Botanical Names: French or bigleaf hydrangea (Hydrangea macrophylla)
Have some fun in your landscape playing with hydrangea colors. These almost magical shrubs have flowers that can shift hue. Hydrangea color change doesn’t require any real hocus pocus; it’s all a matter of soil pH and aluminum content. To change hydrangea color you deal with soil additives, which is easy. Wondering how to make hydrangeas blue? Want to try your hand at shifting colors of hydrangeas? Try some of these techniques.
Before you start trying to succeed with hydrangea color change, it helps to understand which hydrangeas shift colors. Look to the French or bigleaf hydrangea types. This is a large group of hydrangeas that fall under the botanical name Hydrangea macrophylla. Flowers open in a widerange of colors, including white, pink, purple and blue.
In this group, which includes mophead and lacecap hydrangeas, flower colors shift depending on soil pH. It’s not the pH itself that dictates flower color. Instead the pH influences how much aluminum is available to plants. When hydrangea roots absorb aluminum in soil, it brings out the blue tones in flower petals.
It turns out that low soil pH, or acid soil, makes aluminum available to hydrangeas. The plants take up that aluminum, and flowers shift from pink to various blue or lavender shades, depending on the actual soil pH. In high soil pH, or alkaline soil, aluminum is tied up and unavailable to plant roots. In this instance, hydrangea flowers shift toward pink. Hydrangea color change basedon soil pH doesn’t affect white hydrangeas.
To coax pink tints out of your hydrangeas, you need to tie up the aluminum in soil by raising the pH. To do this, add lime pellets several times a year. You’re aiming for a soil pH of 6.0 to 6.2. If soil pH gets too high (above 6.4), leaves turn yellow because iron becomes unavailable to plants.
You can also use a fertilizer that’s high in phosphorus (the center number). Chemically, phosphorus helps keep aluminum from being absorbed by plant roots. Some regions naturally have acidic or high aluminum soil, and most hydrangeas in that area are blue to purple. When that occurs, the easiest way to get pink hydrangeas is to grow them in pots, where you can create and maintain a specific soil pH.
Understanding how to make hydrangeas blue isn’t difficult. In this case, you want to lower soil pH. Usually this is done by adding aluminum sulfate or just sulfur. Aluminum can burn plants, so if you add the aluminum sulfate, follow directions carefully. With garden sulfur, sprinkle pellets on soil and water. Some sulfur is sold specifically as a hydrangea amendment.
Whether you’re changing hydrangea colors from blue to pink or vice-versa, continue your treatments to maintain the soil pH. That’s the secret to keeping hydrangeas blue or pink. Also check the pH of your water supply. If you’re trying to make soil acidic and your water is alkaline, no matter how much sulfur you add to soil, every time you water you’ll be shifting soil back toward higher pH levels, which yields pinker blooms.