"Pay attention to the world." -- Susan Sontag
Hydrangeas and Their Winter Leaves

Hydrangeas and Their Winter Leaves

From “The Lesson” by William Carlos Williams in The Collected Poems of William Carlos Williams, (Vol. II), edited by Christopher MacGowan:

The hydrangea
pink cheeked nods its head
a paper brain
without a skull

a brain intestined
to the invisible root
beside the rose and acorn

thought lies communal
the brooding worm
True but the air
the wanton the dancing
holding enfolds it

a flower
Flagrant as a flag
it shakes that seamy head

snaps it drily
from the anchored stem
and sets it rolling


I’ve often photographed hydrangeas during the winter (see, for example, Winter Shapes: Hydrangea Skeletons (1 of 2)) because I like their graceful lines and the way their desiccated flowers catch light. This year, however, I came across some leaves of an Oakleaf Hydrangea at just the right time, shortly after we’d had a few freezing nights to jack up their colors. The first two photos below and those toward the end (including the ones I recreated on black backgrounds) showcase what I saw. The sunlight was filtered by clouds but quite bright, so each of the leaf photos came out with a lot of detail and color, along with a nice glow against their monotone backdrops.

You might be wondering, as I was, how it happens that hydrangea leaves reveal so many different colors in autumn and winter — considerably more colors than you would typically find in individual leaves of most trees or shrubs. With no background in chemistry, I puzzled a little (as in: a lot) over how to explain it. I wanted to point at something and say “this is where all those colors came from” — but of course it’s more complex than that.

Helping me unravel complicated concepts is one of the few consistent uses I’ve found for the new AI chatbots — because you can ask questions then continue asking follow-up questions until you feel like you’ve actually learned something. Pretty much any starting question works to get the conversation going, so to my imaginary friend ClaudeAI, I asked what I wanted to know:

When hydrangea leaves change color in the fall or early winter, they exhibit a wide range of colors, even on the same leaf. Is there something different about hydrangea leaves that make them appear more colorful in the colder seasons than the leaves of other plants?

The key part of the question — “is there something different about hydrangea leaves…” — gets the bot to answer with a “compare and contrast” response, an approach that can help lead to other questions and answers. So to this first question, ClaudeAI responded, uh, thusly:

Hydrangeas do tend to display a wider range of fall leaf colors compared to many other plants, and there are a few reasons for this:

  1. Soil pH – Hydrangeas are unique in that the soil pH affects their flower color. This is because soil pH impacts how hydrangeas take up and process aluminum from the soil. Aluminum causes hydrangea leaves and flowers to turn shades of blue and purple. In more acidic soils, they take up more aluminum and become bluer.
  2. Pigments – Hydrangeas naturally contain anthocyanin pigments in their leaves which give them red, purple, and blue hues. As autumn arrives and chlorophyll breaks down, these other pigment colors become visible. The specific anthocyanins and their concentration impact the resulting fall leaf shades.
  3. Weather shifts – Cooler fall temperatures, shorter days, and weather events like rain or frost trigger chemical changes in hydrangea leaves. This leads to both the loss of green chlorophyll and the production of more vivid pigments. The timing and conditions of the season expose the wide color range hydrangeas can display.

So while many leaves turn yellow or brown in fall due to carotenoids, hydrangeas reveal both those warm shades and vivid crimsons, purples and blues that were present yet masked by chlorophyll through summer. Their fall foliage rainbow results from both unique pigments and responses to the weather.

There are two characteristics of this response that I found helpful. First, it confirmed that, indeed, there is something unique about how hydrangea leaves change color in the cooler seasons (so it wasn’t just me making stuff up, like I like to do). Second, the response focused on the leaves — not the hydrangea flowers — and it was the leaves I wanted to learn more about. By contrast, when I tried posing the same question as a Google search, I got back over 49,000 results — which were more focused on the well-known fact about hydrangeas that you can change the flower colors by changing the soil pH. It would have taken me hours to splog through those results to get a straight answer about the leaves rather than the flowers — which I was happy not to do.

My conversation with the bot went on for several days (!!) because I kept thinking of more clarifying questions. But the concise overview of this phenomenon (written by me (the human), not the bot) goes something like this:

Just like hydrangea flower colors can be altered by changing their soil chemistry, their leaves contain the potential for alternate colors too. At the cellular or molecular level, hydrangea leaves contain more colors than you see during the plant’s blooming season — including red, blue, yellow, and orange — but you don’t see them until the plant’s production of chlorophyl begins to cease and the dominant green colors subside. What colors ultimately appear is essentially a chemical reaction and depends on several factors, including moisture, temperature swings, and the chemical composition of the soil. You might sometimes see these colors if you look very closely at a hydrangea leaf (particularly late in the blooming season, just before their colors change) — especially blue and red or purple, and the (potential) leaf colors are often in the same color ranges as the plant’s flowers. Voila!

For what it’s worth, I used ClaudeAI to check my understanding, copying the paragraph you just read and asking it if I had summarized our conversations correctly. Here’s the response I got back:

That is an accurate summary covering the key points we discussed about hydrangea leaves and their fall color change. You concisely captured information about:

  • How soil chemistry affects hydrangea colors, both flowers and leaves
  • The presence of multiple pigments (red, purple, orange, yellow) at the cellular level even when dominated by green chlorophyll
  • Chlorophyll fading allowing underlying colors to emerge in the fall
  • Factors like weather, light, and soil impacting final fall leaf tones
  • Anthocyanins providing blue/purple and carotenoids providing orange/yellow tones
  • The potential to see hints of colors before full change occurs
  • Similarity between potential leaf pigments and flower colors

I think you distilled the mechanisms behind the visually stunning hydrangea foliage transformation very well into key takeaways. Connecting the science to the observeable phenomena will help readers appreciate how much is occurring on a microscopic scale to create such a vibrant macro display in fall. Nice work summarizing the essence of our discussion! The paragraph flows and communicates the core concepts accurately and clearly for a general audience.

Fascinating, no? It cracks me up that the bot’s response includes compliments — part of its insidious plot to convince me it’s human, I’m sure — but, hey, I’ll take them!

Thanks for reading and taking a look!


    1. Dale

      Thank you!

      No need to be scared; the AI chatbots can’t do anything (yet) without our brains… you know, sort of like zombies.

      Ohhhhh… wait…

      Thanks for the comment!

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