Grown in acid soil (pH below 6) and properly nourished (see below), most hydrangeas will be blue. In the pH range of 6 to 7, they will exhibit a range of lavenders or purples. At neutral and above, the flowers of the very same plant will be pink or rose red. This "reverse litmus" effect can be observed among the flowers on a single plant (above left) or even on a single flower (right). Not only mophead varieties of bigleaf hydrangea (H. macrophylla) but also lacecaps and cultivars of other species such as the Hydrangea serrata hybrids exhibit this behavior. This seemingly magical color alchemy is not as simple to control as is often represented in gardening articles and books. In fact, much more is involved in getting that precious blue to appear than merely creating an acidic soil. In fact, the blue color appears in the presence of available aluminum in the soil. The acidity plays an important role because the aluminum assumes an available form only under acidic conditions. In neutral to alkaline soil, it is bound up with other atoms in a form unabsorbable by your hydrangea. To complicate matters even further, if you have an abundance of phosphorus present in your soil, the phosphorus will displace the aluminum, making it less available to your hydrangeas. The lesson? Avoid high-phosphorus fertilizers for your hydrangeas. While the relative pink-blue spectrum of your hydrangea is controlled by soil pH and aluminum, the actual intensity and tone of the colors