Last August I wrote a post about Blue Hydrangeas and promised that soon I would write one about Pink Hydrangeas. I didn’t intend it to take seven months, but we have achieved a lot at the nursery in that time.
Anyway lets uncover the mystery of pink and red hydrangeas.
Pink and red flowers are produced when there is no aluminium in the soil. This can be achieved in a number of ways:
- Lime or alkaline soils (pH of 6.5 or above) – these are great for growing beautiful pink and red hydrangeas as there is no available aluminium. In these soils certain plant nutrients are locked up and are unavailable to plants. Aluminium is one such nutrient.
- Applying lime – if you have acid soils (with a pH of below 6) one option is to add a couple of dressings of lime (spring and summer) each year to increase the pH. Care needs to be taken though to avoid other acid-loving plants such as rhododendrons. Note – over application of lime can cause chlorosis or leaf-yellowing.
- Phosphate fertiliser – is another option for acid soils. Application of a fertiliser with an NPK ratio of 10-30-10 should be sufficient to change the flower colour. Phosphate also locks up aluminium making it unavailable to plants.
- Coastal sandy soils – the aluminium is readily leached out of sandy soils.
- Pots and tubs – potting mix generally does not contain aluminium. Check fertiliser to ensure it does not contain aluminium.
- Planting near a spring or running water – the aluminium will be washed away or leached out of the soil.
- Planting near concrete – the cement in concrete contains lime which leaches into the soil raising the pH and locking up aluminium.
Some areas of New Zealand have naturally alkaline (limey) soils such as the peat soils around Cambridge, parts of Te Kuiti and Palmerston North and typically grow brilliant reds and pink without effort.
Note – if you want a pink hydrangea select a good pink variety such as Madame Plumecoq, Madame Cayeux or Masja; rather than attempt to turn a blue variety pink. While some varieties will produce either a good blue or a good pink this is not always the case. Some varieties such as Paris, Harry’s Red and Red Emperor try very hard to stay red in acid soil.
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