Growing Blue Hydrangeas: How to Change and Control Flower Color

Hydrangeas in blue and pink: Endless Summer (left) and Twist-n-Shout. Photo: David Grist

Soil Acidifier enhances blue color in hydrangeas.

You may have heard you can change hydrangea flower color by adjusting soil pH—but there's more to it.

First, not all hydrangeas produce blue flowers. White/cream varieties like Annabelle, oakleaf, and PeeGee family hydrangeas only produce white/cream blooms (sometimes with a pink tinge late season).

Hydrangeas with bloom colors ranging from pink through blue and purple are typically mophead and lacecap cultivars. These change color based on soil chemistry: alkaline soil = pinker blooms; acidic soil = bluer blooms.

Because soil chemistry determines color, variety names mean little for bloom color. 'Nikko Blue', 'Pretty in Pink', 'Forever Pink', and 'Blue Deckle' all have nearly equal chance of blooming pink or blue depending on soil.

Even buying a hydrangea in bloom doesn't guarantee the same color in your garden.

To manipulate bloom color, you must manipulate soil pH and mineral content—repeatedly. Maintaining specific conditions may require applying soil amendments several times per growing season (see Adjusting the pH of Your Soil below).

Blushing Bride produces blooms throughout the season on new and old stems. Pure white flowers up to 8" across blush pink or deep blue as they mature, depending on soil pH.

For serious color experimentation, grow hydrangeas in large pots. Smaller soil volume makes it easier to maintain desired chemistry.

Start by testing soil pH to gauge the challenge. Plant health should be the first priority—dramatic pH changes can harm the plant.

For blue flowers: Grow in soil pH 5.2-5.5. If soil is more alkaline, lower pH with Soil Acidifier at package rates. Acidic organic mulches (pine needles, pine bark) lower pH more gradually.

Naturally high (alkaline) pH makes blue flowers very difficult—alkaline soil "locks up" aluminum, making it unavailable. (But you can grow fabulous pink hydrangeas!)

For pink flowers: Deprive plant of aluminum by growing in alkaline soil pH 6.0-6.2. High-phosphorus fertilizer further discourages aluminum uptake. Raise pH of acidic soil with Garden Lime at package rates.

Adjusting the pH of Your Soil

Blue Billow, a lacecap-style hydrangea. Photo: David Grist

Test soil pH where you'll plant using a kit, meter, or local extension service. Test annually if maintaining specific pH. Effects of amendments aren't immediate; pH reverts to native levels over time.

To Raise Soil pH (More Alkaline)

Most common: add powdered Garden Lime. Dolomitic limestone adds magnesium. Apply in fall—takes months to work.

Wood ash works faster than limestone, adding potassium and trace elements. Use caution: very concentrated, can drastically alter pH and cause nutrient imbalances. Can "burn" foliage. Apply directly to soil in winter, max 2 lbs per 100 sq ft every 2-3 years. Test annually.

To raise pH ~1 point:

• Sandy soil: 3-4 lbs ground limestone per 100 sq ft
• Loam: 7-8 lbs per 100 sq ft
• Heavy clay: 8-10 lbs per 100 sq ft

To Lower Soil pH (More Acidic)

Add Soil Acidifier, ammonium sulfate, or aluminum sulfate per package rates. Incorporate naturally acidic organics: conifer needles, sawdust, peat moss, oak leaves. Coffee grounds are slightly acidic. Retest to monitor effects.

To lower pH ~1 point using elemental sulfur:

• Sandy soil: 1 lb ground sulfur per 100 sq ft
• Loam: 1.5-2 lbs per 100 sq ft
• Heavy clay: 2 lbs per 100 sq ft