Color my hydrangeas blue… or maybe pink!

By Chuck Owens, Director of Horticulture and Manager of Maintenance

This time of year we are often asked about our hydrangeas. Some of the most frequently asked questions are, “How are you able to have hydrangeas bloom in early spring? Don’t they normally bloom in early summer?” Another question we hear frequently is, “What is the difference between the pink and the blue hydrangeas and how do you change or control the color of your hydrangeas?” Overall, lots of planning, pH and aluminum sulfate are the simple answers to these questions, but below is a bit more insight.

Fooling Mother Nature – Florist Hydrangeas

The 800 hydrangeas blooming now in the Gardens are called “florist” or “forced hydrangeas.” They are scheduled to bloom around the same time as several of the spring holidays. The ones that we grow each year are normally scheduled to bloom for Easter.

These florist hydrangea varieties have been selected because of their superior flower color, disease resistance and consistent quality of forcing. Normally we grow 400 pink, 200 blue and 200 white lacecap hydrangeas. We typically receive these in late December by a refrigerated freight truck in a dormant state from a grower in northern California. The hydrangeas come to us in 4-inch pots, are about 5 inches tall and have at least 3 canes or main branches. They are re-potted in 2-gallon pots and put in a heated greenhouse. About 16 weeks later – just in time for Easter – they are ready to go on display in the Gardens.

Pinks and blues – Shaping the color of hydrangeas

Hydrangea macrophylla is one of the few plants in which the color of the “flowers” can be changed by altering the conditions of their soil. This is possible because the colored “flowers” of a hydrangea are not true petals. They are actually sepals and their color is affected by the pH of the soil and the availability of aluminum in that soil. The aluminum and pH level in the soil determine if the hydrangeas are pink or blue.

Aluminum is not present in most soilless potting soils (such as pine bark and peat moss), so the original hydrangea grower in California first adds aluminum sulfate to the soil to begin the coloring process. When we take our hydrangeas to our greenhouses, we add more aluminum sulfate to the soil several times to make the hydrangeas continue to be a blue color. The aluminum sulfate helps acidify the potting soil to achieve the needed pH of 5.5 to produce the blue color. Technically speaking, it is the pH of the soil that is controlling the solubility of the aluminum, making it available for the hydrangeas to use.

Hydrangeas planted in the Gardens should remain, or turn, blue because of our low pH soils, where typically enough aluminum is present in the soil to produce blue sepals. Aluminum sulfate can be added to garden soil to insure or to intensify a blue hydrangea.

In contrast, to grow pink hydrangeas, growers add lime to the soil to raise the soil’s pH and do not add aluminum to the soil. To change your blue hydrangeas to pink, you can add lime to your soil, which will eventually over the course of several years, change the pH of your soil and consequently turn your hydrangeas a pink color.

Of course, another option is to save yourself the growing trouble and come to Bellingrath this spring to see the hundreds of blue, pink and white hydrangeas that we have on display throughout the Gardens.