Which color the lush inflorescences of the hydrangea show is an exciting topic, because you can influence this color yourself using simple means. A hydrangea that is originally pink will change flower color to blue or purple depending on how acidic or alkaline the soil is.
How to dye a hydrangea purple?
To color a hydrangea purple, the soil should be acidic (pH 4-4.5) and contain aluminum. Add aluminum sulfate or hydrangea blue and low-phosphorus, high-potassium fertilizer. If necessary, peat-free rhododendron soil or compost can be used to acidify the soil.
Vivid purple hydrangeas
Hydrangeas prefer acidic soils with a pH value between 4 and 4.5. Many substrates do not have the optimal values, as you can see in the table below:
| Floor | pH value |
|---|---|
| Humus | 4, 0 – 5, 0 |
| Moorland (peat) | 3, 8 – 4, 3 |
| Rhododendron soil | about 4, 5 |
| sandy soil | under 4, 4 or over 8, 8 |
| Sandy soil with little clay | 5, 5 – 6, 2 |
| sandy clay | 6, 3 – 6, 7 |
| potting soil | 6 – 7 |
| clay soil | 6, 5 – 7, 2 |
If you have planted the hydrangea in more alkaline soil, the flowers will have a pink color. However, if the garden soil is acidic, the flowers turn blue or purple. This is due to the fact that the hydrangea absorbs more aluminum from the acidic soil, which is responsible for the sought-after tint.
Color hydrangeas specifically purple
If a purple hydrangea turns back to pink, you must therefore add aluminum sulfate (potash alum, alum) to the hydrangea and acidify the soil.
To make the soil type more acidic you can
- Compost
- composted leaves
- Rhodondrone soil
incorporate into the substrate. For ecological reasons, peat should only be used in exceptional cases.
The aluminum sulfate (€13.00 at Amazon) or alternatively hydrangea blue can be administered together with a low-phosphorus fertilizer with a high potassium content.
Turn purple flowers pink again
If your garden soil contains a lot of aluminum and is acidic, it will turn a pink hydrangea purple, although this is not always desired. Here, too, you have the opportunity to specifically influence the flower color.
First of all, it is important to shift the pH of the soil to the alkaline range. Lime the soil around the hydrangea several times a year and measure the pH value at regular intervals. Ideally it should settle at around 6.2. You can also fertilize the hydrangea for a while with a commercially available flower fertilizer. This contains more phosphorus than special hydrangea fertilizer and therefore inhibits the hydrangea's aluminum absorption, which is responsible for the undesirable color.
Tips & Tricks
You can measure the pH value of the soil using test sticks from gardening stores. These measurements should be carried out at regular intervals, as the pH value can shift back into the alkaline range due to rain and irrigation water.