The most common additive, Aluminum Sulfate is readily available in most garden centers. Aluminum Sulfate should be applied in mid-spring at a rate of 1 tbsp. per gallon of water and can be applied yearly as part of a maintenance program. Extra caution needs to be taken not to over apply and avoid contact with the leaves and stems. The extended continuous use of Aluminum Sulfate can be problematic because the aluminum can accumulate in the soil. A safer alternative may be Elemental Sulfur if the soil already has traces of aluminum present. In mid-spring, one cup sprinkled around the plant's drip line and gently raked in would be sufficient. The last suggestion would be to amend the garden soil around the hydrangeas with sphagnum peat moss. By working one or two inches of peat moss into the top six inches of existing can effectively lower pH. The peat moss application will only last two or three seasons before the pH begins to return to its previous level.
On the opposite end of the spectrum, local gardeners may want to try and get the dark crimson red colors that are found in the Midwest’s alkaline soil. Dolomitic lime is the ideal additive to raise the soil’s pH and by raising the pH stops the hydrangea from absorbing the aluminum. The lime can be applied twice a year (mid-spring and mid-summer) but again moderation is the key when applying since over application will cause leaf-yellowing.
When applying general fertilizers to hydrangeas you want to avoid ratio with a phosphorus (P) number. The fertilizers with higher ratios of phosphorus will actually inhibit the blooms turning blue by limiting the absorption of aluminum. A good general fertilizer NPK ratio would be 25-5-30 while products such as, super phosphate and bone-meal should avoided altogether. For pink and red tones you want to use fertilizers with a higher phosphorus number, such as 25-10-10. Another approach would be to place individual hydrangeas in containers where you have more control over the soil pH.
Keep in mind hydrangeas may take several seasons to settle into their color once planted into the garden. It is also a good idea to test your soil prior to adding amendments to alter the pH. The UMass Extension can provide the testing for a small fee. In a future entry I will further detail a good fertilizer program to keep your hydrangeas healthy and continuously blooming.