How to Grow Hydrangeas: Beloved Shrubs for Part-Shade Spots

Growth Habit: Hydrangeas can grow from 3 to 15 feet tall and almost as wide depending on the species. Some hydrangeas grow more vertical and can be pruned into a tree form. Other hydrangeas have a wider and rounder shape. There is even a vining type of hydrangea.

Staking: Hydrangea that grow in a tree form may need staking after transplanting if grown in a windy location. Remove the staking after one year. Bushy types of hydrangeas may need to have their flowers supported. The large blooms can pull down the branches. Wire cages or plant supports can keep the flowers upright and more visible. Vining hydrangeas will need a strong support to climb on and will have to be attached to the support.

Watering: Hydrangea have large leaves that transpire water easily. Even established plants can wilt during the heat and drought of a summer day. Wilted plants will often recover in the evening and be fine. Add at least 1 inch of water weekly, more during hot, dry stretches.

Fertilizing: Based on a soil test, fertilize in spring with a 2- to 3-inch thick layer of compost and an all purpose organic granule fertilizer. To promote flowering, look for a fertilizer high in phosphorous (the middle number on the bag). Don't apply high phosphorous fertilizers if gardening near bodies of water since it can runoff and pollute streams, rivers and lakes. For big leaf hydrangeas, add sulfur to keep the pH slightly acidic. To get bluer flowers on this type of hydrangea add more sulfur to lower the pH to 6. To produce pinker flowers raise the pH closer to 7 by adding lime.

Trimming & Pruning: Deadhead spent flowers as needed on the bigleaf hydrangeas and smooth hydrangeas. Smooth hydrangeas may send out a second flush of flowers in late summer. Other hydrangea types have the ability to hold their flowers on the shrub for many weeks after full bloom.

The how and when of pruning depends on the type of hydrangeas you're growing. For hydrangeas that flower on new wood produced in spring, such as the panicle and smooth hydrangea, prune in early spring to reduce the size of the shrub, reduce branch crowding and stimulate new growth. The more growth, the more flowers you'll get in mid to late summer. Prune smooth hydrangeas to a few feet off the ground.

For hydrangeas that bloom on old wood, such as the climbing hydrangea, bigleaf and oakleaf hydrangea, prune after flowering in summer. The old wood are branches that survive the winter. Some types of bigleaf hydrangea bloom on the old and new wood. Prune these after the first flush of flowers in summer. Prune to remove dead, diseased, broken and crowding branches and reduce the size of the shrub. Do not prune hydrangeas that bloom on old wood in spring, or you will have no flowers that year.

Mulching: Since hydrangeas need consistently moist soil, mulch in spring with a 2- to 3-inch thick layer of bark mulch. The mulch will keep the soil cool and moist and help prevent weed growth.

Dividing & Transplanting needs: Hydrangeas that grow into bushes with multiple stems, such as the bigleaf and smooth hydrangea, are the best candidates for dividing. In the North, divide hydrangeas in spring while the shrub is still dormant. In the South, divide in spring or fall after all the leaves have dropped. Water the shrub well. With a sharp spade, dig around the root ball and eventually remove the shrub from the ground.

Using the spade or knife, cut the shrub into 2 to 4 sections depending on the size of the plant. Make sure you have a solid mass of roots with each division. Replant each division in a similar location and keep the new plants well watered all season.

Pests/ Disease: Hydrangeas are generally insect and disease free plants. However, they can occasionally be attacked by aphids, spider mites and Japanese beetles. Reduce nitrogen fertilizer to thwart aphids and knock them off the leaves with a strong spray of water from a hose. Keep plants well watered to avoid spider mites. Hand pick Japanese beetles or set up hormone traps placed 200 feet away from the hydrangeas to draw the beetles away.

While hydrangeas may get leaf spots caused by various diseases, powdery mildew is probably the most prevalent disease. Powdery mildew causes a whitish film to form on the leaves. It eventually causes the leaf to yellow and die. If occurring late in the season, don't worry about it. Simply clean up the dropped leaves well in fall. If it needs control, spray Neem oil earlier in the season. You can also use a home remedy of 1 tablespoon baking soda, 1 tablespoon horticultural oil, in 1 gallon of water. This is a preventive spray that stop powdery mildew from forming.

One of the biggest problems with one of the most popular species, the big leaf hydrangea, is lack of flowering. This is particularly an issue in colder climates.

Since bigleaf hydrangeas flower on the old wood, the stems need to survive the winter in order to get early flowers the next year. If they die to the ground, the stems won't form flowers.

The solution is to protect the stems in winter with bark mulch. In late fall, cover the plant stems with a 1 to 2 foot deep layer of bark or wood chips. These will protect the stems in winter and provide mulch in spring as you remove the chips. Don't be in a hurry to remove the wood chips in spring, since late frosts can also kill young buds. Also, avoid spreading high nitrogen fertilizers near your hydrangeas. Nitrogen will create shrubs with large leaves, but fewer flowers.