How to change colors of hydrangea blooms

There's something magical or at least grade school science-fairish about a plant that grows pink flowers one year and blue the next.

Only the mopheads, a type of hydrangea macrophylla, change their blooms from pink to blue or vice versa.

It is much easier to do this if you grow your hydrangeas in containers, said Judith King, a hydrangea lover who worked for a nursery and maintains a Web site devoted to the shrubs.

It's also considerably easier to change a hydrangea from pink to blue than it is from blue to pink, she notes on www.hydrangeashydrangeas.com.

"Changing a hydrangea from pink to blue entails adding aluminum to the soil. Changing from blue to pink means subtracting aluminum from the soil or taking it out of reach of the hydrangea."

To make your flowers pink, she recommends adding dolomitic lime several times a year to raise the pH of the soil.

To turn the flowers blue, add a little aluminum sulfate in water and lower the pH of the soil with organic compost.

Don't do this until your plants are at least 2 or 3 years old.

If you want pink flowers your soil should have a pH of about 6 or 6.2, which is alkaline, said Danielle Ernest, public relations and marketing assistant for Spring Meadows Nurseries in Grand Haven, Mich.

But sometimes your flowers just change because of an environmental change such as being transplanted.