Hydrangeas are sort of the mood ring of the garden landscape. And their mood (or color) is determined by the soil they grow in. The flowers get their color from the pH in the soil. By adjusting the pH, you can turn pink flowers into blue (or blue flowers into pink). Here's what you need to know to change the bloom colors of your hydrangea.
Only the flowers of bigleaf (Hydrangea macrophylla), also known as mopheads and lacecaps, can change color. Other types such as oakleaf hydrangeas or hydrangeas 'Annabelle' only bloom in white or cream.
Look for hydrangeas that are bred to be blue such as 'Nikko Blue', Endless Summer The Original, 'Blue Danube', 'Penny Mac', 'Blauer Prinz', or Endless Summer Twist-n-Shout. The photo on the plant tag will help you choose blue-blooming varieties if the plants are not in flower at the nursery.
Although doing this may sound like complicated chemistry, it's not. All soil has a pH value which measures the acidity or alkalinity. The pH scale is from 0 to 14; 7 is neutral. Soil pH values of less than 7 indicate degrees of acidity. Soil pH values greater than 7 indicate degrees of alkalinity. Use a soil test kit to determine the current pH levels of your soil.
For true blue flowers, the hydrangeas need to be grown in acidic soil (pH 5.5 and lower). For pink flowers, the plants need neutral to alkaline soils (pH 6.5 and higher). For purple blooms (or a mix of blue and pink flowers on the same plant), the pH of the soil must be 5.5 and pH 6.5.
When you get the soil pH result, it will be obvious what you need to do next: to get blue blooms, you need to amp up the soil's acidity. You can do that in a variety of ways. Organic acidifiers include sulfur and sulfate soil additives. There are also easy-to-use soil additives made specifically for hydrangeas. Bailey's Color Me Blue (soil sulfur) or Bailey's Color Me Pink (garden lime) change the pH of the soil so you can enjoy the hydrangea bloom color you want. These all-natural products make the soil more acidic (for blue blooms) or alkaline (for pink blooms). Add the pelleted mixture to the soil when you plant your hydrangea.
Soil pH needs to be regulated to produce consistently blue blooms. Work pH soil additives into the top layer of the soil around the root zone to help keep the flowers the color you want.