top of page
Search

A Science Experiment in Your Own Backyard – Factors Affecting Diverse Hydrangea Colors

  • Writer: Triple Helix
    Triple Helix
  • 5 hours ago
  • 5 min read

Figure 1: Variation in hydrangea colors (1).
Figure 1: Variation in hydrangea colors (1).

Written by: Ava Gell, ’28

Edited by: Yumiko Imai, ‘26


Take a walk around any green space in New England, and you’re sure to stumble across one of nature’s most beautiful science experiments – the hydrangea. They range in size from a small shrub to an eight-foot-tall tree to a lush climbing vine (2). They are perhaps best known for their magical pastel hues, which range from pink to blue. The cause of this range of color is its own kind of magic: chemistry.


What gives a hydrangea color in the first place?


Some flowers, such as tulips, derive their colors from a blend of pigments, the artistic equivalent of mixing multiple paints to create a particular shade (3,4). But a hydrangea’s color is determined by a single pigment – like having a color-changing paint! This pigment is called an anthocyanin, and it’s also responsible for the vibrant colors of trees in autumn. Normally, the anthocyanin reflects wavelengths of red light back into our eyes, causing us to see hydrangea flowers as pink. But its structure changes at different pH levels and in the presence of aluminum. 

Aluminum makes the flower color dependent on pH by interacting with different salts. In damp soil, water pulls these salts apart into charged ions, creating either acidic (pH lower than 7) or basic (pH higher than 7) conditions. Ions that make the soil acidic react with aluminum and turn it into a soluble compound, which is pulled up into the hydrangea with water. The aluminum travels all the way to the flowers, where it interacts with the anthocyanin pigment and changes its structure, making it reflect blue light and turning the hydrangea blue.

On the other hand, two different factors might make a hydrangea pink. First, there might not be any aluminum in the soil to begin with, causing no reaction to occur. Or, the soil could have a high pH, where hydroxide ions are released into the soil, reacting with aluminum to create an insoluble compound. Unlike ions in acidic soil, aluminum hydroxide stays fixed in the ground. Without aluminum in the flower, the hydrangea remains pink.

An intermediate state – a purple flower – is also possible. This happens if there is only a tiny bit of aluminum present or if the pH is mild (not very far from 7). In both cases, a small amount of aluminum reaches the flowers, reacting with only some anthocyanin to create the blend of blue and pink we see as purple. 


Figure 2: Summary of factors affecting hydrangea pigmentation.
Figure 2: Summary of factors affecting hydrangea pigmentation.


What about a white hydrangea? Or a multicolor hydrangea?


However, pink, blue, and purple aren’t the only colors a hydrangea can exhibit. Many hydrangeas are pure white, or only slightly tinted, and some even display a marbling of colors on the same plant or the same flower. The mechanisms underlying these pigments are slightly more nuanced.

A white hydrangea is similar to albinism in animals: just as albino lizards, peacocks, and squirrels don’t produce melanin (the pigment responsible for skin and fur color), white hydrangeas don’t produce anthocyanin (5). Without pigment, the flower has no color. On the other hand, multicolor flowers or plants have the typical amount of anthocyanins, but the sources from which they obtain their water and nutrients differ (6). A large hydrangea plant has a sprawling root system underground, touching different parts of the soil that may differ in their aluminum content. If the soil has variation in aluminum or pH level, the hydrangea will likewise have variation in its flower color. 



What can I do to change the pigment?


If you’re interested in gardening, you may be curious about how to produce the most beautiful blooms in your chosen color. Leveraging this chemistry knowledge is essential to manipulate the flower color. 

Starting with white flowers: you’re out of luck. Since white flowers contain no pigment, alterations to the soil or aluminum content won’t impact your hydrangea’s color. 


Starting with pink flowers (turning them purple or blue): Good news– this is easy! Adding aluminum sulfate to the soil will both decrease the pH and increase the availability of aluminum for your plant to take in (7). For a home remedy, consider acidifying the soil with common household items such as coffee grounds or orange peels. However, be careful not to use vinegar or other additives that could have a negative effect on wildlife. 


Starting with blue flowers (turning them purple or pink): While this requires more time and effort than a color change of pink to blue, it is still possible and necessitates raising the pH. Adding lime (Ca(OH)2) releases hydroxide ions that make the pH basic and keep the aluminum in the soil (7). For an at-home equivalent, crushed eggshells may be effective. 


Other considerations: Other minerals in the soil, especially phosphorus and potassium, can affect whether the aluminum makes it into the plant (8). Phosphorus competes with aluminum for uptake, so adding phosphorus can also create a pink flower. Potassium increases the plant’s ability to pull aluminum out of the soil, so consider methods of adding potassium to achieve a blue flower.


Whether you’re an avid gardener or simply enjoy a morning walk, these jewels of nature are sure to present themselves wherever you find yourself. When you see a hydrangea, pause for a moment to reflect on the minute inner workings and miniature science experiments that create the beautiful world we inhabit.




Bibliography

 
 
 

Comments

The Triple Helix

browntriplehelix@gmail.com

  • Instagram
  • Facebook Social Icon

© 2024 by Triple Helix 

The Triple Helix is Brown University's in-print and online science journal dedicated to reporting scientific and research-based stories to the Brown community and general public.

bottom of page