Changing the Colour of Hydrangeas
Do you know that you can change the colour of your hydrangea? If it is was pink last year you can make blue this year or vice versa.
It is the pH (acidity or alkalinity) that determines the colour of hydrangea flowers. The pH determines whether aluminium is taken up by the plant. The more aluminium taken up the more pink the flowers. Don't be surprised if the hydrangea your bought changes colour when you plant it; the pH may be different in the pot to your flowerbed.
You can test the pH of your soil using a soil pH kit from your local hardware or garden centre. Acidic soil, with a pH lower than 6.0, yields blue or lavender-blue hydrangea blooms. Alkaline soil, with a pH above 7.0, promotes pinks and reds. With a pH between 6 and 7, the blooms turn purple or bluish-pink.
To change the pH of your soil add garden sulphur or aluminium sulphate to your soil. To raise the pH, use lime. See here for more details on how to adjust pH in soils. Note: you may have to apply sulphur or lime several times to change the pH enough to change the colour of your hydrangea. So stating to change pH early, when the plants are still dormant gives you time to get the colour you want when they flower in summer.