Most hydrangeas have a very interesting property--the color of the flower is dependent on the pH of the soil. If the soil is acidic, the flowers are blue with the color of blue getting deeper as the acidity of the soil is increased. If the soil is alkaline, the flowers will be pink. Again, the more alkaline the soil, the deeper the pink color of the hydrangea.

To get more acidic soil for blue hydrangeas, add a 1/4 ounce of aluminum sulphate and a 1/4 ounce of sulphate of iron to one gallon of water. You can apply up to two gallons of this solution in spring and fall. See how your plants response/survive before trying to make a stronger (more acidic) solution.

To get pink hydrangeas, slowly add lime to the soil over time. Again, see how your plant responds/survives before make the soil more alkaline.