Is the color of every type of Hydrangea variable, according to the pH of garden soil? Or are some species or varieties reliably the same color, no matter where they are planted?
I think the main species of Hydrangea whose color gardeners sometimes try to alter is Hydrangea macrophylla, also known as hortensia, bigleaf, or mophead hydrangea. According to Van Gelderen’s Encyclopedia of Hydrangeas (Timber Press, 2004), most cultivars of Hydrangea macrophylla are naturally pink, even in slightly acidic soil. “The key factor in flower color is not the acidity of the soil, but a plant’s accessibility to aluminum sulfate.” The plant will not produce blue flowers if it has access to aluminum sulfate but the soil is strongly alkaline, as the calcium of high pH soils will bind with it, preventing the plant from absorbing it. The authors state that “some cultivars easily turn blue under the right conditions,” while others do not. White-flowered cultivars of different Hydrangea species usually retain their color, though the flowers may become flecked with red or pink toward the end of the season.
Hydrangeas: A Gardener’s Guide, by Toni Lawson-Hall and Brian Rothera (Timber Press, 1995) says that while several species of Hydrangea may have some variation in color, Hydrangea macrophylla ssp. macrophylla and its cultivars (including mopheads and some of the lacecaps) have potential for the most dramatic changes. However, “there are some innate preferences within the individual plants […] and not every mophead or lacecap will change colour in response to a gardener’s alchemy.” The authors do mention a few plants which display red flowers even in acid soil, such as cultivars of Hydrangea serrata: ‘Preziosa,”Grayswood,’ and ‘Beni-Gaku.’ Among the H. macrophylla cultivars that stay red in acid soil are: ‘Alpengluhen,’ and ‘Altona.’ Cultivars which are considered most reliably blue on acid soil also need a low level of phosphate: H. macrophylla ssp. macrophylla ‘Gentian Dome’ and ‘Marechal Foch’ (both deep blue), ‘General Vicomtesse de Vibraye’ (pale blue), and Hydrangea serrata ‘Diadem’ and ‘Blue Deckle’ (blue lacecaps).