How to Keep Hydrangeas Blue

There are about 23 species of hydrangeas, but only five are cultivated in the United States. What makes the hydrangea unique is its huge ball shaped flower clusters that range from white to pink to shades of blue. Depending on the variety of hydrangea, they are hardy from zone four to zones 6 to 9 (see USDA map in Resource Section). In many varieties of hydrangea the flower color can be changed to blue ( or kept blue) by manipulation of the soil Ph and additions to the soil.

Things You'll Need

Ph test kit
Organic matter, such as coffee grounds and pine needles

Aluminum sulfate
One gallon bucket

Instructions

- 1
  Test the Ph level of the soil surrounding your blue hydrangea, using a Ph test kit (available at hardware and garden stores). While kits will vary by manufacturer, a typical test will entail placing a sample of soil in the test chamber and then adding water and the test strip. The strip will change colors according to the Ph balance of the soil; for blue hydrangeas to absorb aluminum and maintain their color the optimum Ph needs to be at a level of 5.2 to 5.5.
- 2
  Lower the Ph level of your soil (if needed) by adding organic matter such as coffee grounds and pine tree needles. This will allow the hydrangea to absorb the needed aluminum more readily.
- 3
  Water the hydrangea plants thoroughly to prevent damage to the root system from the aluminum adding process.
- 4
  Mix one tablespoon aluminum sulfate into one gallon of water and apply to the soil area surrounding your blue hydrangea. Avoid pouring the solution directly onto the leaves and stems of the plant.
- 5
  Repeat the aluminum sulfate application every three weeks for three applications until the blue hydrangea has completed its blooming season.

What Grows with Hydrangeas?

Shrubs That Rabbits Eat

Landscaping Shrubs