Except for the white varieties, the color of the flowers is affected by the amount of aluminum in the soil available to the plant. Aluminum is present in all soils, but its availability to the plant depends on the acidity of the soil. Big-leaf hydrangeas require acid soil -- pH 6.0 to 6.5 for pink flowers and a pH of 5.0 to 5.5 for blue ones. Some big-leaf hydrangeas switch from pink to blue flowers or vice-versa with a change of only 1/2 point on their soil's pH scale.
Plants can be made or kept blue by applying aluminum sulfate to the soil. Treat the soil several times in the spring and fall at weekly intervals with 1/4 ounce of aluminum sulfate to a gallon of water. To obtain or keep pink flowers, add lime to the soil. You can also use fertilizers with a high percentage of phosphorus, which reacts with the aluminum contained in the soil, and the two elements combine in an insoluble form; thus, the aluminum is neutralized and the flowers remain, or become, pink. These treatments will likely not be effective unless they are started before the plant blooms.
Big-leaf hydrangeas make wonderful dried flower displays. Drying them slowly is the key. Once the flower clusters have turned the muted shades of fall, cut them and clip off any leaves or browned flowers. Smash the stems with a hammer to help them absorb water, and place them in a container with 2 inches of water. Leave the container in a cool spot out of the sun. The water will evaporate in a week or so and you will have an attractive bouquet of hydrangeas to enjoy throughout the winter.
This article is part of the Suite101 "Container Gardening" topic area. For the other Suite101 articles and discussion forums on Container Gardening, see the Welcome Page on Suite101.
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