Billows and ruffles of pink flowers are the hallmark of the hydrangea grown on the alkaline soils of limestone-rich areas. Bigleaf hydreangea (Hydrangea macrophylla), which thrives between U.S. Department of Agriculture plant hardiness zones 6 and 9, is known for the blue, pink or purplish flowers that result from varying soil acidity. Pink flowers on a hydrangea result when pH is high, or less acidic, though some gardeners prefer to encourage blue flowers by lowering the soil’s acidity. Knowing when and how to fertilize your hydrangea to keep the blooms pink is a matter of timing and using the proper amendments.
Determining Soil Acidity
Insert the probe of your hand-held pH meter to measure the acidity of your soil. If the readout shows a pH between 5.5 and 7.0, your soil is acidic and will need to be amended to raise the pH.
Decide which method is preferable for raising your soil’s pH, whether by using dolomitic lime for a dry amendment or hydrated lime for a liquid root drench. Flowers may not show any change until the following bloom season. Dolomitic lime takes up to a year, while hydrated lime may act more rapidly but carries the risk of damaging plant roots.
Remove mulch from around the base of the plant. Lime is most effective in raising soil pH when it comes in direct contact with it.
Sprinkle 1/2 cup of dolomitic lime per 10 square feet of area, or dissolve 1 tablespoon of hydrated lime per gallon of water. Dolomitic lime should be applied once in spring around the base and nearby growing area, and watered in well. The pH should be re-checked later in the year to determine if another application is necessary that year. A hydrated lime drench should be applied once a month for three consecutive months starting in March or when buds show signs of breaking dormancy.
Choose an all-purpose plant food that has a higher ratio of phosphorous. Phosphorous ties up blue-flower-promoting aluminum in the soil and is represented by the middle number on the product packaging (i.e., 10-10-10). Phosphorous-only fertilizers are also available.
Calculate how large of an area you’ll need to fertilize by using a tape measure to make a rectangular estimation of your planting bed’s width and depth. Multiply the length by the depth for the overall square footage.
Calculate how much fertilizer you will need by dividing your square footage by 100, then multiplying by 30. Recommended fertilizer application rates for hydrangeas is 1 pound per 100 square feet, which equates to about 30 tablespoons per 100 square feet.
Sprinkle the recommended amount at the base of the hydrangea and nearby areas, and water well. Mulch around the base of the plant does not need to be removed before fertilizing.