Hydrangeas: Chemistry in action

I’ve got some beautiful hydrangeas blooming in my garden right now.  They are one of my favorite flowers, especially because they can be blue, my favorite color.

They are also a fascinating application of acid-base chemistry.  Hydrangeas get their blue color from aluminum, and in acidic soil is it freely available as Al³+ and taken up by the plant; but in basic soil, it is bound up in compounds and therefore not as available for uptake.

Here’s the chemistry for aluminum binding under basic conditions:

from: http://www.soil.ncsu.edu/publications/Soilfacts/AGW-439-50/SoilAcidity_12-3.pdf
from: http://www.soil.ncsu.edu/publications/Soilfacts/AGW-439-50/SoilAcidity_12-3.pdf

Therefore, hydrangeas in acidic soil have blue flowers, and those in basic soil have pink flowers.  So hydrangeas work kind of like a litmus test for you soil, but interestingly the colors are switched from a standard litmus test, where acidic=red and basic=blue.

You can add fertilizers and compounds to your garden soil to manipulate the color of your flowers.  I did not add anything to my hydrangeas, so I got an interesting mix of pink and blue and purplish flowers (sometimes even in the same bloom!)

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