The pretty pink and blue florists' hydrangeas are well-suited to home gardens in many parts of the U.S. but they must have certain soil conditions to perform properly. They also need winter shelter in many Northern gardens. Hues can be changed with a bit of soil chemistry manipulation. One bush can even sport several colors of bloom depending on the variety of hydrangea.
Hydrangea arborescens (Annabelle) and quercifolia (oak leaf) are the only hydrangeas native to North America and, although both families have colorful members, none can be manipulated to change color. The mophead H. macrophylla, also called big-leaf hydrangea, is the only group that can respond to soil chemistry by changing hue.
American native species Annabelle and oak leaf were taken home to England by traders in the 1730s. The big-leaf macrophyllas and other Asian natives became favorite gifts at the turn of the 20th century as French plant breeders developed new garden varieties and hybrids that could grow in pots and containers.
H. macrophylla may be either mophead or lace cap. The mophead flowers in a globular cluster of hundreds of small blooms but the lacecap flower head resembles a pin cushion with tiny flowers in the center surrounded by larger blooms. Mopheads make the best subjects for color-change chemistry because of their size. Like many hydrangeas, mopheads grow best in afternoon shade and evenly moist soil. Yearly soil amendments, watering with diluted solutions of aluminum sulfate and using phosphorus or potassium fertilizer can cause flowers to change color.
Mopheads turn blue when aluminum sulfate is available in the soil. Acidic soil with plenty of organic matter makes it possible for the plant to absorb the chemical; additions of compost and peat moss can help lower the pH in the vicinity of the shrub. Dolmitic lime will raise soil pH; as soil loses acidity and the pH climbs past 6.0 to 6.2, plants are unable to use the aluminum in the soil, so flowers change to pink. High phosphorus fertilizers also help keep the aluminum away from plant roots.
White mopheads do not change color because their flowers lack the pigment that responds to chemical manipulation. The natural acidity of the soil and water supply will also affect the way plants change hue. The determined gardener should complete a soil test at a university extension and test the water with a litmus or water test kit to find this information. Too much aluminum sulfate, a pH below 5.2 or above 6.4 or too much water can result in weak plants and poor flowering.