Growing Hydrangeas in Compost And How it Changes Their Flower Color

June 28, 2017 2 minute read

Soil³ humus compost is not only for vegetables and lawns, woody plants like hydrangeas thrive in it too.

Hydrangeas like the rich, moist soil that Soil³ compost creates, but do you know that the flower color is also influenced by the change in pH that happens when compost is added? In our southern clay soils, compost raises acidic pH into a safe zone for plants and for bigleaf hydrangeas (Hydrangea macrophylla) it also changes the flower color.

Change Bigleaf Hydrangea Flower Color by Adding Compost to Your Soil

The flower color of bigleaf hydrangeas change with soil pH. A low (acidic) pH produces bluer flowers while a high (basic or alkaline) pH produces pinker flowers. A neutral pH produces purple flowers.

Composted material, including Soil³, is neutral in pH.

You may be wondering, "how will the neutral pH of Soil3 compost change the pH of my soil and my bigleaf hydrangea flower color?" 

Acidic Soils

In acid soils, Soil³ compost will neutralize soil so that if you have a highly acidic soil and blue hydrangea flowers, the pH will be raised and the flower color will be influenced toward purple (or bluish purple).

To turn them pink the soil pH must be raised a lot more than compost will raise it; it would take the addition of lime, which is an alkaline additive.

Sometimes a newly planted hydrangea, planted in Soil³, will be pink until the native, acidic clay wins out as the compost decomposes. Keep adding compost and lime to keep them pink.

Alkaline/Basic Soils

In alkaline soil, Soil³ will again neutralize soil, but this time it will lower the pH so that pink flowers colors will again be influenced toward purple (or pinkish purple).

To turn them truly blue, aluminum sulfate must be added - but go easy on how much you add - aluminum sulfate can burn plants.

In clay soils in the Southeast there is seldom a need to lower the pH because our clay is naturally so acidic. 

Not All Hydrangeas Change Color Based on Soil pH

Some species of hydrangea don't change color based on pH. Here's a list of common hydrangeas in the Southeast, group by pH sensitivity.

Hydrangeas That Change Color Based on pH
  • Hydrangea macrophylla - bigleaf hydrangea
  • Hydrangea serrata - mountain hydrangea
Hydrangeas That Don't Change Color Based on pH
  • Hydrangea arborescens - native or wild hydrangea
  • Hydrangea quercifolia - oakleaf hydrangea
  • Hydrangea paniculata - panicle hydrangea

Add Compost for the Heath of Your Hydrangeas

We use the word "influence" because soil pH adjustments to alter bigleaf hydrangea color is more often done with acids or lime, rather than compost. The effects of working compost into your planting bed are gentler and healthier than aggressive pH adjustments using strong additives.

Plus, hydrangeas love the moist, rich soil that compost creates. You've noticed that hydrangeas like a lot of water every week, yes? Soil³ helps your tough clay soil retain the moisture that hydrangeas need.

Most gardeners enjoy both blue and pink bigleaf hydrangeas and accept whichever color the soil pH gives us. The first goal is health of your hydrangeas. By adding compost, the soil that sustains your beloved hydrangea shrubs is improved and nurtured so it can bolster the healthiest hydrangeas you've ever grown. 

Read how Dörte Schmieta incorporates Soil³ into her hydrangea garden in the Soil³ Story: Creating Hydrangea Heaven with Soil³ Compost.

A customer near our Orangeburg, South Carolina, farm sent the office ladies these pictures of his beloved hydrangea flowers he grew in a garden bed amended with Soil³. You can see how Soil³ has turned some of them purplish because it neutralized the soil. He is right to be proud of his Hydrangea macrophylla (bigleaf hydrangeas) and we thank him for sharing his joy in his plants and our compost.

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Did this help you out? Have any questions for clarity? Leave a comment below!