There’s a bit of complicated chemistry behind the fact that the flowers of Hydrangea macrophylla, or mopheads as they are sometimes known, change color from pink to blue or vice versa. For the home gardener, it’s enough to know that the color of this hydrangea is dependent on the soil’s pH. If the soil is acid, the flowers are blue. If the soil is alkaline, the flowers are pink. White cultivars are always white. Changing a blue hydrangea to pink requires tinkering with the soil’s chemistry. H. macrophylla does best when grown in U.S. Department of Agriculture plant hardiness zones 6 through 9.
Type of Lime
Gardening centers and nurseries offer lime products in a variety of grades: ground, pelletized, hydrated and granular. Ground agricultural lime is the one most commonly used, according to horticulture extension agents with Clemson University, who go on to suggest that the finer the particles, the quicker the lime works to change the soil’s pH.
Amount of Lime
Determining how much lime to add to the soil to reach the required pH of 6.0 to 6.2 depends on several factors. For instance, soil texture affects the amount of lime needed. Sandy soils require less lime than clay soils to reach the same pH. Soils high in organic matter may require more lime than those that contain little organic matter. This is why it’s always a good idea to have the soil professionally tested at a Cooperative Extension soil laboratory before adding lime to the hydrangea bed. In lieu of testing, author and gardening expert Walter Reeves suggest using 1 cup of lime per year.
The ideal time to apply lime is in the fall. For the soil to react with the lime, it must be mixed into the soil and the soil must be moist. Scatter the lime around the base of the hydrangea and use a rake to scratch it into the soil. Use the hose to wash off any lime that spilled on the foliage and then water the hydrangea slowly and deeply to activate the lime.
The addition of lime to the soil around the hydrangea won’t change the flower’s color overnight. Depending on the soil’s texture, it may take months or even up to three years, according to Reeves. Monitor the soil’s pH twice a year. If it goes above 6.4, the hydrangea won’t be able to absorb iron and may become chlorotic. If foliage turns yellow, add an iron supplement when you fertilize in the spring. If soil tests show an elevated aluminum level in the soil use a fertilizer with a high middle number, such as 10-25-10. This middle number represents phosphorus which helps prevent the absorption of aluminum, which turns hydrangea flowers blue.
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