How to Change Hydrangea Color: Blue, Pink and Purple

A bigleaf hydrangea's flower color is a readout of its soil. The shrub makes one pigment whether it blooms pink or blue; what differs is aluminum, and soil pH decides whether the roots can reach it. Around pH 5.0 to 5.5, aluminum dissolves, the roots absorb it and the flowers open blue. From about pH 6.0 to 6.5 upward, it stays bound in the soil and the same shrub blooms pink.

So you change the color by changing the soil, in measured doses. Confirm the plant can respond, test the soil, choose the product that suits it and apply it well ahead of bloom, then give it at least a season. Flowers already open will not change; what you are treating is next year's buds.

Why hydrangeas turn blue or pink

Blue flowers carry aluminum and pink ones carry almost none, about one-twentieth as much, and soil pH controls the supply. Every shade in a bigleaf hydrangea's showy sepals comes from a single anthocyanin, delphinidin 3-glucoside. When aluminum reaches the sepal cells, each pigment molecule binds one aluminum ion and one molecule of a colorless helper compound, and that trio reads as blue. Blue sepals hold about 120 micrograms of aluminum per gram of fresh tissue, far more than red ones, and their cell sap is less acidic, near pH 4.1 against 3.3.

The soil decides how much aluminum arrives. Acidic soil keeps aluminum soluble, so roots take it up; as pH climbs toward neutral, it becomes unavailable and the flowers turn pink. Most garden soils already contain plenty of aluminum that high pH simply locks away, which is why lowering pH is the main lever. Commercial growers add aluminum sulfate to the soilless mixes they grow plants in, so a shrub bought blue may owe its color to the nursery mix. Once its roots grow into your garden, your soil sets the color.

Target soil pH for bigleaf and mountain hydrangeas. Published ranges differ by a few tenths of a point.

Purple is the hardest color to hold, because it sits in a narrow band where a small pH swing tips the flowers one way or the other. Cultivar, weather and plant health shade the result as well, so two shrubs in the same bed can bloom differently. A single shrub can even carry pink and blue heads at once when the pH varies across its root zone, for example where one side grows against a foundation that leaches lime.

Which hydrangeas can change color

Only bigleaf hydrangea (Hydrangea macrophylla), in mophead and lacecap forms, and mountain hydrangea (H. serrata) turn blue or pink in response to soil. Every other hydrangea keeps its color whatever you add.

Only the first two rows respond to soil treatment.

The blush that spreads over a Limelight panicle hydrangea or an oakleaf hydrangea as its flowers mature is aging, not the soil at work. Annabelle smooth hydrangea stays white to pale green on any soil, and a climbing hydrangea stays white. White bigleaf cultivars stay white too, although an occasional one shows a pink or blue eye that tracks the aluminum supply.

Mountain hydrangeas behave exactly like bigleafs chemically. They are smaller shrubs, around 3 to 5 feet (0.9 to 1.5 m), and hardier in winter. If you are not sure what you are growing, leaf shape and bloom habit settle it quickly in this rundown of types of hydrangeas.

Sulfur, aluminum sulfate or lime: choosing a treatment

Use elemental sulfur for a lasting drop in pH, a dilute aluminum sulfate drench for a quicker push on an established shrub, and lime to raise pH for pink. Each works at a different speed and fails in a different way.

How the four standard color treatments compare.

Sulfur is the gentle tool. Soil bacteria have to convert it to sulfuric acid before the pH moves, which takes several months, so it suits a fall application and a long-term plan rather than a quick fix. Where soils already hold plenty of aluminum, as in Pennsylvania, sulfur is all a blue program needs.

Aluminum sulfate is faster and riskier. It acidifies the moment it dissolves and delivers aluminum straight to the roots. Used to shift pH in bulk, though, it takes far more product than sulfur. Moving loamy soil from pH 7.0 to 5.5 calls for roughly 2.1 pounds (0.95 kg) per 10 square feet (0.9 m²), against 0.3 pound (140 g) of sulfur. At that scale aluminum accumulates and injures roots. Treat it as a dilute spring program for established shrubs at least 2 to 3 years old, never as a soil conditioner.

Lime works the other way, and more slowly. Raising pH takes longer than lowering it, and lime barely reacts in dry soil. Spread in fall, it has the winter's moisture to work before next year's buds develop. A hydrated lime drench is the spring alternative when you need to correct course.

Fertilizer is a supporting player that can undo a blue program. Phosphorus binds aluminum in the soil so roots cannot take it up, which pulls flowers back toward pink. That is why a blue shrub gets a low-phosphorus feed and a pink one gets a higher-phosphorus feed alongside its lime.

How to change hydrangea color, step by step

Test first, treat before the buds color, then wait.

1. Confirm the plant can respond. Check that it is a bigleaf or mountain hydrangea that blooms pink or blue. Treating a white cultivar or a panicle, oakleaf or smooth hydrangea wastes product and a season.
2. Test the soil. Send a sample to your county Extension office before adding anything, and compare the result with the target ranges above. Knowing the starting pH is what prevents over-correction.
3. For blue, spread sulfur in fall or early spring. Take the dose from the sulfur table below for your test result and soil texture, weigh it, scatter it over the root zone and water it in. Starting at pH 6.0, 0.1 pound (45 g) per 10 square feet (0.9 m²) is enough to bring loamy soil to 5.5. The general rate of 1/2 cup (120 mL) of wettable sulfur per 10 square feet is a heavier dose, meant for soil that tests pH 6.5 or higher. On soil already near 6.0 it can overshoot the target.
4. For faster blue, drench an established shrub in spring. Dissolve 1 tablespoon (15 mL) of aluminum sulfate in 1 gallon (3.8 L) of water. Pour it onto bare soil under the canopy until the root zone is soaked, once in March, once in April and once in May, while new growth emerges and buds form. Use it only on established shrubs at least 2 to 3 years old, and read the warning below before you mix.
5. Feed blue plants low in phosphorus. Choose a fertilizer for acid-loving plants with a low middle number, such as 12-4-8 or 25-5-30, and leave out bone meal and superphosphate. Apply it once in spring, worked into the top 2 to 3 inches (5 to 8 cm) of soil and watered in thoroughly.
6. For pink, lime in fall. Broadcast 1 cup (240 mL) of dolomitic lime per 10 square feet (0.9 m²), then water it in. Alternatively, drench with 1 tablespoon of hydrated lime per gallon of water on the same March, April and May schedule. Feed with a higher-phosphorus fertilizer so leftover aluminum stays tied up.
7. For purple, aim between the two. Purple, mauve and mixed heads come from soil around pH 5.5 to 6.0. Some definitions of the band run as wide as 5.5 to 6.5, others as narrow as 5.6 to 5.9. Move the soil in small steps. From pH 6.5, the lowest sulfur rate in the table, 0.1 pound per 10 square feet, aims for 6.0; soil that tests below 5.5 takes the lime rate your soil test report gives. Retest every season, because a small swing tips the flowers toward blue or pink.
8. Retest before you repeat anything. Judge the flowers over the next season, test again before another round of aluminum sulfate or lime, and after that check pH every 2 to 3 years.

Elemental sulfur in pounds per 10 square feet (0.9 m²) of loamy soil; 0.1 lb is about 45 g. Use two-thirds as much on sandy soil and half again as much on clay. In soil with free lime, no rate will hold the pH down.

Coffee grounds, vinegar and other home remedies

None of the household shortcuts is a dependable way to change color, and none comes with a tested rate. The products in the steps above do the job; these mostly cost you a season.

Popular remedies set against what has been measured.

Coffee grounds change pH as they decompose and can end up alkaline, so a mulch of grounds cannot steer a bed toward 5.5. If you want to use them, they belong in the compost pile at no more than about a fifth of the mix. No test has shown that rusty nails or pennies change flower color, so there is nothing to gain from burying them.

When your soil works against you

Your starting soil decides how hard the job is. In naturally acidic soil blue comes free and pink takes work; in limy soil, blue flowers in the ground may be out of reach.

Regional starting points for bigleaf and mountain hydrangeas.

Soil that contains free lime (calcium carbonate) resists acidification. The pH cannot be pulled down effectively, elemental sulfur often does little, and aluminum sulfate is a poor substitute because the extra aluminum is toxic to roots. Blue flowers need strongly acidic soil, and limy soil will not stay there.

Concrete creates the same problem on a smaller scale. Foundations, sidewalks and patios leach lime into the soil beside them, and a hydrangea planted against a house wall may never turn fully blue. Site blue shrubs well away from concrete, or test both sides of an existing plant and treat the alkaline side.

A container sidesteps native soil altogether, since you control the whole root zone. Fill it with an acidic potting mix and keep phosphorus low. No container-specific drench rate is well established, so test the mix and treat a potted plant more cautiously than one in the ground; this guide to growing hydrangeas in pots covers pot size, watering and winter protection.

Cultivars that change easily, and ones that hold their color

Cultivars differ in how readily they respond, and a few are bred to hold one color in any soil. Matching the cultivar to your soil and winter saves years of adjusting.

Winter matters as much as soil. In Zone 6 and colder, the old-wood flower buds of many older bigleaf cultivars do not reliably survive, which leaves nothing to color. Rebloomers that flower on both old and new wood get around that.

Hardiness and height as published for each cultivar. Nikko Blue can also survive a sheltered site in Zone 5.

Endless Summer The Original flowers on both old and new wood. BloomStruck, another rebloomer, grows wider than tall, 4 to 5 feet (1.2 to 1.5 m). Tuff Stuff Ah-Ha blooms from early summer until frost, with flower buds that hold up in Zones 5 and 6. Let's Dance Rhythmic Blue, rated for Zones 5 to 9, swings between pink and a brilliant blue. Penny Mac, a rebloomer named for Penny McHenry, founder of the American Hydrangea Society, gives a clear blue or a clear pink. Nikko Blue, the long-standing industry mophead, blooms only on old wood, so it needs a winter mild enough to spare its buds. Among mountain hydrangeas, Bluebird is a lacecap of up to 5 feet (1.5 m) that runs from pink to violet-blue. In high-pH soil, Cityline Paris matures to a dark pinkish red.

Other cultivars resist change, and a few never turn blue whatever the pH. Pia, a dwarf of about 2.5 feet (75 cm), holds its pink and turns blue only with difficulty. Among whites, Blushing Bride, an Endless Summer rebloomer, opens pure white and matures to soft pink.

What to expect after treatment

The first shift shows the following season at the earliest, and full, even color often takes two or three bloom cycles. Pink comes more slowly than blue, because raising pH is the slower job. Granular sulfur and lime can take a year to show at all; a spring drench can act sooner, provided it reaches buds that are still forming.

If the result stalls or something looks wrong, the cause is usually one of these:

• Still pink after a year of sulfur. Check the fertilizer label for phosphorus, look for nearby concrete, and retest. If the soil holds free lime, move the plant into a container.
• New leaves yellowing between the veins. The pH has climbed too high, usually from over-liming, and the plant cannot take up enough iron. Stop liming and test before doing anything else.
• Healthy leaves but no flowers. That points to pruning or winter bud loss rather than the soil treatment; the causes are covered in why hydrangeas do not bloom.

Protect the buds you are coloring. Bigleaf and mountain hydrangeas begin forming next year's flower buds in August, so avoid pruning them after August 1, and see when to prune hydrangeas for the timing by type. For siting, watering and routine feeding beyond color, the hydrangea growing guide picks up from here.

If you are planning a new blue or pink planting, sketch it in Sow's garden design studio first so the shrubs sit away from foundations and walks.