I planted seven large hydrangea bushes a couple of weeks back – three white varieties, three blue varieties and one red. The latter is ‘Geoffrey Chadbund’, whose heads are touted as deep red. They’re probably not the red that you and I imagine but, as you can see here, they’re as red as hydrangeas get. However, flower colour is related to soil pH and I reckon mine might eventually turn blue. That’s because my soil is acidic. Plant pink hydrangeas in acidic soil (one with a low pH) and they’ll turn blue. Plant blue hydrangeas in alkaline soil (with a high pH) and they’ll eventually turn pink.
Technically speaking, it’s the aluminium content in the soil that influences the colour. To maintain a blue colour, aluminium must be present. To maintain a pink or red colour, you need to take aluminium out of the picture. Since hydrangeas take up aluminum best at lower pH levels (acidic), raising the pH will help lower its absorption. You can raise the pH by adding lime. On the other hand, adding aluminium sulphate (or Yates Hydrangea Blueing Tonic) will lower the pH and induce a blue colour.
White hydrangeas are not affected either way.
(Photo credit: the white hydrangea in doily vase is from Country Living)