Smooth Hydrangea
Image credit: Yamasa-n, Unsplash
Native Beauty
This fast-growing, native hydrangea has extreme cold tolerance.
The smooth hydrangea, scientifically known as Hydrangea arborescens, is a deciduous flowering shrub native to the eastern United States. It has fertile clusters of flattened lacecap flowers that emerge green in late spring, turn to white in the summer, and fade to brown as the flower heads dry in autumn. Smooth hydrangeas have a loose, open habit and a suckering, spreading tendency, and flower heads can be heavy enough to cause the stems to flop over and bend, especially after a heavy rain. Cultivars aim to help solve this issue while also offering a range of choices in terms of flower color, flower shape, and plant size (read on for a list of popular Hydrangea arborescens cultivars).
Smooth hydrangeas are extremely cold hardy and easy to grow in zones 3 - 8. They prefer partial shade and rich, moist, well-draining soil. To promote the development of strong stems, site this shrub where it will receive at least 3-4 hours of direct sunlight per day. Smooth hydrangeas can withstand full sun if they receive sufficient moisture, but in hotter regions it's advisable to provide protection from the intense rays of the afternoon sun. Smooth hydrangeas flower on new wood and should be pruned back in late winter or early spring.
The fertile flowers of the smooth hydrangea attract bees, butterflies, and other pollinators, and serve as a host plant for the hydrangea sphinx moth by providing food for the caterpillar phase of the moth’s lifecycle. Caterpillars are an important food source for birds - when you use host plants in your landscaping, you double your impact by providing support to both pollinators and birds. Birds also visit the shrub in late summer and fall to feast on the tiny seeds that dry and fall from the flowers. Some smooth hydrangea cultivars produce sterile flowers, which lack the nectar and pollen necessary to attract pollinators - be sure to choose the straight species or a fertile cultivar if your gardening goal is to support wildlife.
Smooth hydrangeas are not considered to be a deer-resistant plant. Deer will eat hydrangea flower buds and leaves, especially the tender young growth. If you would like to grow a flowering shrub that deer tend to leave alone, try Ruby Spice summersweet clethra as a deer-resistant alternative to the smooth hydrangea.
What is an ‘Annabelle’ Hydrangea?
The Annabelle hydrangea (Hydrangea arborescens ‘Annabelle’) is a popular cultivated variety of Hydrangea arborescens. It is not synonymous with the straight species. Instead, the Annabelle hydrangea represents a specific genetic variation of Hydrangea arborescens, with its own unique characteristics. Many people consider the Annabelle hydrangea to be a nativar - a cultivated variety of a native plant species, chosen for having particular genetic traits.
‘Annabelle’ was discovered by Harriet Kirkpatrick in 1910, in Southern Illinois. Harriet noticed a hydrangea with unusually large, rounded blooms growing wild alongside a horseback riding trail. She subsequently dug up this unusual shrub and transplanted it into her own garden. After intense admiration from friends and neighbors, she propagated the plant and shared the offspring locally. Through word-of-mouth, Harriet’s discovery spread regionally and the shrub became a popular smooth hydrangea variety in Illinois. In the early 1960s, Dr. Joseph McDaniel of the University of Illinois collected and propagated samples of this shrub discovered by Harriet, leading to the start of its commercial production in 1962 under the name ‘Annabelle’.
The Annabelle cultivar and the straight species of smooth hydrangea share many similarities, yet also have distinct differences:
Both grow 3 -5 feet tall and wide and have an upright suckering habit.
Both prefer partial shade and moist, well-draining soil.
Both have flower heads comprised of clusters of smaller white florets.
Both have stems that tend to flop over from the weight of their flower heads, especially after a heavy rain.
The size and shape of their flower heads is different - Annabelle has large, rounded, ball-shaped flower heads growing up to 12 inches in diameter; the straight species has flattened flower heads that grow 6 - 8 inches wide.
Their ability to sustain pollinating insects differs - Annabelle has mostly sterile florets and brings no value to pollinators, while the straight species has mostly fertile florets and is rich in nectar and pollen, which attracts and supports pollinators.
Annabelle hydrangeas and Hinoki cypress. Image credit: Yuliya May, Unsplash
What is the Difference Between a Smooth Hydrangea and a Bigleaf Hydrangea?
The smooth hydrangea (Hydrangea arborescens) and the bigleaf hydrangea (Hydrangea macrophylla) are two distinct species of plants belonging to the Hydrangeaceae family, with several key differences:
Native Range: Smooth hydrangeas are native to the eastern United States, while bigleaf hydrangeas originate from Japan and Korea.
Hardiness: Smooth hydrangeas can tolerate colder garden temperatures, growing happily in zones 3 - 8. Bigleaf hydrangeas are hardy in zones 6 - 9, and can also grow in zone 5 when winter protection is provided.
Leaves: Smooth hydrangeas have heart-shaped, slightly rounded, matte green leaves; bigleaf hydrangeas have large, oblong, fleshy leaves that are glossier in appearance.
Flower Color: Smooth hydrangeas have white, green or pink flowers regardless of the soil pH. The flower color of the bigleaf hydrangea is influenced by soil pH, with acidic soils producing blue flowers and alkaline soils producing pink flowers.
Incrediball® Smooth Hydrangea (Hydrangea arborescens ‘Abetwo’). Photo courtesy of Proven Winners.
How to Plant and Grow the Smooth Hydrangea
Planting: The best time of year to plant a smooth hydrangea is in the spring or fall, giving the roots time to establish before the onset of extreme hot or cold weather. Water deeply immediately after planting.
Location: Choose a location with well-drained soil that is rich in organic matter. Smooth hydrangeas prefer soil that retains moisture but does not become waterlogged. Select a spot with at least 3-4 hours of direct sunlight and protection from the intense afternoon sun.
Soil: Amend the soil with organic matter, such as compost, shredded leaves, or Leaf-gro® to improve drainage and soil fertility.
Mulch: Apply a layer of organic mulch around the base of the plant to retain soil moisture, regulate temperature, and suppress weeds. Make sure the mulch does not touch the base of the plant.
Watering: Keep the soil consistently moist but not waterlogged, especially during periods of drought. Deep watering is preferable to encourage the development of a strong root system (Are you in a drought? Check the U.S. Drought Monitor).
Fertilization: Hydrangeas typically do not need fertilization when they are given the rich, fertile soil they prefer. Each spring, spread a layer of compost, shredded leaves or Leaf-gro® around the base of your hydrangea, being careful to keep it away from the base of the shrub.
Pruning: Smooth hydrangeas flower on new wood and can pruned in late winter or early spring. Plants may be hard pruned back to 12 inches, and suckers can be clipped off below the soil line.
Pest and Diseases: Smooth hydrangeas can suffer from fungal diseases such as powdery mildew, cercospora leaf spot and rust. Scale is an occasional pest problem, as are aphids, spider mites and nematodes. Ensure good air circulation around your plants to minimize the risk of disease, and practice good watering habits (see “How to Water your Plants” to brush up on your watering skills).
Do Smooth Hydrangeas Like Sun or Shade?
Smooth hydrangeas thrive in partial shade, requiring 3 - 4 hours of direct sunlight per day to ensure strong stems and abundant flowering. They can tolerate full sun (6+ hours of direct sunlight per day), as long as adequate moisture is provided, and prefer protection from the intense hot afternoon sun. In hot climates, providing them with morning sun and afternoon shade can help prevent their flowers and leaves from wilting and scorching. In cooler regions, they may tolerate more sun, but protection from harsh afternoon sunlight is still recommended.
Invincibelle® Ruby Hydrangea. Photo courtesy of Proven Winners.
Landscaping Ideas for the Smooth Hydrangea
Mixed Foundation Planting: Plant smooth hydrangeas along the foundation of your home. Combine with a mix of evergreen and deciduous shrubs, creating four seasons of interest and enhancing the curb appeal of your home.
Native Garden: Plant the straight species of Hydrangea arborescens in a native garden, alongside oakleaf hydrangeas, woodland phlox, summersweet clethra, and other native plants.
Mixed Borders: Create a mixed border planting by combining the smooth hydrangea with small trees, shrubs, grasses and perennials. Plants like the white wood aster, creeping phlox, azaleas, and the Eastern redbud tree combine nicely with the dark green leaves and white flowers of the smooth hydrangea.
Mass Planting: Plant smooth hydrangeas in large groupings to provide maximum visual impact. This approach works well in large garden beds and along pathways or fences.
Pollinator Garden or Bee Garden: Plant fertile, pollinator-favorite lacecap cultivars like ‘Haas’ Halo’ and ‘Pinky Pollen Ring’™ in the partially shaded areas of a pollinator garden.
Woodland Gardens: The partial shade underneath the canopy of tall trees is an ideal spot for the smooth hydrangea. Combine with other partial-shade loving plants like camellias, Japanese andromeda, azaleas, and rhododendrons.
Accent Planting: Use smooth hydrangeas as a focal point in the landscape. Plant strategically to draw attention to specific areas of the garden like entryways, water features, pathways, or seating areas.
Vintage Cottage Garden: For a charming, informal look, plant smooth hydrangeas alongside old-fashioned cottage garden favorites like foxgloves, garden phlox, hollyhocks, catmint, hibiscus, and roses.
Edging: Dwarf cultivars like Invincibelle Mini Mauvette® and Invicibelle Wee White® make great edging plants along walkways, sidewalks, and driveways.
Smooth Hydrangea Cultivars
Here is a list of popular Hydrangea arborescens cultivars - keep in mind there are numerous varieties available with more coming on the market every few years. This is by no means an exhaustive list!
Annabelle (Hydrangea arborescens ‘Annabelle’): By far, the most popular and widely planted cultivar. Large, round flower heads grow up to 12 inches wide. Mature size is 3 - 5 feet high and wide. Stems are prone to flop, especially after a heavy rain. Mostly sterile.
Haas Halo (Hydrangea arborescens ‘Haas’ Halo’): The white lacecap flowers of this cultivar were given top ratings as a pollinator favorite in the Mount Cuba Center’s trial of Wild Hydrangeas for the Mid-Atlantic Region. Worth seeking out to add to any pollinator garden.
Incrediball® (Hydrangea arborescens ‘Abetwo’): An offspring of ‘Annabelle’, with large white flowers and stronger stems that resist flopping. Grows 5-8 feet tall and wide.
Incrediball® Blush (Hydrangea arborescens ‘NCHA4’): Large pink fertile flowers cover a compact, 4-5 foot tall and wide shrub. Does best in full sun.
Invincibelle Limetta® (Hydrangea arborescens 'NCHA8'): Soft green flowers darken as the summer progresses. Grows 3-4 feet tall and wide.
Invincibelle Mini Mauvette® (Hydrangea arborescens ‘NCHA7’): A dwarf cultivar with deep mauve pink flowers atop sturdy, flop-resistant stems. Grows 2.5 - 3 feet tall and wide.
Invincibelle® Ruby (Hydrangea arborescens 'NCHA3'): Two-toned ruby red and pink flowers top darker green foliage. Grows 3 - 4 feet tall and 2 - 3 feet wide.
Invincibelle® Spirit II (Hydrangea arborescens 'NCHA3'): Dark pink mophead flowers and a robust habit. Grows 3 - 4 feet tall and wide and attracts some pollinators, but not as many as other lacecap varieties.
Invincibelle Sublime™ (Hydrangea arborescens 'SMNHRL'): Green mophead flowers on a 3 - 5 foot tall and wide shrub.
Invincibelle Wee White® (Hydrangea arborescens 'NCHA5'): A dwarf shrub, growing only 12 - 30 inches tall and wide. Large white flowers top strong stems.
Lime Rickey® (Hydrangea arborescens 'SMNHALR'): Flat, disc-shaped flowers change color during the season, from soft green to a darker green accented with pink pollen. A larger shrub, growing up to 6 feet tall and 8 feet wide.
Pinky Pollen RIng™ (Hydrangea arborescens 'SMHALP'): This pollinator favorite has pastel pink lacecap flowers. Grows 4 - 5 feet tall and wide.
Are Smooth Hydrangeas Poisonous to Dogs?
Smooth hydrangeas (Hydrangea arborescens) contain compounds that can be toxic to dogs if ingested, specifically cyanogenic glycosides. These compounds can release cyanide when broken down in the digestive system. However, the level of toxins in smooth hydrangeas are relatively low, and instances of dogs getting poisoned by consuming these plants are rare.
While the risk of severe poisoning is low, it’s important to monitor your dog's behavior in outdoor spaces. If you suspect your dog has ingested a toxic plant, seek immediate veterinary care or contact the Pet Poison Hotline (read “Which Plants are Toxic to Dogs” for more information on plants poisonous to dogs).