Hydrangea flower color follows soil pH. Soil with a pH below 5.5 gives blue flowers. Soil with a pH above 6.5 gives pink flowers. Soil between 5.5 and 6.5 gives purple flowers, or blue and pink flowers on the same plant. The pH matters because of aluminum. Acidic soil frees aluminum for the roots, and aluminum in the petals turns them blue. Bigleaf and mountain hydrangeas are the two types that change color. White cultivars stay white. A color change takes months, and it can take a year to show.
Start with a soil test. If the pH already sits in the range for the color you want, leave the soil alone. To go blue, lower the pH with sulfur and make sure aluminum is in the soil. To go pink, raise the pH with garden lime. The hydrangea color chart below shows where each color falls for Hydrangea macrophylla.
Hydrangea color chart
The chart shows the estimated flower color at each soil pH level. Soil with a pH under about 5.5 tends to give blue hydrangea blooms. Soil with a pH above 6.5 tends to give pink blooms. Flowers tend to be purple in the soil pH range of 5.5 to 6.5, which is also a good range for overall nutrient uptake.
The color of hydrangea flowers depends on the amount of aluminum in the flower petals. When aluminum is available and mobile, the flowers tend to turn blue. Aluminum is most available in acidic soils. It stays locked up in alkaline soils.
The pH levels in the chart refer to the pH of the soil, not the pH of the flowers.
Types of hydrangeas that can change colors
Of the half-dozen popular species of garden hydrangea, two species have flowers that change color with soil conditions. Most cultivars that change color belong to bigleaf hydrangea (Hydrangea macrophylla). Some mountain hydrangea (Hydrangea serrata) cultivars change color too.
Nature Hills sells bigleaf hydrangeas and mountain hydrangeas, the two types that change color.
Hydrangeas that won’t turn blue
Smooth hydrangea (Hydrangea arborescens), panicle hydrangea (Hydrangea paniculata), oakleaf hydrangea (Hydrangea quercifolia), and climbing hydrangea (Hydrangea anomala subsp. petiolaris) do not respond to soil pH. ‘Annabelle’, ‘Limelight’, and ‘Skylands Giant’ belong to this group. Their flowers open white or cream. Some age to pink or green as the season goes on. Soil amendments won’t turn them blue.
White bigleaf cultivars stay white, too. The Mississippi State University Extension notes that white-flowered cultivars do not respond to soil pH, though some show a pink or blue eye. Some bigleaf cultivars also lean pink or blue no matter the soil, according to UMass Amherst Extension.
‘Summer Crush‘ is a good example. It blooms raspberry red in alkaline soil and turns purple in acidic soil, according to Northern Gardener. It does not reach a deep blue. For true blue flowers, start with a cultivar sold as blue, such as ‘Nikko Blue‘. This list of blue hydrangea varieties has more options.
How to change hydrangea flower color
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What you’ll need:
- A soil test kit, such as the Luster Leaf Rapitest soil test kit
- Wettable sulfur, to lower the pH for blue flowers
- Aluminum sulfate, such as Hi-Yield aluminum sulfate
- Dolomitic lime, such as Espoma Garden Lime
- A low-phosphorus fertilizer for acid-loving plants, such as Espoma Holly-tone 4-3-4
To change hydrangea color, test the soil first to get a baseline pH. A home kit such as the Luster Leaf Rapitest soil test kit gives a quick reading. Then adjust the pH toward the color you want. A soil test also shows nutrient gaps. The amount of lime or sulfur you need depends on your starting pH and your soil type.
Here is a guide for soil pH levels and the flower color they tend to give:
- Blue flowers: pH of 4.5 to 5.5 (some nutrients become hard to absorb below about pH 5.0)
- Purple flowers: pH of 5.5 to 6.5 (a good range for the health of the plant)
- Pink flowers: pH of 6.5 to 7.5 (some nutrients become hard to absorb above about pH 7.5)
The pH of the soil does not change the petal color on its own. The pH changes what happens to soil minerals. Aluminum ions must be in the soil, and they must be in a form the roots can absorb. The roots then move the aluminum up to the flower heads.
For blue hydrangea flowers
For deep blue blooms, aim for a soil pH around 5.0 or lower. Soil this acidic is not always the best for plant health. Lower the pH in small steps and watch how the whole plant responds.
To make hydrangeas blue, add elemental sulfur or aluminum sulfate to the soil. The University of Georgia Extension gives two methods:
- Slow method: broadcast 1/2 cup of wettable sulfur per 10 square feet and water it in.
- Faster method: dissolve 1 tablespoon of aluminum sulfate in 1 gallon of water. Drench the soil around the plant in March, April, and May. Keep the solution off the leaves.
Take care with aluminum sulfate. Too much of it burns roots and injures the plant. Do not increase the amount, and follow the label on any product. The University of Maryland Extension suggests sulfur or iron sulfate over aluminum sulfate for this reason. Aluminum sulfate is the one to reach for when the soil is acidic but the flowers still won’t turn blue, since it adds aluminum as well.
Avoid high phosphorus fertilizers on blue hydrangeas. Phosphorus binds aluminum in the soil so the plant can’t use it. UMass Amherst Extension suggests a fertilizer for acid-loving plants with a low middle number, such as 12-4-8, once in spring. Espoma Holly-tone 4-3-4 is one widely sold option with a low middle number.
For pink hydrangea flowers
For bright pink flowers, aim for a soil pH of about 6.5 or higher. This is close to the neutral pH of 7.0. Gardeners with alkaline soil almost always get pink flowers.
To make hydrangeas pink, raise the soil pH with garden lime (dolomitic limestone), such as Espoma Garden Lime. The University of Georgia Extension suggests 1 cup of dolomitic lime per 10 square feet, watered in. For a faster change, dissolve 1 tablespoon of hydrated lime in 1 gallon of water and drench the soil in March, April, and May. Too much lime can turn new growth yellow, because it blocks iron.
For purple hydrangea flowers
Purple sits in the middle of the chart, at a soil pH of 5.5 to 6.5. Many gardens land here without any help. To make hydrangeas purple, test the soil and nudge the pH into that range with small doses of sulfur or lime.
How long it takes hydrangea color to change
Hydrangea color takes time to change. Granular sulfur or lime can take a year to show a clear change, according to the University of Georgia Extension. Liquid drenches in spring work faster.
Soil also drifts back to its natural pH over time. Plan to repeat the treatment each year to hold the color. This guide on how long it takes to change hydrangea flower color covers the timing in more detail. If a plant was blue and is fading, these reasons for hydrangea losing color can help you find the cause.
The science behind changing the color of hydrangeas
Hydrangea color varies with soil pH because soil pH controls how much aluminum is available in the soil. Soil pH may not be the main factor behind petal color. It is still a reliable sign of how much aluminum is available, and so of the color the petals will be. The soil must hold enough aluminum, and the aluminum must be in a mobile form that the roots can absorb.
In acidic soil, aluminum ions are mobile. The ions react with other molecules, and the roots absorb them and carry them up to the flowers. The aluminum shifts the pink pigment in the flower to a blue tone.
In neutral to alkaline soil, aluminum ions combine with hydroxide ions to form aluminum hydroxide, which roots can’t absorb. Hydrangeas with red-pink flowers tend to hold little aluminum in their petals. Chemist Henry Schreiber explains how aluminum binds the pigment in Curious chemistry guides hydrangea colors in American Scientist:
“At a threshold of only about 40 micrograms of aluminum per gram of fresh sepal, hydrangea sepals turn blue, but they don’t become bluer with yet more aluminum.”
American Scientist, Curious chemistry guides: Hydrangea colors
The color of hydrangea petals comes from a water-soluble anthocyanin pigment called myrtillin (delphinidin-3-glucoside). The molecular structure of the metalloanthocyanin (the myrtillin chemical complex) sets which wavelengths of light it absorbs. The complexes lose hydrogen ions as pH changes, and that shifts which wavelengths they absorb.
Hydrangeas move aluminum in a citrate complex. Citrate ions and citric acid form around the roots, depending on soil pH. They react with aluminum ions to form a stable complex that the roots can absorb.
American Scientist, Curious chemistry guides: Hydrangea colors
Hydrangea color and soil pH FAQ
What soil pH makes hydrangeas blue?
A soil pH below 5.5 gives blue flowers on most bigleaf hydrangeas. A pH near 5.0 gives the deepest blue. The soil also needs aluminum the roots can reach.
What soil pH makes hydrangeas pink?
A soil pH above 6.5 gives pink flowers. Neutral and alkaline soils keep aluminum locked up, so the petals stay pink.
Why are my hydrangeas purple?
Purple flowers mean the soil pH sits between 5.5 and 6.5. Some aluminum reaches the petals, but not enough for a full blue. A plant in this range can carry blue, pink, and purple flowers at once.
Can I change the color of a white hydrangea?
No. White cultivars do not respond to soil pH. Smooth, panicle, oakleaf, and climbing hydrangeas do not change color with soil pH either.
Do rusty nails or coffee grounds turn hydrangeas blue?
No. Burying rusty nails does not supply the aluminum that blue flowers need, according to the Mississippi State University Extension. Coffee grounds, pennies, and aluminum foil won’t change the flower color either, according to Proven Winners ColorChoice.
When should I add sulfur or lime?
Granular sulfur and lime can go down any time of year, according to the University of Georgia Extension. Liquid drenches go on in March, April, and May. Test the soil again before each new treatment to see how far the pH moved.
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