How to keep hydrangeas blue: soil and pruning combination

Why do blue hydrangeas turn pink?

This is the question that frustrates many hydrangea lovers. You buy a lovely blue hydrangea (for example 'Nikko Blue'), plant it in your garden, and two years later the flowers are pink. Not frost damage, not disease - just pink.

This happens because hydrangeas use their color to express their health. The pigments (anthocyanins) that make hydrangeas blue appear only in acidic soil with plenty of available aluminium. In neutral or alkaline soil (higher pH) the plant cannot take up the aluminium, and the blue color vanishes. Instead, the flowers turn pink.

This is completely natural and not unhealthy. Your hydrangea still blooms beautifully. But if you want to keep it blue, you must maintain acidic soil AND adjust pruning accordingly.

Soil pH and blue color

pH below 5.5: Very acidic. Hydrangeas become deep blue (almost purple in extreme acidity).

pH 5.5-6.0: Acidic. Hydrangeas become blue (lovely sky-blue to navy).

pH 6.0-6.5: Weakly acidic. Hydrangeas become purple-pink mixed (in-between tone).

pH above 6.5: Neutral to alkaline. Hydrangeas become pink (fuchsia in very alkaline soil).

Your hydrangea probably sits in pH 6.0-6.5 or higher. That is why it is pink.

How do you test soil pH?

Cheap: Buy pH test kit from garden center (about 5-10 euros). Follow instructions: collect soil, add water, compare color. Not super accurate, but gives direction.

Better: Buy pH meter (digital, about 15-25 euros). Measuring moist soil gives accurate reading.

Best: Send soil sample to university or horticultural research station (sometimes free, sometimes 10-20 euros). They measure pH plus nutrient content, giving more information.

Measurement tip: acidity varies by location in garden. Test multiple spots around your hydrangea, not just one.

Making soil more acidic: practical steps

Option 1: Add acid (most direct)

The quickest way to make soil more acidic is to add aluminium sulphate (also called "garden alum").

• Recommendation: 30-50 grams per liter of water, give around your hydrangea
• Timing: May-June, repeatedly (every 2-3 weeks) until bloom
• Amount: initially 1-2 times per season, then 1x per season to maintain pH
• Store portions dry (keeps for years)

You will see result in same bloom (color deeper, more intense blue) if you start early enough (May).

Option 2: Add acidic soil (gradually)

For larger hydrangeas or permanent solution:

• Buy "bluing soil" or "acidic garden soil" (garden centers, about 20-40 litre bags)
• Gently dig 10-15 cm of soil away around your hydrangea
• Replace with acidic soil
• Repeat next year
• Gradually your soil becomes more acidic

This works slower (1-2 years) but is more permanent.

Option 3: Mulch with acidic material

• Cover soil around hydrangea with 5-10 cm of peat moss or acidic leaves (oak, beech)
• This slowly releases acidity and helps against drying
• Effect subtle, but helps long-term

Pruning and blue color: what is the link?

Soil pH determines color, BUT pruning influences the INTENSITY of color. How?

Hard pruning = more intense blue When you cut hard (for example Paniculata in March to 30-40 cm), you stimulate lots of new, young growth with many leaves. These new shoots produce more anthocyanins (blue pigments) because they are "hungry" for energy. Flowers on young wood are usually more intensely colored.

Light pruning = subtler blue When you prune minimally (Macrophylla, dead wood only), you mostly grow on old wood. Old flowers are sometimes less intense in color because old wood has less vigor.

This works especially for Paniculata:

• Cut hard in March (to 30-40 cm)
• Give acidic soil or sulphate
• Young shoots plus acidic soil = more intense, vivid blue

For Macrophylla the pruning effect is subtle (because you do not cut hard). Soil pH dominates.

Feeding strategy

Nitrogen strengthens green leaves and weakens blue color. Phosphate and potassium (potash) encourage flowers and color.

Feeding schedule for blue:

• May-June: Bloom feed (low nitrogen, high phosphate/potassium)
• July-August: Continue bloom feed
• September: Stop feeding (plant enters rest, no more need)

Avoid:

• Lawn feed (high nitrogen stimulates foliage, not flowers)
• Compost in summer (energy boost helps growth, not flowers)

Use specialized hydrangea bloom feed if available (garden centers sell this). They are formulated for color enhancement.

Combination strategy: soil plus pruning plus feeding

This is how you keep hydrangeas blue and intense:

March:

• Check soil pH (test kit or meter)
• Prune Paniculata hard (to 30-40 cm), Macrophylla minimally
• Check for dead wood

April-May:

• Start acidic feeding (aluminium sulphate or bluing feed)
• Give first dose
• Repeat every 2-3 weeks until bloom appears

June-August:

• Bloom appears
• Watch color: becoming blue? Good. Still pink/purple? More acidity needed.
• Supplementary alum/feed if needed
• Repeat until end August

September-October:

• Stop acidic feeding
• Plant enters rest
• Color is now set for this season

Next year:

• Repeat process
• Results accumulate: years of acid management make soil permanently more acidic

Frequently asked questions

Can I turn my pink hydrangea back blue?

Yes, but slowly. Add acidic soil and give aluminium sulphate. First year sometimes shows some blue, but complete transformation (really deep blue) usually takes 2-3 years. Patience.

How long does aluminium sulphate effect last?

Effect lasts about 1 season. Repeating each season needed to keep pH low. If you stop alum, soil will slowly become neutral again (especially in rainy regions where acid washes out).

My hydrangea is half blue, half pink. What now?

This happens if soil is unevenly acidic (one side more acidic than other). Or you have two different cultivars that handle color differently.

Option A: Add acidic soil evenly around whole plant (all sides) Option B: Accept that you have a "pink-blue" hydrangea (some find this beautiful!)

Why is my blue hydrangea suddenly green?

Not normal. This suggests nutrient deficiency (often iron-chlorosis). Give IRON feed, not aluminium sulphate. Iron deficiency makes leaves yellow with green veins. Acidic soil helps iron uptake, so this often fixes itself if you acidify.

Can pruning alone make my hydrangea blue?

No. Pruning affects color intensity, not base color. You must keep soil acidic. Pruning helps only if soil is already acidic, then color becomes more vivid.

Step-by-step

Step 1: Test soil pH

Buy pH test kit or meter. Measure around your hydrangea. Note result.

Step 2: pH lower than 6.0?

Good, your soil is acidic enough. Continue to step 5.

Step 3: pH 6.0-6.5 or higher?

Your soil is not acidic enough for blue. Start acidifying strategy.

Step 4: Start acidifying (May)

Give aluminium sulphate (alum) or bluing soil. Repeat every 2-3 weeks until end August.

Step 5: Adjust pruning

Paniculata: cut hard in March (30-40 cm). Macrophylla: prune minimally, leave old wood.

Step 6: Monitor

Watch color development in May-June bloom. Becoming blue? Perfect. Still pink? More alum or patience until next year.

Cultivars for reliably blue

Very reliably blue (stay blue even in weakly acidic soil):

• Nikko Blue (classic, very stable)
• Générale Vicomtesse de Vibraye (robust blue)
• Ayesha (dark purple-blue)

Risky (turn pink more easily if pH not perfect):

• Forever Pink (can paradoxically be pink anyway... despite name)
• Lancelot (unstable, does both colors)

Different type (no color change, always white):

• Paniculata (white, no pH effect)
• Quercifolia (white, no pH effect)

Want blue guarantee without fuss? Choose 'Nikko Blue' - it stays blue longest.

Frequently asked questions

Is it safe to give aluminium sulphate?

Yes. Aluminium sulphate is safe in garden dosages. Many professional blue flower lines use it. As long as you follow instructions (do not overdose), no risk.

Can I use bluing soil instead of alum?

Yes. Bluing soil usually contains alum plus extra nutrition. It works the same, just more expensive. Advantage: you do not need to dose repeatedly (soil absorbs slowly). Disadvantage: cost.

My neighbor has the same hydrangea, but his is blue and mine is pink. Why?

Soil! His garden probably sits on more acidic soil (acidic sand, low-lying, lots of leaf humus). Your garden on more alkaline soil (lots of lime, elevated, clean sand). This is one of the most common reasons for color difference.

Discover your own garden design

At [gardenworld.app](https://gardenworld.app) you can upload your front yard and see how blue hydrangeas fit in your garden plan. Plant them in groups for color saturation and ensure soil is acidic beforehand.