Hydrangea Flower Color - Pink or Blue
The following information does not apply to white hydrangeas.
USE ONLY PLANT STARTER FERTILIZERS ON NEWLY PLANTED HYDRANGEAS. THE FOLLOWING FERTILIZER SUGGESTIONS ARE FOR ESTABLISHED HYDRANGEAS.
Changing Pink to Blue
Many hydrangeas that are pink in bloom can be made to change to blue. The addition of aluminum sulfate in most cases will generally change the blossom to blue. You may also use Mir-Acid, (the acidic form of Miracle-Gro) or Holly-Tone. These soil acidifiers will enable the hydrangea to absorb aluminum from the soil. It is this absorbed aluminum that turns the flowers from pink to blue. Below is a more detailed explantation for those interested.
The PH of soil is measured on a scale of 1-14. 1 is very acidic, 7 is neutral, and 14 is very alkaline. Water is very close to 7. Soil in the our area tends to be slightly acidic with a ph around 6.3-6.5. This lends itself towards a more pink bloom. Blue flowers in hydrangeas are most easily attained with soil ph in the 5.2-5.5 range. This requires the addition of a soil acidifier such as aluminum sulfate, Mir-Acid, or Holly-Tone to produce a more blue bloom. Coffee grounds can also be used to acidify. It is best to add these acidifiers before the plant starts to bloom, even better in the preceding fall. Be patient, acidifying the soil enough can take several years.
Soil in our area generally contains aluminum. If the soil is of the proper ph the aluminum will be available to the plant. If the ph is too high the plant will be unable to absorb the aluminum and the blooms will remain pink.
Intensity of Blue
The intensity of the blue in the flower is controlled by health of the plant and weather conditions. Normally fertilizing, by aiding the health of the plant will usually increase the intensity of the blue.
Changing Blue to Pink
This is generally not a problem in our area but if your soil is very acidic you can do the following. Add lime to sweeten the soil. Getting the PH above 6.5 usually will turn your blue flowers pink. You can also add a high phosphous fertilizer such as bone meal. Phosphorus will inhibit the plant's absorbtion of aluminum. Again, be patient, soil changes work best over a couple of seasons.