The Magic Behind Hydrangea Colors

 

Did you know?

The color of hydrangea isn't just genetics — it's the soil they grow in.

Specifically, soil pH affects the availability of aluminum, which in turn influences flower color. pic.twitter.com/OgYRPrshLL

— Massimo (@Rainmaker1973) April 7, 2026

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How it works

The flower color depends on soil pH and aluminum availability:

• Acidic soil (low pH, below ~6)
  → Aluminum becomes available to the plant
  → Flowers turn blue
• Neutral soil (around pH 6–7)
  → Mixed availability
  → Flowers may be purple or bluish-pink
• Alkaline soil (high pH, above ~7)
  → Aluminum is “locked away” and unavailable
  → Flowers turn pink

Why aluminum matters

It’s not just the pH alone — the key is a soil chemistry interaction:

• In acidic conditions, aluminum dissolves and is absorbed by the plant
• Inside the petals, aluminum reacts with pigments, producing blue tones
• Without aluminum, the natural pigment appears pink

Important note

• This effect mainly happens in bigleaf hydrangeas (Hydrangea macrophylla)
• White hydrangeas do not change color
• Some varieties are bred to stay pink or blue regardless of soil

A fun gardening trick

Gardeners sometimes:

• Add coffee grounds or sulfur → to make soil more acidic (bluer flowers)
• Add lime → to make soil more alkaline (pinker flowers)