Hydrangeas (Hydrangea spp.), produce showy, billowing blossoms that range in color from white, pink and blue to lavender and rose. Hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on the species and variety, hydrangeas usually bloom during summer and fall. Manipulating certain growing conditions can force hydrangeas to blossom early, but in only some instances. In general, poor plant care causes late and/or low flower production.
Shocking the Plants
Stabbing the roots of established inground, outdoor hydrangeas shocks the plants into blossoming. Drive a straightedge blade into the ground at the edge of each plant's foliage, doing so in six places that are evenly spaced around each plant. This technique kick-starts the plants' reproductive cycle, causing them to produce buds and blooms. The method can work for hydrangeas growing in containers as well, but be careful not to push the straightedge blade too deeply because the roots of potted hydrangeas are less hardy than those of inground hydrangeas.
Warming the Plants
A sudden change in temperature can force indoor bigleaf hydrangeas (Hydrangea macrophylla) into blossoming because they are very temperature-sensitive. Bigleaf hydrangeas include the cultivar "Nikko Blue" (Hydrangea macrophylla "Nikko Blue" USDA zones 6 through 9). Move indoor bigleaf hydrangeas to a warm location, such as a greenhouse or sun-room, to force early blossoms. You can also loosely cover the indoor shrubs with a clear or white plastic bag and place them in a sunny location for three to five days, temporarily raising the temperature around the plants.
Other than shocking them, outdoor bigleaf hydrangeas cannot be forced to blossom earlier than usual. The easiest way to ensure they have early blooms is to protect them from winter damage by wrapping them in burlap and fleece. Do not unwrap them until all chance of frost has passed. When bigleaf hydrangeas' buds and branches are protected, warm spring temperatures will naturally induce an early bloom.
Pruning methods depend greatly on the type of hydrangea. Bigleaf hydrangea and oakleaf hydrangea (Hydrangea quercifolia, USDA zones 5 through 9) blossom on old wood. Therefore, pruning them excessively can lead them to have poor flower production the next growing season. Remove only portions of bigleaf and oakleaf hydrangeas that were damaged by snow or that have weak growth, and do so in early spring.
Smooth hydrangea (Hydrangea arborescens, USDA zones 4 through 9) and panicle hydrangea (Hydrangea paniculata, USDA zones 4 through 7) produce blossoms on new wood. Prune those plants heavily in late fall after all their blooms have faded or died.
Providing Growing Conditions
Hydrangeas are among the easiest plants to grow, and they bloom with minimal effort when given proper care. Plant your hydrangeas in partial shade; the farther south you are, the less direct sun exposure your plants need to thrive. Hydrangeas thrive in mildly acidic and mildly alkaline soils, and the soil pH level affects the flower color of some hydrangeas, namely bigleaf hydrangea. In those instances, acidic soil leads to blue blossoms while alkaline soil results in pink blossoms. Keep the soil around hydrangeas moist but not soggy, providing about 1 inch of water per week.