How to Force Hydrangeas

Provide hydrangeas with more sunlight if they receive too much shade to help force them to bloom. Hydrangeas ideally should receive at least four to six hours of sunlight a day to bloom well. Trim nearby trees and shrubs to help increase sunlight, if possible.

Fertilize hydrangeas once a year in the spring with a balanced fertilizer, such as 8-8-8, applying a total of 1 pound for every 50 square feet. In addition, water hydrangeas at least once a week during the hot summer months or during drought-like conditions to prevent the shrubs from drying out. These cultural practices should encourage hydrangeas to bloom.

Prune hydrangeas at the right time; otherwise, you may inadvertently cut off the flower buds. Prune bigleaf and oakleaf hydrangeas, for example, after they flower in the spring or early summer because these types of hydrangeas bloom on old wood. On the other hand, prune smooth and panicle hydrangeas, for example, in the spring before new growth because they bloom on new wood.

Change the pH of the soil to force hydrangeas to bloom a different color. Test your soil’s acidity using a home pH testing kit, or send a sample to a laboratory for testing. Raise the pH with lime, or lower it with aluminum sulfate; use it per manufacturer's label to achieve the desired acidic levels. In general, hydrangeas grown in soil with a pH level between 5.0 and 5.5 bloom blue. Hydrangeas grown in soil with a pH level between 6.5 to 7.0 bloom pink. Hydrangeas grown in soil with a pH level between 5.5 and 6.5 bloom purple. Note that white cultivars will usually boom white, regardless of the soil's acidity.