Hydrangeas have been one of the most popular shrubs in home gardens for countless years, which comes as no surprise to those familiar with their fantastical color changing properties. While most garden plants 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 with a natural color range of pink to blue can be manipulated. Hydrangeas that bloom white or cream may pick up a pink hue as the flowers mature and dry but cannot be manipulated to bloom in an entirely different color.
The color of the flowers is determined by the pH of the soil that the plant is growing 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 will bloom in shades of pink without human intervention. If you're looking to encourage blue blooms on your hydrangeas, we suggest lowering the pH to acheive a slightly more acidic soil.
Before changing the soil acidity however, 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 consulting your local garden experts for the best method and product to achieve your goal soil pH.
Aluminum sulfate can be directly applied to acidify your soil, but the amount and application method is determined by the current pH and the desired level. There are many pre-formulated, easy to use products that you can purchase to make this task easier. For best results, begin your soil amending in Fall or early Spring well before flowers emerge. It’s important to note that it may take time and repeated applications 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