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Hydrangea Macrophylla - a Science Project in Bloom: How Soil Affects Color

Updated on December 31, 2012

Our bush staked its claim to a corner of the flowerbed long ago. It grew from cuttings from our last home, brought when we moved here seven years ago; and each year it has become fuller and lusher, more confidently in charge of backyard esthetics. Hydrangea Macrophylla is its name; my husband pronounces it hy - drain - ja; I just say hydran ja.

This afternoon, he thrust a bouquet from the bush under my nose – under my eyes really – and marveled at the various colors we can enjoy from the same plant: four distinct shades, at a minimum, from this plant. More if you have an artistic eye.

We oohed and aahed for several minutes, and then he took the flowers to the dining table, and I returned to the computer to continue writing and re-writing, editing and re-editing.

But Ron has always had a scientific bent, first earning degrees in chemistry and electronics before entering the ministry, and it is really not in his nature to overlook a teachable moment, especially when it comes to a chemical reaction or formula or to the Great Outdoors. Or Both. Especially Both.

Suppertime Discussion

And so it was, at supper, that he launched into his discussion of the real reason a hydrangea can serve as a literal litmus test for soil pH. “It’s all about aluminum,” he said. “Acidity and alkalinity, yes. But it’s the aluminum that makes a hydrangea blue.”

We chewed on our hamburgers, the ones that he had grilled outside while I was busy writing and rewriting. He explained that the color of the blossoms will tell us how acidic or alkaline the soil is. But how? he asked, Do you know how?

Of course we didn’t, and he wasn’t about to leave us in the dark. The acid in soil makes it possible for the plant to take up and use the aluminum that is present in the soil. The aluminum has to be there, but if the soil is not acidic enough, the aluminum can’t be taken up and used by the plant. So, when the soil is acid, the aluminum is taken up into the plant and causes the blossoms to turn blue. Lacking acid -- when the soil is alkaline -- the plant does not take in aluminum, and the blossoms remain pink.


And then, there are the situations like our plant, in which the same bush produces numerous quite different shades of color.  That happens when the soil is comparatively neutral in acidity.  The level of aluminum taken in can fluctuate over time.


He mentioned that he had added some aluminum sulfate to the soil last week – to help turn the blossoms blue.  Sulfate is acidic and thus would help the plant absorb the extra aluminum that the compound also provides.  But evidently it was taken in unevenly, in differing amounts at different times; or it arrived when the blossoms were at different stages of development.


Whatever the case, the result was splendid, in my opinion, and we’ll enjoy the blossoms outdoors and on the dining table for a long time.

Perspective from Science

Our youngest son, the rising senior, had a thoughtful look on his face. “You said alkaline soil makes it pink, and acidic soil makes it blue? That’s just the opposite of litmus paper.” We tossed this interesting contradiction around for awhile without discovering any explanation, and then I rummaged around to find the camera and make use of it.

I thought it worthwhile to see what others had written on the subject before trying to organize my own thoughts, and in doing so I came across an interesting item in a Purdue University horticultural newsletter. Someone had sent the question, “Are you sure about the blue and pink hydrangeas?” The writer/questioner explained that her mother’s pink hydrangeas had turned blue over the years, and she attributed the change to years of alkaline dishwater that had been used to water the plant. She pointed out the same contradiction with litmus paper that our son had mentioned.

The Horticultural Specialist from Purdue in responding completely verified Ron’s explanation. “Aluminum tends to be more available for plant uptake in acid soils, less so in alkaline soils. Aluminum sulfate … lowers pH and provides aluminum,” wrote Rosie Lerner. She also explained that the color change on litmus paper is caused by a reaction with dyes that come from extracts from lichens, a completely different chemical process from the one in hydrangeas.

I’ve had enough experience with Ron’s knowledge that I should just accept it as gospel by now. But he’s human too, and so it’s good to be able to check with a regional expert – just in case he ever slips.


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    • Aficionada profile image

      Aficionada 7 years ago from Indiana, USA

      I am so glad to hear about the black leaves, Pcunix! Not glad that they happened, but glad to know as a warning for the future, and glad to know about the baking soda - very smart. It's good to hear that your begonia is better. When I have resorted to poisons, it has often been too late for them to help. All success is encouraging!

    • Pcunix profile image

      Tony Lawrence 7 years ago from SE MA

      We love ours, I almost killed the poor thing this year by adding too much aluminum sulfate. The leaves starting turning black. I immediately added baking soda, which stabilized it and it is happy again and giving us lovely blue blossoms.

      We also almost lost our begonia by moving it. I watered it faithfully but it got attacked by insects. I hate using poisons, but it got to the point where most of the poor thing was eaten, so I reluctantly sprayed it. It is pretty well recovered now but I am watching it closely.

    • Aficionada profile image

      Aficionada 7 years ago from Indiana, USA

      Thanks, Shalini. You're very kind to comment on the lushness of the blooms. I leave that work up to my husband; I don't really know what he does, but I'm very happy with it. And I do like having different colors on the same bush. I agree that the shades of nature are just incredible! Have you ever tried to count the number of different shades of green you can see at one time? Impossible!

    • Shalini Kagal profile image

      Shalini Kagal 7 years ago from India

      Wow! Incredible how Nature has so many different sides - and shades to her! Those blooms look great! Somehow ours never come out looking so lush - something about the weather? the soil, maybe?

    • Aficionada profile image

      Aficionada 7 years ago from Indiana, USA

      I love the different colors myself, and I'm actually rather glad the aluminum sulfate didn't cause a complete change in our one plant. I'll bet the varying colors made a stunning sight in your garden!

      Thanks for stopping by and reading.

    • LeanMan profile image

      Tony 7 years ago from At the Gemba

      One of my favorite plants, used to have so many in my garden in the fens, different areas of the garden being different colours and shades...