The Secret's In the Soil
The trick to changing hydrangeas.
Hydrangeas, Bluestone Perennials
Wouldn't it be nice to control the color of flowers on your shrubs? If you're gardening with bigleaf hydrangeas, you won't need a magic wand to wave over your garden.
You can make the typical blossoms of this unique plant change it a deep pink or striking blue. The secret is in the soil.
Soil pH tells you if your soil is acid, neutral or alkaline. This can sometimes be difficult to change. The difference in numbers might be small, but it takes huge amounts of energy and materials to change them. So don't give up if at first you don't succeed.
Lime is used to raise pH in acid soils, and sulfur is often used to lower pH in alkaline soils. It's important that you test your soil before trying to change the pH. It could take years to repair soil if it was given an improper application when trying to change the pH.
Acidic soil (pH below 5.5) brings out the flower's blue coloring, while less acid to alkaline soil (pH of 6.0 or higher) enhances its vibrant pink color. If you'd like the blue flowers, add aluminum sulfate (alum) to the surrounding soil. If you prefer pink blossoms, add agricultural lime.
Don't try to go against nature, though, especially if your soil is already set. For example, if you tend to have acidic soil, you shouldn't expect pink blooms. Instead, use the acidic nature of your soil to your advantage and enjoy the blue hydrangea blooms that come naturally. The same is true for alkaline soils. Don't expect blue blues if you have alkaline soil. Just enjoy the vibrant pinks of hydrangeas.
Bigleaf varieties are the only commonly grown types of hydrangea that have to ability to change their flower color.