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