Why are my hydrangea blooms pink instead of blue?

TL;DR

Hydrangea flower colour depends on soil acidity (pH), not plant variety. Acidic soil (pH below 6.0) gives blue blooms. Neutral to alkaline soil (pH above 7.0) gives pink or red blooms. You can change this by adding sulfate or lime.

Why are my hydrangeas pink instead of blue?

This is the biggest disappointment for hydrangea lovers. You buy a plant with pictures of brilliant blue, plant it with care... and it blooms pink. Not disease, not damage, just the wrong colour.

The reason: hydrangeas are "chameleons". Their blooms take the colour from the chemical makeup of your soil. Specifically: how acidic or alkaline your soil is, measured in pH value. This is not a mystery of nature; it is pure chemistry.

In acidic soil, the hydrangea root system takes up aluminium. This aluminium gives the blooms their blue colour. In neutral or alkaline soil, aluminium is not available to the plant, so no blue. Instead you get pink, purple or red. The plant itself is the same; only the ground chemistry determines what you see.

How do you test your soil acidity?

Before you start adjusting, you need to know what you have. Your soil pH determines everything.

Quick self-test: Go to the garden centre and buy a simple soil pH test kit (roughly 10-15 euros). These are usually small field kits with colour indicators. You add some soil sample, wait a couple of minutes, and compare the colour against the scale on the packet. This gives you a decent quick impression.

More accurate: Take a soil sample from 3-4 spots in your garden (mix them), put it in a plastic bag, and bring it to your local garden centre or agricultural quality lab. They do a professional analysis. This costs around 20-30 euros and gives you exact pH.

What the numbers mean:

• pH below 6.0: Very acidic. Blue blooms result.
• pH 6.0-6.5: Acidic. Blue to purple blooms.
• pH 6.5-7.0: Neutral. Purple to pink blooms.
• pH 7.0-7.5: Lightly alkaline. Pink blooms.
• pH above 7.5: Alkaline. Deep pink or red blooms.

How do you make your soil more acidic for blue hydrangeas?

Your hydrangea likes sour better than your wine. Fortunately, this is relatively easy to adjust.

Aluminium sulfate (most effective):

This is the classic remedy. Aluminium sulfate contains both the acid maker (sulfate) and the pigment (aluminium) that makes blooms blue. The dose is usually 60-120 grams per plant per season, depending on how alkaline your soil is.

Practice: Mix the sulfate in water (follow the label carefully) and pour it around the roots of the hydrangea in May-June, when the plant is growing. Repeat this every 2-3 weeks until late July. After three weeks you will see the blooms gradually turn blue. This is not immediate; it takes several weeks and sometimes until the next season before you see the full effect.

Warning: Make sure you do not add too much. Overdose can scorch the roots. Always follow the label instructions.

Iron sulfate (weaker alternative):

This is less effective than aluminium sulfate, but still helps. Iron sulfate makes the soil more acidic, but does not contain the aluminium itself. Some soil types respond better to this than others.

Sulfur (slow but good):

You can also simply add elemental sulphur to your soil. This acidifies the soil gradually (it works through bacterial decomposition). This is slower (takes weeks to months) but very effective long term. Around 60-120 grams per square metre per season, worked into the top layer of soil.

How do you make your soil more alkaline for pink hydrangeas?

The reverse: you want pink blooms? Then you want more alkaline soil.

Garden lime (limestone powder):

This is the classic. Garden lime raises pH and makes soil more alkaline. Around 150-250 grams per square metre per season. Work this in March or October, at the end of one season or the start of a new one.

It works slowly. It takes several months before you see the full effect, usually not until the next blooming season.

Wood ash (also alkaline):

Actually wood ash is somewhat alkaline (due to its calcium-rich composition). This does not help directly, but does contribute to less acidic soil.

Why does this work better in some gardens than others?

Good question. The same plant in two different gardens can give very different blooms.

Garden soil varies: Much depends on your base soil. If your soil is very acidic (sandy soil in North Europe, for example), you need much effort to get pink blooms. Conversely, if you live on lime-rich soil (for example, in much of Belgium and South Netherlands), it is very difficult to get blue.

Rainwater: Rainwater is usually slightly acidic. If you water a lot with rainwater from a barrel, that also acidifies the soil around the plant.

Organic material: Lots of compost, leaf mould or peat (all acidic) gradually increase acidity. Conversely, old stone remnants, concrete mortar or other material can be alkaline.

When do you start adjusting?

This should happen before blooms appear (March-May). If your plant is already in bloom, it is better to wait until after flowering. The plant cannot change colour this season.

Timing:

• March: Start of growing season. Make your soil acidic or alkaline for the first time.
• May-June: Carry out all your adjustments while the plant is growing.
• July: Last chance; after July adjusting does not help this season.
• August-September: Stop adding. Let it work.

Can you change a plant's colour mid-bloom?

Unfortunately no. Blooms that are already coloured do not change further. The pigmentation is already set. You can only influence next season's blooms.

So this is really a preparation matter. You start now, and you see the result next summer.

Step-by-step

Step 1: Test your soil acidity

Buy a pH test kit and test your soil around the hydrangea. Note the pH.

Step 2: Decide your target colour

Do you want blue (pH below 6.0) or pink (pH above 7.0)?

Step 3: Add the right substance

For blue: Aluminium sulfate (60-120 grams per plant) in May-June, every 2-3 weeks. For pink: Garden lime (150-250 grams per square metre) in March or October.

Step 4: Repeat annually

This is not a one-time job. Your soil gradually becomes neutral again. Repeat the treatment every spring.

Frequently asked questions

Can I make my hydrangea any colour?

Almost. Some cultivars are "colour-resistant" and always give pink, regardless of soil. The famous "Nikko Blue" listens well to adjustments. Check the label or ask at the garden centre. Blue cultivars (e.g. "Endless Blue", "Ocean Blue") respond best to acidification. Pink cultivars ("Oakhill", "Pink Poodle") follow better toward red/pink if you add lime.

Does this work on white hydrangeas?

No. White hydrangeas (such as "Annabelle") do not change colour. This applies only to the large-flowered (macrophylla) types and a few others (e.g. serrata).

How long before I see results?

Minimum 2-3 weeks if you actively add (aluminium sulfate). Garden lime works slower; sometimes up to months. Next season you will see the big difference.

Can you overdose on aluminium sulfate?

Yes. Too much sulfate (more than 120 grams per plant per season) can scorch the roots and damage the plant. Always follow the label.

My hydrangea is not blooming - what helps?

This is something different from colour. Ensure your plant gets 3-4 hours of sun per day, stays well moist (not soggy), and is not pruned too hard. Also wait until year 2-3 after planting; young hydrangeas flower lightly.

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At [gardenworld.app](https://gardenworld.app) you can upload your front yard and see how your hydrangeas fit with your garden and other plants - with realistic bloom patterns. Plan and visualise before you add chemicals.