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So, I've heard that people sometimes mix a bit of lemon juice in when they water blue hydrangeas, they say it keeps them blue if the soil is lacking in acid. I was wondering, I have a pink indoor hydrangea plant; would it be possible to alter the color by putting the lemon juice in? Would it possibly make it blue in time, or purple?

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It really depends on the specific species of hydrangea that you have. It's mostly Hydrangea macrophylla, also called "big leaf" hydrangea (that's the Latin name literally!) that changes colour depending on the pH level of the soil.

In acidic soils (pH less than 7), the flowers are blue in colour (see pic 1), in neutral soils (pH = 7), it is very pale pinkish to nearly whitish (or light cream) and in alkaline soils (pH greater than 7), it is a bright pink or sometimes purple in colour (although, I've never managed to get it to turn purple). Wikipedia gives the following as the reason for the change in colour:

[The change in color with acidity] is caused by a color change of the flower pigments in the presence of aluminium ions which can be taken up into hyperaccumulating plants.

Usually, people throw their coffee grounds in the soil next to a hydrangea plant to increase the acidity and get it to turn blue. However, I've never heard of anyone using lemon juice to achieve this effect. Note that lemon juice has a higher acidity than coffee grounds (and I guess stronger than what hydrangeas would prefer) and I'm afraid you might actually damage the roots. If you must use lemon juice, then you might have to dilute it before feeding it to the plant.

If you'd like to play it safe, most gardening centers sell mixes to turn hydrangeas either blue or pink and you could get one of those and feed your plant. It really is worth trying it and a garden with multiple hydrangea plants, each a different shade is very beautiful.

1. Blue flowers (acidic soil)

enter image description here

2. Bright pink flowers (alkaline soil)

enter image description here

Images courtesy Wikimedia commons

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    Yes a lot of modern varieties have been bred to be specific colours - typically, blue, pink, or white. If you have one of these, tricks with limestone, coffee grounds, lemon juice, etc are not going to work. – winwaed Aug 19 '11 at 8:40
  • Great answer, thanks! My plant is the 'big leaf' hydrangea, but since they're bright pink at the moment and I'm unaware of how they've been bred, I think I'll just keep them the way they are. – BKaylor Aug 19 '11 at 20:42
  • My wife uses a mixture from the garden store that is "acidic" hydrangea food. Keeps them nice and blue. – Larian LeQuella Aug 21 '11 at 2:51
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I would definitely not use lemon juice for the reasons already mentioned, but provided you don't have one of the varieties that have been bred to be a specific color, you could use aluminum sulfate to acidify your soil after testing its pH. I have used it myself and it is very effective, but it has to be applied carefully, as an overdose can be toxic.

Here is the advice given by the University of Minnesota Extension:

After planting:

1) Periodically retest your soil pH. Do not add any acidifying amendments if your soil pH is 5.0 or less.

2) To maintain good vegetative growth and acid soil conditions, use ammonium sulfate as the nitrogen source at the rate of 1 lb (or 2 cups) per 100 square feet (or a little less than a ¼ cup per 10 square feet). Ammonium sulfate is the best nitrogen source to help maintain soil acidity.

3) If your soil pH is greater than 5.5, use aluminum sulfate to help lower your pH and supply available aluminum at the same time. Note that aluminum sulfate is generally not recommended for lowering soil pH because high rates can injure most plants. Hydrangeas are the exception. Mix about 1 lb (2 cups) of aluminum sulfate per 5 gallons of water and then apply the solution around the drip line of the plant. Apply enough of the solution per plant to just saturate the soil. Repeat the application on a monthly basis as long as your soil pH is greater than 5.0. Caution - over application of aluminum sulfate can be toxic even to hydrangea. Monitor your soil pH periodically and only apply aluminum sulfate if your pH is greater than 5.0.

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