Even when they completely understand the simple soil chemistry involved, taking steps to change the color of hydrangeas still makes most gardeners feel a bit like magicians. Hydrangea color is determined, in part, by the availability of aluminum compounds in the soil, and it is easy to make aluminum more or less available. The principles may be simple and the directions may be easy, but the results remain firmly within nature's province. Every plant, every yard and every year will yield slightly different results.
Determine that your hydrangea plants possess the capacity to change colors. Older, pale, here-when-we-bought-the-house varieties often can. Hydrangea-lovers' websites and local nurseries can help you determine whether your hydrangeas can be turned. Some hydrangeas, however, such as Alpengluh (red) and Annabelle (white), are genetically programmed to keep their colors true and consistent. Names are not always a perfect guideline (Nikko Blue, for instance, can be turned pink with changes to soil chemistry). When you purchase hydrangea plants, those that stay true to color are usually labeled as such. Those that are changeable do not usually contain color information. Note also that color-change is a quality prized in big-leaf or macrophylla hydrangeas. PeeGee tree-type and oakleaf hydrangeas are usually bred for other qualities and are therefore not part of this discussion.
Be prepared to persist with changes in soil chemistry. Hydrangea bloom color depends on the aluminum content in the soil, and this cannot be done only once. If you have determined that your hydrangeas look best in blue, prepare to add necessary soil acidifiers on a regular basis (fall and spring, every year). Like fertilizer or other soil additions, supplies need regular renewal to keep soil chemically balanced.
Add aluminum sulfate or another high-acid fertilizer if you seek blue flowers. Mix 1 to 2 cups in with planting soil if you are starting a new hydrangea. Side-dress established plants by mixing 1 to 2 cups of aluminum sulfate with soil all the way around the plant. Loosen and mix soil with a trowel or hand fork, taking care not to disturb or break plant roots. Water thoroughly after you have added acidifiers. Repeat this process at least twice a year, to keep soil consistently acid.
Apply powdered lime by the same method if you want to turn blooms pink. Raising the pH of soil makes it harder for hydrangeas to access available aluminum. In a strongly alkaline soil with a pH of 6.5 or greater, blooms will turn pink. Repeat for consistent results.
Expect results to vary, depending on the variety of hydrangea, general soil conditions, and even weather conditions over the year. Heavy rains wash away all kinds of soil nutrients; a cool, cloudy summer can affect all kinds of flower blooms, making them paler and less vigorous. The exact variety of hydrangea will also make a difference. Some will take on a pale shade of blue or pink, irrespective of how much and how often they are fed. Other varieties may color deeply. Overall soil condition impacts results, too. In a yard where soil is definitely alkaline, adding acid to hydrangeas may produce purple, a little blue, a little pink--or all three.