Remember litmus paper? If you dip it in a liquid and the paper turns blue, the liquid is an acid. If the paper turns pink, the liquid is alkaline. Well, hydrangeas are a lot like litmus paper. Their blooms are blue in acidic soil and pink in alkaline soil.
The fun part about this trait is that you can, to some extent, change the color of the blooms by changing the pH of the soil around the plant. This allows you to alternate blue and pink flowering plants for a striking display.
Hydrangeas grow best with morning sun and afternoon shade. They need moist but not wet soil. Hydrangeas have large leaves and will wilt if the soil dries out because they lose lots of moisture through the pores on the leaves.
Pruning hydrangeas can be tricky. They bloom on old wood. The buds for next year’s flowers grow in late summer after the blooms are finished for the current year. Pruning must be done right after the blooms for the year die. Any later, and you are actually cutting off next year’s flowers.
In soil with a neutral pH, hydrangeas are a lavender color. If the soil is acidic, they bloom blue. You can acidify the soil right around the hydrangeas by using aluminum sulfate. Dissolve one pound of aluminum sulfate in five gallons of water. Water the plant with one gallon of the solution every two weeks from when the plant starts growing in the spring until it blooms. Many garden stores carry aluminum sulfate.
There is not an easy way to make the soil more alkaline so the blooms turn pink. You can add agricultural lime, but it may work rather slowly than the aluminum sulfate. It is possible, however, to do this over time and make the soil where the hydrangeas are more alkaline.
One caveat is that the blooms of hydrangeas with white flowers cannot be manipulated to turn a different color. They only bloom in white. If you want to be able to manipulate the color of the flowers, choose a hydrangea with a blue, lavender, or pink blossom.
