Expert: C.L. Fornari - 8/1/2010

Question
One year ago, I planted a pink hydrangea.  This year the plant has huge royal blue flowers.  It is beautiful and the talk of my street.  How did this happen?  The only food I've given it was some miracle grow in the Spring.

Answer
Mary,
This type of hydrangea changes from pink to blue because of the pH of the soil.  You can alter the color of many,
but not all, of the pink and blue flowering types. In acid soils the plants can absorb aluminum from the soil and the flowers will be blue, in alkaline soils they won't absorb the aluminum and they will be pink. Light blue can be changed to light pink but not to dark blue or deep purple. Dark pink will turn dark blue to purple in acid soils.

Hydrangeas are often grown in neutral or alkaline soils so are pink when you buy them.  Where you live there must be naturally acid soils, so they darkened to the blue/purple range.

White (H. aborescens and H. paniculata) will always be white. Contrary to old gardeners tales, color has nothing to do with planting nickels, rusty nails or pennies. If you want to play with hydrangea color, use sulfur or Aluminum Sulfate to acidify soils but only according to directions.  Put lime around the plant 2x a year to make soil alkaline and turn the flowers pink again.

I hope that helps,
C.L.