While most plants that we grow in our gardens bloom in one specific color, hydrangeas have the unique ability to change colors: often even exhibiting multiple colors on one plant! This color changing ability is observed only in two specific species of hydrangea: Big Leaf (Hydrangea macrophylla) and Mountain (Hydrangea serrata). It’s important to note, that within these two species only those that have a bloom color range of pink to blue can be manipulated to achieve a desired color. Hydrangeas that bloom white or cream may pick up a pink hue as the flowers mature and dry but cannot be physically altered to bloom in a different color.
The color of hydrangea blooms is determined by the pH of the soil that it is planted in. The pH level controls the plant’s ability to absorb aluminum, which is the mineral that turns the flowers blue. Acidic soils, with a pH of 5.2-6.0 will yield flowers in blue tones, while alkaline soils with a pH of 6.0-7.0 generally produce pink blooms.
Because Iowa soils naturally fall between 5.5-7.5 pH, most hydrangeas in the pink to blue range will lean towards the pink side. We often receive customers asking how they can encourage blue blooms on their hydrangeas, the answer to which is lowering the pH for a slightly more acidic soil.
Before changing acidity, the first step is testing your soil to determine the current pH level. An at home soil pH test kit can be purchased at your local garden center and will help you understand the best next steps to reaching your desired color. Once you have results, we recommend you consult your local garden experts for the best method and product to achieve your goal soil pH.
Aluminum sulfate can be directly amended into your soil, but the amount and application method is determined by the current pH and how much it needs to change. There are many pre-formulated, easy to use products that you can purchase to make this task easier, or simply work with your local garden experts to determine the best amount and method for your garden. For best results, begin your soil amending well before flowers emerge, Fall or early Spring is ideal. It’s important to note that it may take time and more than one application to achieve blue flowers as the aluminum must travel from the soil into the plant and be metabolized throughout.
Remember to follow the directions as instructed – if the soil becomes too acidic or too alkaline, it can inhibit the uptake of essential nutrients, causing your hydrangea and surrounding plants to suffer.
Soil pH test kits and aluminum sulfate products available at Blooms Garden Center and Gift Shop