wilted hydrangea may have its hydration restored by first having its stem immersed in boiling water; as the petals of the hydrangea can also absorb water, the petals may then be immersed, in room-temperature water, to restore the flower's hydration. Colors and soil acidity Hydrangea flower color can change based on the pH in soil. As the graph depicts, soil with a pH of 5.5 or lower will produce blue flowers, a pH of 6.5 or higher will produce pink hydrangeas, and soil in between 5.5 and 6.5 will have purple hydrangeas. White hydrangeas cannot be color-manipulated by soil pH because they do not produce pigment for color. In other words, while the hue of the inflorescence is variable dependent upon cultural factors, the color saturation is genetically predetermined. In most species, the flowers are white. In some, however, (notably H. macrophylla), they can be blue, red, or purple, with color saturation levels ranging from the palest of pinks, lavenders & powder blues, to deep, rich purples, bordeauxs, and royal blues. In these species, floral color change occurs due to the availability of aluminum ions, a variable which itself depends upon the soil pH. For H. macrophylla and H. serrata cultivars, the flower color can be determined by the relative acidity of the soil: an acidic soil (pH below 7), will have available aluminum ions and typically produce flowers that are blue to purple, whereas an alkaline soil (pH above 7) will tie up aluminum ions and result in pink or red flowers. This is caused by a color change of the flower pigments in the presence of aluminum ions which can be taken up into hyperaccumulating plants. Partial list of species Hydrangea anomala – (climbing hydrangea) Himalaya, southwest China Hydrangea arborescens – (smooth hydrangea) eastern North America Hydrangea aspera – China, Himalaya Hydrangea bretschneideri – China Hydrangea chinensis – China and Taiwan Hydrangea chungii – China Hydrangea cinerea – (ashy hydrangea) eastern United States Hydrangea coenobialis – China Hydrangea davidii – China Hydrangea glaucescens – China, Myanmar and Vietnam Hydrangea gracilis – China Hydrangea heteromalla – Himalaya, west and north China Hydrangea hirta – Japan Hydrangea hydrangeoides – Ulleungdo, Japan, Kurils Hydrangea hypoglauca – China Hydrangea integrifolia – China Hydrangea involucrata – Japan, Taiwan Hydrangea jelskii – Andes Hydrangea kwangsiensis – China Hydrangea kwangtungensis – China Hydrangea lingii – China Hydrangea linkweiensis – China Hydrangea longifolia – China Hydrangea