Hydrangea Biochemistry – pH, Aluminum, and Bigleaf Hydrangea Flower Color
Did you know that our beauty queen hydrangea, Hydrangea macrophylla, combats aluminum toxicity with its flowers?
Same for H. serrata (mountain hydranga) and Dichroa febrifuga (evergreen hydrangea), but they're not in the magazines (dare I say, garden tabloids?) as much.
It’s well known that pH influences flower color on bigleaf and mountain hydrangeas, but how this happens is even more interesting than pH alone. Here’s the science behind the flower color.
Potential of Hydrogen
Potential of Hydrogen is written shorthand as pH and it tells us the amount of hydrogen ions present that make something (in our case, soil) either acidic, neutral, or alkaline/basic.
As bigleaf hydrangea aficionados, we know acid soils produce blue flowers and alkaline soils produce pink. pH is where the story formerly began and ended. While what we know about pH and bigleaf hydrangeas is all still true, there’s much more to it.
The next piece of the puzzle is aluminum.
Aluminum Toxicity and How Bigleaf Hydrangea Handles It
Aluminum is toxic to plants. I’ve seen the toxicity myself in a flower bed with an acidic pH of 5.0. The plants were stunted, warped, and sick.
In neutral (pH 7.0) and alkaline soils (above 7.0), with less hydrogen ions to dissolve aluminum into water, any present aluminum is significantly less available for plants to absorb through their roots, so plants are therefore spared toxic effects.
However, in soils below neutral (pH 7.), the increasing concentrations of hydrogen renders aluminum increasingly available for uptake, so plants are therefore affected by aluminum in acid soils.
The phenomenon of pH affecting availability and uptake applies to all essential plant nutrients in the soil, not just toxic metals such as aluminum. Uptake of other nutrients is an aside to this article, but you can read about it here.
Some plants have biological tricks that allow them to combat aluminum and its toxic effects.
Bigleaf hydrangea is one of those plants with tricks. The species has a specialized mechanism that accumulates aluminum in the flowers (both mopheads and lacecaps), thereby sequestering it and inactivating toxic effects. This sequestering turns them from pink to blue. We’ll explain how that happens right now.
Bigleaf hydrangea flowers are naturally pink or reddish due to the presence of an anthocyanin pigment called delphinidin-3-glucoside.
Anthocyanins are a group of red, purple, or blue-colored pigments that give many berries their red, purple, or blue colors and provide the red colors of autumn leaves.
If there’s no aluminum in the soil, bigleaf hydrangeas are their natural pink or reddish colors in all soil types: acid, neutral, and alkaline.
If there is aluminum in the soil and pH remains alkaline, then flowers remain pink because that alkaline pH has less hydrogen and won’t dissolve the aluminum into water. As explained above, the plant won’t and can’t absorb the aluminum.
If you see a pink flower, you can assume either the soil is A) alkaline and therefore the hydrangea won’t take up aluminum or B) the soil is acidic or neutral, but there’s no aluminum present.
In acid soils with a high concentration of aluminum, the pink/reddish pigment we talked about, delphinidin-3-glucoside, binds with aluminum sequestering it in the flowers – thereby protecting the plant from aluminum’s toxic effects.
The delphinidin-3-glucoside bound with aluminum turns from pink to blue. This process both protects the plant from toxicity and turns the flowers blue! As gardeners, we love the result.
Remember the aluminum availability chart above? At neutral pH, even with aluminum present, the flowers are likely to be purplish because there is less uptake of aluminum at that pH.
In acid or weekly acidic (6.5 or nearing a neutral ph of 7.0) soils with lower concentrations of aluminum, bigleaf hydrangea flowers turn purple.
Again, remember the aluminum availability chart above? Even in soils with less aluminum, as long as there is some aluminum, the flowers are likely to turn purple-blue to blue as the pH decreases.
That was a lot of science we went through, but I wanted to explain the facts. What’s fun for gardeners like us is how bigleaf hydrangea flowers act like a litmus test telling us if a soil is alkaline, neutral, or acidic. Now we know it’s more complicated than pH and we can muse about aluminum being present or not.
If this information about aluminum is a surprise to you, you’re not alone - it's fairly new to me too. Surprisingly, the activity of aluminum in Hydrangeas has been known for a while, thanks to research in the 1930’s by a scientist in Great Britain.*
Within the last few decades, scientists, especially in the US and Japan, have been having what looks like great fun studying this mechanism and have explained the phenomenon thoroughly since it was first discovered.
If you're interested in learning more, there's a lot more information for you to read. Here is some of the material I studied to write this basic (no, not alkaline! I mean basic as in simple) blog post:
Dirr, Michael A. Hydrangeas for American Gardens. 2004. Timber Press. Pages 174-176.
*Haworth-Booth, Michael. The Hydrangeas. Fifth Revised Edition. 1984. Constable London. Pages 158-159.
Schreiber, Henry. 2014. Curious Chemistry Guides Hydrangea Flower Color. American Scientist V. 102 6:444
Most of our southeaster soils are totally safe for growing hydrangeas. Rarely is a soil at pH 5.0 (it's typically at a nice 5.5 to 6.5) and when it is too low, the pH can be raised with lime, but my favorite method is liberal additions of organic matter in the form of well-aged compost.
Bigleaf hydrangeas love the moist, rich compost. Compost can be purchased in bulk and delivered in trucks to make the process more efficient. Compost is neutral in pH and when incorporated into clay soils works magic on raising the pH and improving growing conditions in general.
A word of caution about applying Aluminum Sulfate
Aluminum Sulfate is a product marketed to lower pH, add aluminum, and turn Hydrangea flowers blue. Apply this product conservatively. Even though hydrangea flowers sequester aluminum, it’s still possible to damage a plant by applying this product. How do I know? I burned out a few hydrangeas! Apply less than the recommended amount to cautiously move forward turning flowers blue.
All drawings in this post are thanks to the talents of our niece and CEP Chief Potter, Carolina Angulo. Carolina is a Masters of Landscape Architecture student who is learning her plants in school and from a nursery perspective here at our place.