Experts and BHG readers answer.
helping my hydrangas bloom the best they can
Hydrangeas bloom blue in acid soil and pink in alkaline soil. The product that you used last year to lower the pH (make the soil more acidic), was likely some form of sulfur or sulfate. The amount needed depends on the starting pH of the soil and the soil type, so having a soil test done to get some baseline information is a good idea. In regions where the soil tends to be alkaline (high pH), you may have to add sulfur every year to keep the pH acidic enough to produce clear blue flowers on the hydrangea.