Color of Hydrangeas in Basic Soil

Alkaline Soil Measurement

• The acid-alkaline balance of all soils is measured on a 14-point pH scale, with a neutral midpoint of 7.0 and scores below 6.5 for acid soils and above 7.5 for alkaline ones. PH stands for potential hydrogen ions, the activity of which determines the availability of soil nutrients to plants. In neutral soil, nutrients are maximally available. Ratings are logarithmic, which means that a soil with a pH of 7.5 is 10 times more alkaline, or basic, than a soil with a 6.5 pH. Within their growing range of 4.5 to 7.5 pH, hydrangeas bloom blue in soils with a pH of 5.0 or below; above 5.5, they turn pink, depending on the availability of aluminum.

Aluminum Availability

• One of the soil nutrients regulated by pH is aluminum. Aluminum is present in many soils but readily available to plants in acid soils. Aluminum controls blue color in hydrangeas, which is why those planted in acid-to-neutral soil flower in shades of blue to purple. Even varieties of mophead hydrangea bred to bloom pink, like "Alpenglow" (Hydrangea macrophylla "Alpenglow") or "Kardinal" (Hydrangea macrophylla "Kardinal") may turn purplish in strongly acid soil. Gardeners with neutral soil sometimes add aluminum sulphate to hydrangea soil to produce the desired blue color. In alkaline soil, aluminum may be present but is not readily available to plants; blooms therefore turn or remain pink. Judith King of the Georgia/South Carolina Hydrangea Society notes that it is more difficult to turn a hydrangea pink than blue, because you are reducing or restricting aluminum rather than simply adding it.

Preparing for Color-Change

• A soil pH test is essential to manipulating hydrangea color. Choosing susceptible varieties is also critical to success. Mophead or lacecap hydrangeas are the only species that respond to soil conditions with color change. Panicled hydrangeas (Hydrangea paniculata), hardy between zones 3 and 9, oakleaf hydrangea (Hydrangea quercifolia), hardy in zones 5 through 9 and smooth hydrangea (Hydrangea arborescens), hardy between zones 5 and 9, all undergo changes in bloom color as summer changes to fall, but changes are unrelated to soil chemistry. Even among the macrophylla, not all varieties are responsive. "Nikko Blue" (Hydrangea macrophylla "Nikko Blue") may take on purple tones but is difficult to transform into a pure pink; hence the development of "Nikko Pink." Further, a color change does not intensify bloom pigment; the pale blue of "All Summer Beauty" (Hydrangea macrophylla "All Summer Beauty") can be nudged toward pale, but not deep, pink, as is also the case with "Endless Summer" (Hydrangea macrophylla "Endless Summer"). The white blooms of "Variegata" (Hydrangea macrophylla "Variegata") remain white. Your local County Extension and garden centers can advise on color-responsive varieties for your specific growing conditions.

Maintaining Color Change

• Several additional factors affect the consistency of hydrangea-blossom colors. Water runoff, drainage and soil composition can all play a part in color variation. Adding ground limestone to the soil surrounding your hydrangeas several times a year can help maintain alkalinity. A high-phosphorus fertilizer can interfere with aluminum uptake. If results remain unpredictable, you can grow pink hydrangeas by planting them in very large pots with a soilless potting mix, augmented with regular applications of dolomite lime. Horticulturalists suggest that you postpone any active color-change strategies until plants are at least 2 years old with well established root systems. Patience and frequent attention will keep your blossoms pink.