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I'm actually studying for a chemistry exam, and in the Wikipedia page for Hydrangeas, it states that

In these species the color (flower) is affected by soil pH.

So the plant evidently takes the nutrients from the soil, and depending on the pH the flower will be different colors. However me and my friend were having a discussion on whether the flower can change color after forming. For instance, if I had a red hydrangea flower, and then suddenly acidified the soil, would it be possible that this red flower change color to blue? Or would the red flower remain, and only new flowers be blue?

  • In some cases pollination can do the thing for the next generation, but if you need a fast result you can add a big amount of vegetable colorant to the soil (just an idea) – Alexander Leon VI Oct 22 '15 at 19:10
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The previously existing, fully formed and open flowers would retain the original colour, although natural fading or slight alteration can be seen over time - application of substances to alter colour between pink and blue should be made 6 weeks prior to the flower being fully open, and that will be when the buds are forming. That's the latest time to attempt to influence flower colour deliberately. Generally, this response of colour change is only true of Hydrangea macrophylla varieties, though H. involucrata and H. serrata also demonstrate some ability to change the colour of their sepals according to conditions. White varieties cannot have their flower colour altered by the addition of substances to change the ph. More info in the links below

http://www.hydrangeashydrangeas.com/colorchange.html

https://www.rhs.org.uk/advice/profile?pid=122

As a point of interest, although we talk about hydrangea flowers, and imagine the typical mopheads seen everywhere, in fact, the mophead is not a flower - the sepals which give the colour of red, pink or blue are a bit like litmus paper, changing colour according to soil ph, or the availability of aluminium, but the actual flowers are tiny, held within the coloured sepals.

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We have roses in our yard. When one of the bushes first blooms, the flowers are red, but they're orange and/or pink when they're done blooming. So, yes, I believe they can change color. I don't think the soil PH has anything to do with this rose bush's flower colors, particularly, though.

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