I just purchased a beautiful dark purple hydrangea with a label from your farms. When I transplant it outdoors in a container (after the bloom has faded), what should I add to the soil to keep the dark purple color?
The color of hydrangeas is dependent on the pH of the soil. The more acidic the soil is (below 6 pH), the bluer the flowers will be. Soil that is more basic (above 6 pH) with have pink flowers. Since your plant has dark purple flowers, its color will stay dark no matter what the pH is. When the soil is more acidic for the darker flower varieties, the flowers will be dark purple. This means that to keep the beautiful purple color, your soil should be more acidic. A basic soil will cause your flowers to be red or dark pink.
It is easier to alter the pH of soil in a pot than it is in the ground. Get some pH testing strips and find out what pH you are growing in. You should aim for a pH of around 5.2-5.5. If you need to lower your pH, you should add aluminum sulfate to the soil. A table spoon of aluminum sulfate per one gallon of water is an acceptable rate to use. Apply it every so often throughout the growing cycle. It will help if you check the pH multiple times to make sure it is where you need it.