The Smooth hydrangea (Hydrangea arborescens) is a is a species of hydrangea that is deciduous in nature. It is a small- to medium-sized, multi-stemmed, growing to heights of 3 to 8 feet tall. Smooth hydrangea was first documented in the United States, where it is found growing wild amid woods, slopes, and stream banks throughout the Mid-Atlantic and Eastern states.
Smooth hydrangea is widely distributed across the eastern United States—from southern New York to the panhandle of Florida, west to eastern Oklahoma and southeastern Kansas. It is mainly found in moist soils under a hardwood forest canopy and is often common along woodland road banks and streams. It is common in the Delaware River Valley and in the Appalachian Mountains.
Due to their popularity as ornamental plants, smooth hydrangeas have been introduced to other parts of the world, including Europe, Africa and Asia. They have even become naturalized in some areas, like parts of the United Kingdom and Japan.
The flowers of Hydrangea arborescens bloom prolifically throughout the summer, with bloom time typically occurring in the summer months. The flowers are white and have a rounded shape. Unlike other hydrangea species, the smooth hydrangea’s flower color is not influenced by soil pH. This is a hardy plant, with a hardiness zone range of 3 to 9 in the United States. It can tolerate cold temperatures down to -37.2°C (-35°F).
Other than smooth hydrangea this species is also referred to by names like: wild hydrangea, sevenbark, or in some cases sheep flower.
Characteristics of Smooth Hydrangeas
- These hydrangeas are very cold tolerant, thriving in USDA zones 3-9.
- They have a rounded, mounding growth habit. They develop multiple stems that create a bushy appearance.
- Mature smooth hydrangeas typically reach 3-6 feet tall and wide, although some cultivars can grow larger.
- The foliage consists of dark green, ovate leaves with serrated edges. They turn yellow in fall before dropping.
- They have a moderate growth rate, typically growing 1-2 feet per year once established.
- They are long-lived shrubs, with a lifespan of 20 to 50 years or more with proper care.
- Features large, flower clusters in shades of white or occasionally pink, depending on the cultivar. These blooms typically start green, transition to white (or pink), and then turn tan in fall.
- A few large sterile flowers usually appear at the cluster margins (usually not enough for a quality lacecap effect). Flowers give way to dehiscent seed capsules which ripen in October-November.
- While they can tolerate full sun, smooth hydrangeas prefer partial shade, especially in hot climates.
- They thrive in moist, well-drained soil but can adapt to a range of soil conditions.
- They are relatively low-maintenance shrubs. They benefit from regular watering, especially during dry periods. Pruning in late winter or early spring can encourage bushier growth and larger blooms.
Ways to Use Hydrangea In Landscape
- Borders and Landscapes: Grow Smooth Hydrangeas in a mixed border, woodland setting, or as a foundation planting. They can also be used to create a hedge or in mass plantings for a dramatic effect.
- Slopes and Hillsides: Mass along a slope, hillside, or steep bank to help stem erosion. The extensive root system of Hydrangea arborescens can help stabilize soil on these challenging sites.
- Containers: A smaller specimen can be featured in a container as an accent on a deck or patio, or as a focal point in the landscape.
- Pollinator and Native Gardens: Include Smooth Hydrangeas in a pollinator or native garden to support beneficial insects and other wildlife.
- Shade Gardens: This hydrangeas thrive in partial shade, making them a great choice for shade gardens.
- Specimen Plant: Due to their showy flowers, these hydrangeas can be used as a specimen plant to draw attention and provide a focal point in the landscape.
Cultivars of Smooth Hydrangeas
Cultivars
| Cultivar Name | Flower Color | Size |
| ‘Annabelle’ | White, turning green | 3-5 ft |
| ‘Incrediball’ | White | 4-5 ft |
| ‘Invincibelle Spirit’ | Pink | 3-4 ft |
| ‘Hayes Starburst’ | White | 3-5 ft |
| ‘White Dome’ | White | 4-6 ft |
| ‘Abetwo’ | White | 4-5 ft |
| ‘Grandiflora’ | White | 3-5 ft |
| ‘Pink Sensation’ | Pink | 3-5 ft |
| ‘Emerald Lace’ | White | 4-6 ft |
| ‘White Diamonds’ | White | 3-5 ft |
| ‘Bounty’ | White | 3-5 ft |
| ‘Strong Annabelle’ | White | 4-5 ft |
Growing Hydrangeas
Most hydrangea varieties require three to four hours of sunlight per day. Because too much sun can wilt the blooms, a location exposing them to morning sun and providing shade to protect them from hot, sunny afternoons is best. Planting in deep-shaded areas may result in flower failure. During winter, the plants are covered with a sheet or cardboard box to protect them from cold damage.
Hydrangeas thrive in moist, well-drained soil. Compost added to the native soil can add nutrients and help promote drainage. Weed-free soil cultivated to a depth of 12 inches is optimal for planting a hydrangea with the top of the root ball level with the soil surface. The soil pH determines the flower color of bigleaf hydrangea varieties; a soil pH of 7.0 or higher results in pink flowers and a soil pH of 5.5 or lower results in blue flowers. Regular lime or aluminum sulfate applications during the growing season can increase or lower the soil pH. When planting, consider the mature size of the shrub (3 to 10 feet tall, with a similar spread) to ensure it has enough room to grow.
Hydrangeas don’t tolerate drought very well. When watering, the plants are drenched so a puddle forms on the soil surface. About one inch of water per week is sufficient, increased to 2 inches per week in hot weather. A 3-inch layer of organic mulch spread on the soil around the plants can help promote moisture retention and suppress weeds. A 10-10-10 fertilizer is applied about one month after planting when the plant is established, two months after this and again two months thereafter. As an alternative, use a one-time application of a timed-release fertilizer, about one month after planting.
Pruning hydrangeas includes removing old blooms and dead stems to reshape the plant and promote growth. Because some hydrangeas flower on old wood, some flower on new wood and some varieties flower on both old and new wood, they must be pruned according to cultivar. Regardless of the variety, pruning must occur in early summer, after the blooms fade and before the plants set new bloom buds for the following year. Pruning the plants late in summer may result in flower failure.