Unlike other flowers that are bred to be a certain color, the shades of pink, blue or purple on some varieties of hydrangea are determined by the chemistry of the soil in which they are growing. But if you are trying to grow a certain color, it’s not just a simple matter of adding the correct fertilizer.
If you want to change the color of your blossoms or keep the color you have, you will first need to check which cultivar of hydrangea you have planted. Some, such as oakleaf hydrangea and all of the white-blossomed varieties, will not change color no matter what soil they are grown in.
Hydrangea blossom colors are affected by the pH of the soil and by the aluminum levels in the soil. Since these elements can change over time, you will need to keep up on maintaining them at the level you want, as often as several times a growing season. You can test your soil’s pH with an easy at-home kit or with litmus strips. Keep in mind that you won’t see an immediate change in the flowers when you change the soil, and the process of adjusting your soil’s nutrient contents should be done gradually so you don’t shock your plants.
It is easiest to control the soil conditions in a container than it is in the native soil. For container plants, use potting soil for blue flowers, but soilless potting mix for pink flowers, as many potting soils contain some aluminum.
Blue hydrangea flowers occur when the soil has a pH of 5.2 to 5.5, and when there is aluminum available to the plant.
Alkaline soil tends to “lock up” aluminum, preventing it from being used by your hydrangeas. If you have plenty of aluminum available but your pH is too high, you will not get blue flowers – both pH and aluminum must be at the correct levels to make the flowers turn blue!
Phosphorus also can prevent aluminum from being used, so be cautious about adding “Bloom” type fertilizers or bone meal, both of which have high phosphorus levels – while they will help your plants bloom, they may also cause them to bloom pink!
Be careful where you plant a blue hydrangea! Those planted near concrete walkways, driveways or walls may turn pink because of the lime that leaches out of the concrete.
Pink hydrangea flowers occur when the soil has a pH of 6.0 to 6.4, and has very low aluminum levels. Contrary to popular belief, hydrangeas do not need truly alkaline soil to turn pink – they just need soil that is less acidic than for blue flowers.
If your soil has a lower, more acidic pH, you can add lime fertilizers such as oystershell or dolomite to raise the pH to the desired level. Be careful not to raise your pH above 6.4, as this would make your hydrangea unhealthy and may cause an iron deficiency.
It is difficult to exclude or remove aluminum if it is already present in your soil. However, adding a high phosphorus fertilizer can discourage the plants from using the aluminum that is present in the soil. This is why it is much easier to turn a pink flower blue than it is to turn a blue flower pink. For soils with high aluminum levels, it is often easier to grow pink flowers in containers using soilless potting mix.
Purple and mixed colors
Purple flowers and plants with multiple colors are a result of less-than-ideal conditions for blue or pink flowers. The exact color or mixture of colors will depend on the precise pH and aluminum in your soil. For some in-between soils, you may even see flower clusters with two colors present.
If your soil is too acidic for pink and not acidic enough for blue, you will get purple flowers or plants with both pink and blue flowers.
If your soil has plenty of aluminum but soil is not acidic enough for blue, you will probably get purple flowers.
The fertilizer you use cannot affect the intensity of the pink, blue or purple in your flowers. Whether your flowers are bright or pale will vary by the environmental conditions such as humidity and temperature, health of the plant, and other natural factors out of your control.