Nurserymen usually force hydrangeas into flower in the spring and this is the best time to buy plants, which are usually available with flowers in shades of pink, red, white and blue. The pink-flowered varieties range from light pink to deep rose; and these can be changed into blue flowers by treating the soil with a hydrangea 'blueing compound' or aluminium sulphate. The lighter shades of pinks convert to the most attractive shades of blue. Only pinks can be changed to blue, because reds turn a mauvy-purple. It is apparently the absorption of aluminium by the plant which changes the colour from pink to blue; and when the soil is neutral or has a good lime content this seems to prevent the assimilation of aluminium by the plant. Consequently, in order to keep pink shades pure in shade the compost should have a good chalk or lime content. White flowers are unaffected by the acidity or alkalinity of the soil.
