The flower color of a bigleaf hydrangea is determined by the soil pH and by the amount of aluminum in the soil unless it is a white-flowering variety, which has no pigment and is not affected.

Note: Not all hydrangeas can change color. Hydrangea macrophylla and Hydrangea serrata are species that are best known to respond to soil pH changes.

The Science of Hydrangea Color

Hydrangeas can readily absorb aluminum only when the soil pH is low. Hence, the more aluminum in the soil and the lower the pH (acidic), the bluer the flowers will be.

The less aluminum in the soil and the higher the pH (alkaline/more basic), the pinker the flowers will be.

How to Determine Hydrangea Color

It sounds simple enough, but since there aren’t really any soil tests available to the average gardener to determine aluminum content, the easiest way to determine how your soil will impact your hydrangea is to let your plant be your guide.

1. Allow your plant to grow in the ground for a year, through one flower cycle.
2. If your flowers are blue, you have aluminum present in your acid soil.
3. If your flowers are pink, you either have no aluminum OR the pH is alkaline.
4. To get the color you want, test your soil pH and amend it accordingly in early spring. Watch this video to learn about soil testing

Quick Guide to Amending Soil to Change Hydrangea Color

Once you have the results of your soil test and know whether your soil is acidic or alkaline, you can determine how to amend your soil for the flower color of your choice.

Acidic soil with aluminum = blue flowers

› For pink flowers, add lime and phosphorus to raise the pH and block aluminum uptake, or grow in raised beds or containers using a soilless or aluminum-free planting medium.

Acidic soil with no aluminum = pink flowers

› For blue flowers, add aluminum sulfate to increase aluminum content.

Alkaline soil with aluminum = pink flowers

› For blue flowers, add organic matter like compost or composted manure around the base of the plant to lower the pH.

Alkaline soil with no aluminum = pink flowers

› For blue flowers, add aluminum sulfate to lower the pH and increase aluminum content. Note: In general, aluminum is available in soils and adding too much aluminum sulfate can damage plants and the environment.