Some hydrangea, like Endless Summer, Twist and Shout, and Nikko Blue can change color between pink and blue. These hydrangeas are also known as Big Leaf Hydrangeas. I know, it's cool right!? Unfortunately, they don't just have an app that lets you change the color.
Their color is reliant on soil pH. More specifically, an enzyme in the plant called the delphinidin-3-glucoside-aluminum-3-caffeoylquinic acid complex (I know....I'm a nerd). Remember back to science class when you learned about acids and bases? No? OK here's a quick refresher of the pH scale.
Now, here's the real science behind why they change colors. Aluminum makes the flowers of Hydrangeas turn blue. But, Hydrangea plants can't absorb Aluminum through their roots at a pH above 6. It is bound too tightly to the soil particles.
When we lower the pH and make the soil more acidic, the bonds between the soil and Aluminum weaken, making it available to Hydrangeas. They absorb the aluminum which activates the enzyme we talked about above, and the flowers turn blue. When the Aluminum isn't present in the plant though, the enzyme isn't activated and the flowers remain pink. Mind blown. I know.
So if your soil is basic, Hydrangeas will be pink. If your soil is acidic, they will be blue.
Many people ask how to make them lavender, which you can do, it's just very hard to maintain.
For your hydrangeas to be blue, we want a pH below 6. Pink requires a pH above 7. Lavender on the other hand, must be maintained between 6 and 7. There's very little wiggle room there. Some varieties are also very sensitive to pH and may display both pink and blue flowers in that range.
Illinois soils tend to be just slightly basic, so if you simply plant your Hydrangea in the soil and don't add anything to it or aren't already starting with an acid soil (like those found under pine trees) then yours will be pink.
OK, so how do you make them turn BLUE?
If you want your Hydrangea to be blue, you need to acidify your soil.
If you aren't sure what your soil pH is, you can test it with one of these simple at-home pH test kits. They're really easy to use and only cost a few dollars.
We have 3 main types of fertilizer we can use to acidify our soils:
Aluminum Sulphate, Miracid, and Sulfur.
Whatever product you choose to use, be sure to follow the proper application instructions listed on the back of the package.
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