The Colorful Chemistry of Hydrangeas: Why Your Soil Might Be the Artist Behind the Blooms

A gardener’s guide to painting with pH

Hydrangeas are the mood rings of the garden world. These blousy, romantic blooms seem like something out of a fairytale — delicate, showy, and somehow a little mysterious. But there’s a not-so-secret trick behind their beauty: it all comes down to your soil.

That’s right. Hydrangeas — particularly the bigleaf variety (Hydrangea macrophylla) — can actually change color based on the pH level of the soil they're growing in. Acidic soil tends to produce blue flowers, while alkaline soil pushes the blooms toward pink. In between, you might get a gorgeous muddled purple.

Unless you're dealing with white hydrangeas. Those rebels keep their crisp bridal white blooms no matter how much soil amending you do. They’re beautiful too, just not as emotionally reactive.

The Science of Soil pH (And Why It Matters)

Let’s break it down without getting too classroom-y.

Soil pH is a measure of how acidic or alkaline your soil is. It’s measured on a scale from 0 to 14:

7 is neutral

Below 7 is acidic

Above 7 is alkaline

For hydrangeas, the pH level changes how available aluminum is in the soil. Aluminum is what influences the color shift.

In acidic soils (pH 5.5 or lower), aluminum becomes more soluble, and hydrangea blooms turn blue.

In neutral to alkaline soils (pH 6.5 and higher), aluminum gets locked away, and blooms turn pink.

In that sweet middle ground (roughly pH 5.5–6.5), you often get those swoon-worthy lavender or purple shades.

It’s not magic — it’s science. But it feels like magic when it works.

Hydrangeas and My Azorean Roots

My family came from the Azores, those lush volcanic islands off the coast of Portugal, and let me tell you — they do hydrangeas right. Whole hillsides are blanketed in billowing clouds of blue, thanks to that mineral-rich volcanic soil. The naturally acidic conditions caused by volcanic ash mean blue hydrangeas are just... normal over there.

As a kid, I remember seeing photos of the island roads lined with sapphire-colored blooms taller than a person. It wasn’t something you planted for curb appeal. It was just nature doing her thing.

So it’s no surprise that I’ve got a soft spot for hydrangeas in every shade — but especially those rich, oceanic blues that remind me of where my people came from.

How to Adjust Your Soil to Change Hydrangea Color

If you're feeling experimental and want to play garden alchemist, here’s how to nudge your blooms in the direction you want:

To turn your hydrangeas blue:

Add aluminum sulfate or sulfur to the soil.

Mulch with pine needles or oak leaves (which increase acidity).

Use acidic fertilizers, like those designed for azaleas or camellias.

To turn your hydrangeas pink:

Add lime (like garden lime or dolomitic lime) to raise pH.

Avoid aluminum-based fertilizers.

Use compost that leans more neutral, like well-rotted manure.

Just remember: color changes don’t happen overnight. It may take a full growing season (or two) for the shift to show up.

Planting Tips from a Busy Mom Who’s Definitely Got Dirt Under Her Nails

As an office manager, homeschooling mom, and backyard hobbyist who still somehow finds time to deadhead blooms while sending invoices, I promise you — hydrangeas are low-drama plants once you get their pH preferences dialed in.

Here are a few quick tips to keep them happy:

Morning sun, afternoon shade is ideal, especially in hot climates.

Don’t over-prune. Some types (like macrophylla) bloom on old wood, so cut back too much and you'll miss the flower show.

Water deeply — especially in their first couple of years. Shallow watering won’t cut it.

And don’t be afraid to plant a few and see what color they want to be. Sometimes the best plan is to let the soil — and your garden — speak for itself.

Hydrangeas Are Storytellers

At the end of the day, I love hydrangeas not just for their beauty, but because they carry stories. Of the Azorean islands. Of my grandmother’s garden. Of backyard summers filled with pool mermaids and lemonade.

Whether you’re a landscaping professional looking to impress a client, or just a tired mom sneaking a moment of peace among the flowers, these moody blossoms have a way of making you feel like you're home.

So grab a trowel, test your soil, and let your hydrangeas show their true colors.