Hydrangea macrophylla of the mophead or lacecap varieties are chameleon color shifters, able to produce pink or blue blooms depending on the pH level of the surrounding soil. Altering soil pH is an involved, trial-based experiment requiring dedication and consistent vigilance on the part of the brave gardener. However, the results of playing with color can be rewarding as you watch your blue hydrangea blossoms of last season morph into gorgeous, lacy pink clusters before your very eyes. Expect that soil changes may require several months to take effect, so begin the process of testing and amendment in the fall prior to the coming growing season.
Use a home soil testing kit as one way to determine the pH of your hydrangea bed. Tests can be purchased at local gardening suppliers or through online resources, including electronic versions that give almost instant read-outs and traditional core sampling devices.
Contact a local university extension office to locate a lab that will test your soil for you. The office will either mail you the sampling kit and instructions, or require you to deliver a sample. When sampling, take 6 inches of soil with a clean trowel from up to 12 distinct planting zones and mix them together in a clean bucket. The result should be about 2 cups of soil that will be examined at the extension office by experts for pH and amendment recommendations.
Check the results of your pH test to determine if your soil is acidic or alkaline. 7.0 pH indicates neutral soil, below 7.0 is acidic and above 7.0 is alkaline or basic. The key to blue blooms is acidic soil pH, ideally between 5.0 and 5.5 pH. For those coveted pink blooms, alkaline soil above 6.0 pH is needed. The likely result of your soil test if you have been experiencing blue flowers will be below 7.0 pH. Acidic soil requires amendment to change those boy blues to a feminine pink.
Amend acidic soil by the addition of finely ground lime to raise the pH above 6.0 into the alkaline ranges. Your soil test kit or university extension office recommendations will indicate the precise rate of lime application over the total square feet of your hydrangea bed. Spread the lime over the surface of the bed and carefully cultivate it into the soil, avoiding the roots as you incorporate the lime evenly.
Water the soil well. Although lime is not highly water soluble, moist soil will accept the lime and provide more soil-to-lime contact that is necessary for the pH change to take place.
Test the soil again in the spring to check for altered pH. Don't be disappointed if the pH results or the hydrangea bloom color change are not as dramatic as you had hoped. Adjusting soil pH can take time and may require different strategies in order to produce the pink color of your dreams.