Hydrangeas: Nature’s Litmus Test

Ever wonder why these big, beautiful flowers seem to spontaneously bloom pink or blue?  Their color is actually a result of the pH (acidity or basicity) of their planting soil!

NOTE: If you are already feeling your eyes begin to glaze over at the mention of chemistry terms, please just scroll down and simply enjoy the beautiful shots of hydrangeas I took during my walk to the Post Office this week and ignore the text :)

Hydrangeas caught on camera on my walk to the P.O.

So how does this work?

In more basic soil (above pH = 5.5), aluminum ions (which are toxic to most other plants) are bound to other ions like oxygen and phosphate in the soil, the hydrangea is unable to take up the aluminium and more pink-colored blooms are observed.  In more acidic soil (below pH = 5.5), the aluminum ions roam free within the soil and are able to be more easily absorbed by the hydrangea roots.  Once absorbed, the aluminum travels throughout the plant, including to the vacuoles within the cells of the flower petals.  Within these vacuoles, the aluminum binds with a chemical called anthrocyanin, changing the reflection of light off the petals, making them appear blue!

In summary …

  • More Acidic Soil (pH < 5.5) = More Blue
  • More Basic Soil (pH > 5.5) = More Pink

More Hydrangeas!

Pretty nifty, huh?  I bet you will never look at a hydrangea bloom the same way again … Have a lovely weekend!



  1. Pingback: Hydrangeas: Nature’s Litmus Test (via What. No Mints?) « Wonderful Shrubs

  2. Hi, Just discovered your blog. I love your etsy store! As a biochemist, I really appreciate your plushes. Really creative! And it is nice to see someone explaining the science of every day things.

    • Thank you! I used to tutor organic chemistry in college and I loved the challenge of making chemistry accessible and understandable to everyone – Glad you enjoyed it!

  3. wooww…
    interesting, actually I wondered before read this post, why Hydrangea have different color…
    There are many Hydrangea around my Lab.
    Nice info …

  4. Thanks for the explanation — and for the great photos. People have been telling me that hydrangeas turn color based on the acidity of the soil (and that you can change the colors by tossing rusty nails in the soil). But nobody could offer the chemical explanation behind it. So, thank you!

  5. Excellent blooms and a nice post on it. It indeed is a nice way of checking the soil pH.

    I also read that the presence of aluminium decides the color but the aluminium is available only at lower pH.
    I wish the hydrangeas in my garden bloom like this.

    Enjoyed your post.


  6. Congratulations on being Freshly Pressed!

    I love hydrangeas, but I didn’t know what caused the color differences. Thanks for the lesson! Many of the hydrangea bushes around my area bloom bright white, and are nicknamed ‘snowball bush’. But there’s also purple, and the lighter shades as shown in your beautiful pictures.


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