By Jim McCausland, Sunset senior garden writer
Garden hydrangeas naturally come in the white/pink/red/purple/blue range, but in some measure, flower color depends on soil pH. Here are the principles.
White-flowered hydrangeas will always be white.
Some reds and blues will always be red or blue. Some two-tone (red- or pink-and-blue) will always be two-tone.
Some hydrangeas (like 'Masja', 'Nigra', and 'Tricolor') will be either pink or blue, depending on soil pH and aluminum content. More acid soil (lower pH) with more aluminum tends to produce blue flowers, while more alkaline or basic soil (higher pH) with less available aluminum tends to produce pink or reddish blooms.
For blue flowers, water aluminum sulfate in around the base of established hydrangeas at the rate of 1/2 ounce per gallon of water through the growing season. Only apply the solution after youâve already watered the plant. You can buy aluminum sulfate at local nurseries or online from Hydrangeas Plus.
For pink flowers, mix lime or superphosphate into the soil at planting time (easiest if you're planting hydrangeas in big pots). Virtually every nursery and garden center sells these amendments. You can also scratch them into the soil around already planted hydrangeas, but in such cases the soil pH is only altered slowly, so you may have to keep at it for two or three years to see much change.
