‘Endless Summer’ Hydrangea

Bigleaf Hydrangea at pH 6.4 or higher

Bigleaf hydrangeas (Hydrangea macrophylla), aka “hortensia” hydrangeas, bear pink flowers in low acidic and alkaline soil and blue flowers in mildly acidic soil. Some tips about regulating soil pH and flower color in bigleaf hydrangeas follows.

First, a short review about soil pH. Soil pH measures soil acidity/alkalinity on a scale of 0-14, with 7.0 being neutral. Numbers above 7.0 indicate alkaline soil while numbers below 7.0 indicate acidic soil. Bigleaf hydrangeas flower blue if the pH is between 5.0 and 6.2. Flowers are pink if soil pH rises to 6.4 and higher.

Flower color is an expression of the amount of aluminum ions (Al+++) in the soil. Aluminum is most available to the plant in acidic soil, and less available in alkaline soil. Gardeners typically purchase aluminum sulfate to lower soil pH. Unfortunately, aluminum toxicity to plants is a big concern because hydrangeas need only small amounts of Al+++.

Garden and shrub beds near cement or concrete foundation walls, sidewalks and driveways, often exhibit low acidic or slightly alkaline soil pH. Over many years limestone leaches from the cement or concrete. While limestone is ideal for most vegetables, flowers or lawn grasses, bigleaf hydrangeas will bloom pink instead of blue.

Pink Flowered Hydrangeas: Gradually add hydrated or dolomitic lime to raise soil pH above 6.2. Be careful on the amount of lime that you add. Above pH 6.4, the quantity of available iron in the soil may become deficient or unavailable to the plant. The hydrangeas take up aluminum best at lower (more acid) pH levels. By raising soil pH, Al+++ is less available to the plant.

Blue Flowered Hydrangeas: For bigleaf hydrangea to flower blue, add aluminum sulfate to garden soil or garden container. Distribute even amounts of aluminum sulfate into soil around the drip line of each shrub. Use of pine needle or bark mulch aids to keep soil slightly more acidic.

Directions: Apply 1/2 oz (1 Tbsp) aluminum sulfate per gallon of water be applied to plants monthly. Irrigate plants immediately following application to prevent burning roots.

Caution: using high amounts of phosphorus fertilizer may result in pink flowered hydrangeas. Soils low in phosphorus and high in potassium bloom more.