Hydrangea flower colors range from cobalt blue to white and rosy pink. Not all hydrangea species can change the color of their flowers; many species bloom white regardless of growing conditions. Three species are known to change flower color by manipulating soil pH, but flower color and beauty in all shrub types can be diminished if the soils are not moist and organic-rich or sunlight is too intense.
Common Garden Species
There are more than 80 species of hydrangeas, but far fewer have been chosen by gardeners over the centuries for ornamental use. The wild or smooth hydrangea (Hydrangea arborescens) is native to the eastern United States, as is the oakleaf hydrangea (Hydrangea quercifolia). Eastern Asia has provided species of interest, including Hydrangea aspera, the peegee hydrangea (Hydrangea paniculata) and the bigleaf or florist hydrangea (Hydrangea macrophylla). The villosa hydrangea (Hydrangea villosa) is often regarded as a very closely related type of Hydrangea aspera, but ornamentally important nonetheless.
Hydrangea Flower Colors
Many hydrangea species naturally bloom in tones of white , regardless of growing conditions . In stark contrast, three species' flowers can be manipulated with changes in soil pH: those of Hydrangea aspera, Hydrangea villosa and Hydrangea macrophylla. Their flowers can range from blue to lavender, white, or pink.
Environmental Factors on Flower Color
Naturally white-flowering hydrangea species, such as peegee (Hydrangea paniculata) and oakleaf types (Hydrangea quercifolia), will always produce blooms in shades of white or ivory. Age of the petals can cause temporary blushing to tan-pink before fully drying to light beige.
In Hydrangea aspera, Hydrangea villosa and Hydrangea macrophylla, flower color varies based on the soil pH, or amount of hydrogen ions present. Acidic soil readings are less than 7.0, while alkaline soil is greater than 7.0. Those near 7.0 are regarded as neutral.
The more acidic the soil, the more intensely blue these three species' flowers will be, especially if aluminum ions are is present. Conversely, the more basic or alkaline the soil, the deeper the tint of pink. If soil pH is close to neutral, the flower color is creamy with hints of either pale pink or pale blue, sometimes lavender.
To make white-blooming hydrangea species flower with their best color, ensure that their soil is consistently moist and rich in humus. An organic mulch over the roots promotes a nutritious soil and healthy shrub.
In the species affected by soil pH, intensifying flower color requires experimentation and diligence. First, know the pH of the native soil. Changing pH with fertilizers and other amendments can be pricey. Maintaining the desired pH level in the soil is necessary year-round. The pH must be perfect three to six months in advance of the formation and opening of flowers for best display. Time is needed for fertilizers to dissolve, be absorbed and distributed in the hydrangea.
Bigleaf hydrangeas grown in excessive light can create flowers with faded colors. Too much direct sun or the intensity of heat or reflected light and heat can cause petals to wilt or bleach regardless of soil moisture, fertility or pH.
Relocating plants with pale colors to a site that receives more shade or filtered light can improve the depth and duration of color. Shadecloths can protect opening flower buds from premature fading.
Generally speaking, nutrient-poor and dry, sandy soils will not create good flowers. The addition of fertilizers that aim to change soil pH and flower color may be short-lived because the sandy soil drains and dries too quickly, allowing fertilizers to leech.