Hydrangeas (Hydrangea spp.) are deciduous shrubs that bloom in the spring or summer. The flowers are usually white but some hydrangeas bloom in pink or blue, depending on the soil pH. Hydrangeas are generally hardy in U.S. Department of Agriculture plant hardiness zones 3 to 9, depending on the species. Failure to bloom can be attributed to a number of cultural and environmental causes.
Give Them Time
If the hydrangeas were just planted a year or two ago, wait another year or two before becoming concerned about a lack of flowers. Hydrangeas often take a year or two to get established in a new planting site. It is not unusual for hydrangeas to wait until the third or fourth year to bloom, even if there was a flower or two on the shrubs when they were purchased.
Give Them Plenty of Sunlight
Make sure hydrangeas are getting enough sunlight. Ideally, they should get four to five hours of sunshine every day. Morning sun exposure with afternoon shade is best. Hydrangeas can be grown successfully in shadier areas, but they will grow more slowly and bloom very little, if at all. Dappled or bright shade cast by high trees should provide enough sunlight, although they will not bloom as profusely. Full sun is okay in cool-summer climates, but the soil must be kept uniformly moist.
Give Them Plenty of Water
Provide hydrangeas with supplemental water when the top inch or two of soil starts to become dry. This is especially important during the first two years while hydrangeas are getting established. Young hydrangeas need between 4 and 6 gallons or 1 1/2 to 2 inches of water per week, while larger, more mature hydrangeas use 8 to 10 gallons per week. During dry periods or when there has been little rainfall, check the soil once or twice each week to see if supplemental water is needed. Spread 2 to 3 inches of mulch over the soil to keep the soil moist and reduce weeds.
Prune Them Correctly
Prune hydrangeas at the correct time of year. Most hydrangeas that bloom on "new wood," or stems from the current season, should not be pruned except to remove stems or leaves with black winter damage in the spring. Smooth hydrangeas (Hydrangea arborescens), climbing hydrangeas (Hydrangea anomala petiolaris), Endless Summer hydrangeas (Hydrangea macrophylla 'Balimer') and panicle hydrangeas (Hydrangea paniculata) all fall within this group. However, panicle hydrangeas and smooth hydrangeas can be cut back to a height of 1 to 2 feet in late winter. They will grow new stems and bloom reliably that same year. This sort of drastic pruning on these two species will result in shorter shrubs with bigger flowers.
Hydrangeas that bloom on "old wood," or stems that grew the previous summer, should be pruned right after they finish blooming. If they are pruned in late winter or early spring, the flower buds will be removed and they may not bloom at all that year. Bigleaf hydrangeas (Hydrangea macrophylla), Nikko Blue hydrangeas (Hydrangea macrophylla 'Nikko Blue'), oakleaf hydrangeas (Hydrangea quercifolia) and serrated hydrangeas (Hydrangea serrate) all fall within that pruning group.
Always use sharp pruners and sterilize them in household disinfectant before using them. If there are any signs of disease on the branches, disinfect the pruners between cuts. Always rinse the disinfectant off before cutting stems to prevent possible damage to healthy plant tissue. Make pruning cuts about ¼ inch above a growth bud or set of leaves. Cut diseased and dead branches off at the base at any time, preferably as soon as they are noticed.
Watch Out for Cold Weather and Deer
Pay attention to weather forecasts in the spring and cover hydrangeas with burlap or a bed sheet when late cold snaps are predicted. Frost can damage flower buds and prevent them from blooming.
Protect hydrangeas from hungry deer that will eat the flower buds and young leaves. Deer repellant made from 20 percent whole eggs and 80 percent water works well for up to a month per spray. Remove the membrane that surrounds the yolk prior to mixing to prevent clogging. Mix it up, pour it into a sprayer and spray the egg mixture all over the hydrangeas when temperatures rise above freezing and the hydrangea begins putting on new leaves. Do this on a dry day so the mixture will dry quickly.
Leave the Fertilizer in the Shed
Don't give hydrangeas fertilizer and keep lawn fertilizer well away from them. They usually do not need any fertilizer at all. Nitrogen, which is the primary nutrient in lawn fertilizer, causes hydrangeas to grow lush, green leaves at the expense of flowers. If lawn fertilizer is used, do not spread it within 3 feet of the edge of the hydrangea branches.