Hydrangea is a large genus with over 80 species of deciduous and evergreen shrubs and climbers, though it is generally the deciduous shrubs that are common in our gardens. Hydrangeas are among the easiest ornamentals to grow, thriving in a variety of climates and conditions from north to south and coast to coast. They want a humus-rich soil, ample moisture and relief from midday sun. Add an annual pruning specific to the species and the needs of these garden favorites are fairly well met.
Hydrangea macrophylla is the litmus test plant of the garden. They generally prefer a slightly acidic soil, more so if you want blue blooms. The color change from blues to reds is actually caused by the amount of available aluminum in the soil.
Since aluminum availability strongly depends on soil pH, it is generally assumed that pH alone is the determining factor of flower color. Generally, the aluminum is already in the soil and will become available once the soil pH is low enough to unlock it.
Plant genetics also plays a big role in just how far you can stretch those color changes, as the different varieties tend to lean one way or another on the color spectrum. The lesson here is not to buy ‘Red Star’ or something similar if you want a vibrant blue – you just can’t stretch the color that far and will usually end up with more of a purple (can be nice) or sickly mauve color (not so nice).
To supplement the fertilizer and increase the acidity of your soil you can use egg shells, coffee grounds, or ground up orange or grapefruit peels. Rhody/Azalea fertilizers tend to make the soil more acidic also.