value around 5.5): The blue color no longer forms and the blue or purple hydrangeas turn pink or red again.
- Strongly acidic soil (pH value around 4): the blue hydrangeas keep their blue color, pink hydrangeas turn blue.
Coloring hydrangeas blue: instructions and the right fertilizer
To achieve the color change of the hydrangea flowers from pink to the popular hydrangea blue, essentially two steps are important. The first thing to do is to acidify the soil in which the hydrangea grows. A low pH value is a prerequisite for the uptake of aluminum and iron ions, which can be supplied secondly by adding alum dissolved in water. Finally, maintaining the acidic soil pH is important to avoid back discoloration.
How to color pink hydrangeas blue
- To lower the pH value directly when planting, you can either use special (usually peat-containing) hydrangea or rhododendron soil or mix your garden soil directly with peat. Since hydrangeas and other bog plants can no longer efficiently absorb essential iron when the pH value is too high, this measure is necessary in any case to prevent hydrangeas from iron deficiency. The acidic pH makes the aluminum in the soil available and the hydrangeas turn blue.
- To achieve a faster and more intensive color change in the hydrangea flowers, aluminum can also be added artificially to the soil. More precisely, one uses potassium aluminum sulfate (alum) or aluminum sulfate. You can even get these aluminum compounds in pharmacies.
To get a blue color right from the start, water the hydrangea with a solution of three to four grams of alum in one liter of water during the phase of sprouting, or until the beginning of June. You should repeat this about four to five times with an interval of 7 days.
- You can get “Blue Hydrangeas” from garden shops. These agents lower the pH value for a short time and the aluminum turns the hydrangeas blue. “Hydrangea blue” unfortunately does not keep the flowers permanently blue, but acidifying the soil has a lasting effect. Please do not confuse hydrangea blue with blue grain. Blue grain is a universal fertilizer that got its name because of its own color.
Are there home remedies for coloring hydrangeas blue?
If you don’t want to buy environmentally harmful peat and hydrangea blue or alum, you can try using home remedies to turn the hydrangea flowers blue. Grape marc (a by-product of winemaking), the needles of conifers, or acidic primary rock flour (basalt, granite) are suitable for lowering the pH value of the soil. Iron and aluminum ions are present in sufficient quantities in most natural soils and do not have to be supplied.
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With potted plants, you should therefore mix in a good amount of garden soil to bring in iron and aluminum. The acid bog soil, of all things, which is present on former, drained bog areas, is an exception here – it is very poor in iron.
This is how you can keep hydrangeas blue
When planting or transplanting blue hydrangeas, the most important thing is choosing the right substrate. The pH value of the soil must then be kept permanently low to avoid discoloration. You can obtain acidic soil with a pH value of 4.5, among other things, by using only low-lime rainwater for watering, which is acidified by peat, vinegar, or bark mulch.
In addition, you should regularly mulch with leaf compost, lawn clippings, or needle litter, as the organic material releases acids into the soil, which have a pH-lowering effect. Regular use of grape marc and sour rock flour also helps keep the soil acidic.
With a soil pH meter, you can check the pH value of the substrate to be able to react in good time to an increase in pH value.
Summary of coloring hydrangeas blue :
Two conditions must be met for your hydrangeas to turn beautiful blue:
- Only pink or red hydrangeas of the species Hydrangea macrophylla can be colored blue or dark purple.
- The soil must be acidic, i.e. the pH value must be around 4.5 and kept low: For this purpose, mulch regularly with acidic material and water with acidified rainwater.
- The blue color is easier to achieve in the bucket than in the garden soil due to the smaller volume of soil.
- Aluminum ions must be available in the substrate: The hydrangeas can be poured with aluminum sulfate (alum solution or special fertilizer), but aluminum is usually present in natural soils.