Through the years I have heard many myths and half-truths about Hydrangea colors (this is for Hydrangea macrophylla i.e. Mophead and Lacecap Hydr.).
I have been asked many times in our garden center what makes the color of Hydrangea flowers turn blue or pink. Most gardeners want them to be blue which can be harder to achieve than pink particularly if your soil won't play along.
A lot of people might tell you that it is iron that will make the flowers turn blue and that adding rusty nails to the soil will do the trick. That is a myth though ...and it is totally wrong! The issue is a little bit more complex than that!
There are several factors that play a role with Hydrangea colors.
1. You need a blue variety! Not all of them will turn blue even if the soil conditions are correct. Some pink varieties will stay pink no matter what (and some can turn blue or purple!). White Hydrangeas usually stay white. Red varieties can be changed to purple but not blue.
These varieties simply don't absorb Aluminium from the soil which is responsible for the blue flower colors. There are many beautiful blue Hydrangea varieties to choose from. Have a look here if you are interested.
2. The soil needed for blue Hydrangea colors has to be in the correct pH range. The ideal range is a pH 4.3 - 5 (you can test your soil with a pH tester).
The soil also needs a sufficient available amount of Aluminium which is only absorbable for plant roots if the pH is below 5.
If you are gardening on alkaline soils (high pH) it is not so easy to keep the flowers blue. You can try to lower the pH by mulching the soil with bark mulch. This is not a quick fix though. It might take a long time to change the soil but it also helps to keep the soil moist which is essential for healthy hydrangea plants.
You can use an organic soil acidifier like this one to lower the pH. Depending on how alkaline your soil is this might take several applications. Work it into the surface around your plant and water well. If you have hard water use rainwater instead for watering.
You might have to use the soil acidifier several times during the growing season to keep the color blue.
Aluminum sulfate is also often recommended to acidify the soil. You need to be more careful though with it. It can be toxic if it is used too much and it can kill the plants by burning the roots.
Keeping the flowers pink can be achieved in two ways. You can simply buy a pink Hydrangea variety that won't turn blue...and you are done!
Or just work some garden lime into your soil to keep the pH above 5. It might take several weeks to adjust the pH so keep checking your soil with a pH tester from time to time. Apply more lime as necessary. You should aim for a pH of 6-7.