Hydrangeas are those large, bush-type perennials with the giant flower heads in either white, pink, or blue. They’re also called “mophead” or “lacecap.”
Fossil records show that hydrangeas grew in North America between 40 and 70 million years ago, and about 25 million years ago in Asia.
They bloom in mid to late summer and keep their flowers long after their leaves drop off in fall. The flowers look spectacular on the plants for winter interest in the garden, or cut and dried for indoor flower arrangements.
Plant hydrangeas in full sun to partial shade in moist, rich, loamy soil that is well drained. Add generous amounts of compost when transplanting and top dress with compost every spring.
The name “hydrangea” comes from the Greek words hydro (water) and angeion (vase) or “water vase.” The name doesn’t refer to the flowers; it refers to the shape of the seed capsule.
Hydrangeas are unusual in that you can change the color of their flowers by changing the pH of the soil in which they grow.
To make hydrangeas flowers pink like the ones in the photo above, raise the pH of their soil. Do this by adding dolomitic lime to the soil several times a year (available at garden supply stores). Follow the manufacturer’s instructions for correct quantities and add that amount to the soil around each plant in spring, summer and fall.
Another thing to do to keep your hydrangeas blooming pink is to use a fertilizer that is high is phosphorus (the second number on the fertilizer label—for example 10-15-10).
If blue hydrangeas are what you are after, add aluminum sulfate (a soil additive available at garden supply stores) to the soil in which they are growing. Mix 1 tablespoon aluminum sulfate with one gallon of water and apply a half gallon of this mixture to the soil around each plant. Caution: water your hydrangeas well the day before you do this so the roots can more easily take up the aluminum sulfate without getting burnt. Apply the aluminum sulfate mixture to the soil in spring, summer and fall.
To help keep your hydrangeas blooming blue, use a fertilizer that is low in phosphorus (the second number on the fertilizer label) and high in potassium (the third number on the fertilizer label). For example, use a fertilizer labeled 25-5-30. Avoid using superphosphates or bone meal if trying for blue hydrangeas.
If your best efforts to turn hydrangeas either pink or blue result in them turning the opposite color, the culprit is most likely your water. Water that has a high pH will tend to produce pink hydrangeas; conversely, water with a lower pH will tend to produce blue hydrangeas—both in spite of soil amendments to the contrary.
Finally, hydrangeas planted near a concrete foundation or walk will tend to bloom pink because of the considerable amount of lime leaching out of the concrete, which raises the pH of the soil.
One last note on changing the color of hydrangeas: white hydrangeas will always be white and cannot be changed to pink or blue.