Hydrangeas are lush shrubs with enduring flowers. They are easy to grow, bloom all season long and can be planted in spring or fall. Few shrubs can rival their full form, variety of sizes and unique bloom. These plants are perfect for bordering the edge of natural, wooded settings or along the side of the house that receives morning sun followed by some afternoon shade.
Most varieties can be coaxed to change their bloom color by altering the soil acidity. Add elemental sulfur or aluminum sulfate to produce blue blooms; add lime for pink blooms. Be patient as it could take several months for the full color effect. There is one exception: White hydrangeas will not change color.
The most popular hydrangea is the Macrophylia group, consisting of lacecaps and mopheads. When people think of hydrangeas, they typically visualize the mophead or hortensia variety with large rounded pink or blue blossoms. Lacecaps are flat, open flower heads and are considered more graceful, but not as showy. The oakleaf hydrangea is a hardy shrub grown as much for its lush foliage as its flowers.
If planted in late spring/early summer, hydrangeas will need more water in the beginning to fully establish a strong root system. Most varieties thrive best in morning sunlight and afternoon shade. Plant in well-drained soil that is moist and rich in nutrients and organic matter. Water deeply once a week, and more if the weather is particularly hot and/or dry.
Many a gardener has asked why their hydrangeas produced bountiful leaves, but no blooms. There are several things that can cause this, either individually or collectively, such as:
- Too much nitrogen: Use a slow-release fertilizer high in phosphorus.
- Too much shade: The plant needs 4-6 hours of mostly morning sun.
- Late spring frost/freeze or very cold winter: Tender buds can freeze.
- Time: Some young specimens might take two to five years to bloom.
- Animals: Deer searching for food might eat the blooms and stems.
- Over or ill-timed pruning.
Appropriate pruning for your type of hydrangea is especially important. A key secret to having blooming hydrangeas is to first understand what kind of hydrangea you have before pruning. Buds on some hydrangea varieties will form and bloom on old wood (stems from the previous growing season); some will form and bloom on new wood (stems formed during current growing season), and sometimes on both types of stems. Pruning too early might remove buds that are destined to be blooms. A good practice is to prune only in the spring just as new growth appears. And even then, don’t prune much — just old blooms and dead stems. Do not prune in the fall or during the growing season. Even if you prune correctly, occasionally a late spring freeze may kill the buds. Because of this, newer varieties (e.g. “Endless Summer”) have been developed to be more cold hardy.
As for the pruning of flowers:
- Lacecap hydrangeas: Remove flowers as soon as they have bloomed to keep them from going to seed. Cut the stems just above a bud or pair of buds beneath each flower head.
- Mophead hydrangeas: Prune as flowers fade. Cut stems back to strong laterals or buds. In early spring, remove damaged wood and cut about one-third of the oldest flowered shoots to the ground.
- Oakleaf hydrangeas: Consider your ultimate size. For a small shrub, prune all the way to the ground each spring. For other shapes, thin and shape as desired in the spring.
As for the annual sequence of blooming, oakleaf hydrangeas bloom first in the spring, followed by the mophead and then lacecaps in mid-summer.
Thank you to Tulsa County Master Gardeners for their expertise in this subject matter. Allen Robinson has been a Master Gardener since 2010.