Reactions | How Do Hydrangeas Change Colors? | Season 5 | Episode 22

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.

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