Articles, Blog

How Do Hydrangeas Change Colors?

September 4, 2019

Thanks to CuriosityStream for supporting PBS
Digital Studios. Hey so remember that time in the 3rd grade
when you drowned a carnation in food coloring and… this happened ? We’re about to try
something similar…but not. This experiment shows that carnations take
up water and whatever’s dissolved in it, which is cool. But what if we switch out the carnations for
some hydrangeas, and instead of food coloring, use this completely colorless solution. Will they change color? But why would they? Hydrangeas have an unusual property. Their blossoms can change color depending
on the pH of the soil they’re planted in. Acidic soil grows blue flowers, while more
basic soil grows red ones — and borderline conditions turn out purple. By the way, before we go any further I am
contractually obligated by all botanists in the world to tell you that these things that
look like petals — they’re not actually petals, they’re sepals — same as the little
star-shaped thing at the base of a rosebud. OK, so anyway: You can change the color of
a hydrangea’s sepals in your garden by adjusting the soil pH. You can add stuff like garden sulfur to lower
the pH and get bluer blossoms, or you can add stuff like calcium hydroxide to raise
the pH and get redder blossoms. But it’s not the pH that’s making the
color change — not directly. See, soil contains some aluminum. Which is not that surprising since the Earth’s
crust is full of it. In acidic soils, that aluminum dissolved in
the water in the soil, so it’s in its ionic form. This dissolved Al3+ is toxic to plants; it
can be taken up by the roots and damage the plants. To avoid that damage, some plants, including
hydrangeas, secrete chemicals into the soil that can bind to that Al3+ — in this case,
citric acid. That citric acid forms a complex with the
Al3+ and that complex is not toxic. Now I know, his seems like it has nothing
to do with color. But stick with me. If the soil is neutral or basic, the aluminum
binds to hydroxide ions, forming solid aluminum hydroxide. Unlike aluminum citrate, aluminum hydroxide
cannot enter the plant’s roots. So, quick recap: Hydrangeas in acidic soil
take up aluminum, though they have to detoxify it. Hydrangeas in basic soil don’t have to deal
with aluminum at all. Store that in your memory banks, it’s gonna
be important in about 15 seconds. But in the meantime, let’s talk about why
pink hydrangeas are pink in the first place. This pink — or red, if you like — is actually
a type of anthocyanin — a family of pigments found in tons of plants from grapes to berries
to eggplants. This particular one is called delphinidin-o-glucoside. In basic soil, where hydrangeas — remember
— don’t take up aluminum, delphinidin-o-glucoside takes this form (flavylium cation). Let’s call it red version. It reflects red light, which means to our
eyes, it has a red color. But in acidic soil, where aluminum citrate
does get taken up by the roots, red version binds to the aluminum ion, loses a few hydrogens
and shifts a few double bonds, and gets a new name…which we’ll call blue version. Those rearrangements mean it now reflects
blue light instead of red… and voila, blue sepals. Now that seems complicated, but… it’s
actually even more complicated in real life. The blue you’re seeing is blue version,
but it’s stabilized by a bunch of buddies called co-pigments. While scientists aren’t sure exactly how
they all interact, they do know that blue version is the main source of the color. So all this means that the color of hydrangea
sepals depends on how much aluminum the plant can take up through its roots. Sepals that contain less than 10 micrograms
of aluminum per gram are red. Over 40 micrograms per gram are blue, and
in between gives you purple. The complex of aluminum with blue version
is very stable. And it’s been speculated that complexing
toxic aluminum in its sepals like this is the plant’s way of preventing damage to
itself from free aluminum in the soil: first transporting it using citrate, then using
the pigments to safely tuck it away. Changing the soil pH in your garden can take
about a year. But some scientists who published their work
in a 2011 paper in the journal Biometals found a way to speed up the color change by a lot. They incubated cut hydrangeas in a solution
of aluminum and buffered citric acid, and they were able to turn red hydrangeas blue
in just a couple of days. Now, when we read this paper, we thought,
well, we have to do this on camera. So we got in touch with the researchers and
tried to replicate their experiment and… We managed to get a tinge of blue around the
edges of the sepals before our flowers died. Yeah these results are pretty underwhelming. So like good scientists should, we went back
to the drawing board… and we’ll tell you all about that in part 2, so come back in
a couple weeks to see if we pull it off. Thanks to CuriosityStream for supporting PBS
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  • Reply Pi June 19, 2018 at 8:06 pm


  • Reply Blank June 19, 2018 at 8:06 pm


  • Reply Green Cat June 19, 2018 at 8:07 pm


  • Reply Reactions June 19, 2018 at 8:15 pm

    We’ve got another video coming up where we repeat our little experiment, and (spoiler alert) it went a bit better the next time around. But in the meantime, what would you do to get a better color change in vitro?

  • Reply Shirin Rose June 19, 2018 at 8:35 pm

    I can't tell what's different here from the previous upload…

  • Reply Robert Fletcher June 19, 2018 at 8:56 pm

    What happened to the first video? I was hoping to get an answer to a question not going to write it again. I am not liking or disliking this video it looks the same to me and others I noticed.

  • Reply Venomous Raga June 19, 2018 at 10:22 pm


  • Reply LOVE OF PLANTS 🌻 June 19, 2018 at 11:09 pm

    Thanks for an interesting plant video!

  • Reply B B64 June 19, 2018 at 11:24 pm

    Mine changed in 2 weeks.

  • Reply Jia Hong Situ June 21, 2018 at 2:43 pm

    Introductory bio kicking in

  • Reply ZenZaBill June 25, 2018 at 1:04 am

    Interesting and informative — for what ends up being an ad for their website.

  • Reply Ireallyreally Hategoogle June 28, 2018 at 8:47 pm

    No likes for the unfinished video.

  • Reply HERO X X October 9, 2018 at 7:01 pm

    How does calcium hydroxide raise the level of ph on the soil ?

  • Reply Mina JeetjeMineetje January 26, 2019 at 7:07 pm


  • Reply Star Dust March 16, 2019 at 4:05 pm

    How long does it take to change the color from pink to blue during Spring?

  • Reply Mit Nosnah August 31, 2019 at 9:16 am

    Does anyone know how much sulphate of iron to put in soil to turn hydrangea back blue ?

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