What Can You Add to Your Soil to Get a Deeper Blue of Hydrangea?

Anthocyanin

• The hydrangea petals are actually modified leaves called vacuoles that contain anthocyanin. This is the pigment that causes the pH dependent color shift in the French hydrangeas. When the pH of the cellular fluid is more alkaline and contains a higher concentration of metal ions, such as aluminum, the hydrangea vacuoles appear blue. Since the pH of the vacuoles moves opposite the soil pH, inducing a higher pH internally requires a lower soil pH.

Bluest Hydrangeas

• Due to differing amounts of anthocyanin, some hydrangea bushes will become more blue than others. Enziandom produces the deepest blue flowers. Brestenburg, Kasteln, Kuhnert, Merritt's Supreme and Red Star are medium blue hydrangeas. Several hydrangeas should not be grown as blue. These include Bottstein, Schenkenburg, Strafford and Todi.

Lowering pH

• Check the soil pH before attempting to manipulate the hydrangea flowers of an existing plant. French hydrangea are the only type of hydrangea that will change colors readily. If your soil pH is already in the correct range to create a blue hydrangea, lowering the pH more will not help. However, if your soil pH is higher than 6.5, you can easily adjust it by mixing one ounce of aluminum sulphate, one ounce of ferrous sulphate and one gallon of water. Water the hydrangea with this mixture twice yearly, once in the fall and once in the spring before the plant blooms.

Blue Hydrangea Tips

• If you know that you want a very blue hydrangea long before you plant it, properly preparing the site will help ensure that the soil pH stays lower for a longer period of time. Incorporate organic amendments like pine bark, coffee grounds, fruit peels or grass clippings to the soil before planting. As these amendments slowly break down, they lower the pH of the soil. Be sure to fertilize with a fertilizer that is low in phosphorus and high in potassium. When your hydrangea begins showing signs of loosing its blue color, you can side dress with an aluminum solution.