All About Gardening With Native Smooth Hydrangeas

Bigleaf hydrangea (Hydrangea macrophylla) and panicle hydrangea (Hydrangea paniculata) have long been popular in yards, gardens and public landscapes, but a North American cousin is currently gaining ground, thanks to a slate of cultivars that play up its natural appeal. Make way for the smooth hydrangea (Hydrangea arborescens)!

Hydrangea arborescens can be found growing wild amid woods, slopes and stream banks throughout the Mid-Atlantic and Southeast states. It’s a deciduous shrub with rounded, toothed green leaves lining upright canes—a growth habit reminiscent of H. macrophylla. (Its foliage is noticeably thinner than the bigleaf species’s, though.) Smooth hydrangea grows three to five feet tall and wide, but it can spread by suckers to fill available space. It blooms greenish white in summer. Its infloresences are of the lacecap type—a dense, flat cluster of tiny but numerous fertile flowers ringed by a scattering of showier sterile flowers. 

In its natural form, smooth hydrangea faces a few challenges for great garden performance. Its flowers, while pretty, can be heavy enough to make the stems flop. These same stems can die back in winter, to be replaced by weak growth the next season, which in turn easily breaks under snow and ice—creating a vicious cycle. Both flowers and foliage can be marred by sunscald and damaged by drought.

But the species has plenty of redeeming qualities, such that breeders have taken notice and worked to improve it for garden use. Its excellent points include its summer bloom time. Plus, its flowers occur on the new growth of the year, so the buds aren’t endangered by harsh winters or mis-timed pruning (the way big leaf hydrangeas’ can be). The flower color is consistent—not related to soil pH like the bigleaf—and the lacecaps feed many pollinators. The compact size of the plant suits residential landscapes. It can take shade, and its foliage provides coarse texture to offset smaller-leaved companions. Finally, as the flowers fade, they persist on the stems and provide understated interest into fall and winter. All of these points combine to make smooth hydrangea a native species well worth tweaking.

For many, the antique cultivar ‘Annabelle’ is synonymous with smooth hydrangea. Here’s its history: A woman named Harriet Kirkpatrick discovered a mophead variety of the native hydrangea growing naturally near the southern Illinois town of Anna. She transplanted it to her yard and shared cuttings with anyone interested. It persisted as an unnamed local favorite until 1960, when Joseph C. McDaniel, a horticulture professor at the University of Illinois, spotted it in an Urbana garden. He learned its history, propagated it, named it ‘Annabelle’ and introduced it to the trade in 1962.

‘Annabelle’ hydrangea remains popular today. Its size is that of the species (three to five feet tall and spreading) but its flowers are bigger, pure white and mophead—that is, made primarily of the larger sterile florets. A drawback: These large flowers can make the stems flop, particularly in shade or under heavy rains. They also do not support pollinators, since the fertile flowers are absent or few and hidden.

For a bee-friendly smooth hydrangea, there’s ‘Haas Halo’, selected by Pennsylvania horticulturist Frederick Ray, who was drawn by its sturdy stems, blue-green foliage and large lacecap flowers. The flower heads span an impressive eight inches at least. ‘Haas Halo’ (named for Joan Haas, the gardener who gave Ray seedlings) rose to the very top of a five-year native hydrangea trial at Delaware’s Mt. Cuba Center. This cultivar received a perfect score of 5 stars, the sum of its average rating for floral display and overall quality (looking at the whole plant and the foliage) plus bonus points for high pollinator traffic.

Another standout in the Mt. Cuba trial was Lime Rickey (‘SMNHALR’), the runner-up to ‘Haas Halo’ with a score of 4.6. A mophead, this cultivar was bred at Spring Meadow Nursery in Michigan and introduced by Proven Winners ColorChoice Shrubs. Not a pure H. arborescens, it came about through crosses between the smooth hydrangea cultivar ‘Pink Pincushion’ and another North American species, H. radiata. Ornamentally, Lime Rickey stands out because of its changing colors. Early in the bloom season, the sterile florets open pale green. Tiny pink fertile flowers lightly accent the green color as it darkens to jade.

Pink takes on a starring role in other smooth hydrangea introductions. The first pink-flowered mophead was the 2010 introduction H. a. Invincibelle Spirit (‘NCHAI’), developed in Dr. Thomas Ranney’s breeding program at North Carolina State University. Its rosy color represented a true breakthrough. However, the stems proved floppy enough to warrant the introduction of Invincibelle Spirit II (‘NCHA2’) several years later. This improved version is a stronger plant, with darker pink blooms that bear enough fertile flowers to make this cultivar more popular among pollinators than any other mophead in the Mt. Cuba trial. (It drew fewer visits than the average lacecap cultivar, though.)

Related: Read about another smooth hydrangea in the Invincibelle series: Invincibelle Lace.

The NCSU breeding program worked on smooth hydrangeas’ size as well as flower color, resulting in Invincibelle series members like Wee White (‘NCHA5’), which tops out at two feet tall and bears pale-pink, fading-to-white mopheads, and the similarly sized Ruby (‘NCHA3’), blooming dark rose. 

Not to be confused with the Invincibelles, there’s also the Incrediball series, which began with a seedling selected from a batch of ‘Annabelle’. Recognized for its stronger stems that better support the expected huge mopheads, it was branded Incrediball. There’s also a pink-toned version called Incrediball Blush (‘NCHA4’). These two cultivars tied for third place with Invincibelle Spirit II in Mt. Cuba’s trial, each earning a score of 4.5.

Clearly, the market has concentrated on mophead introductions, creating eye-catching flowers on sturdy stems. Finding lacecap cultivars for sale can prove tricky, but they exist, including the aptly named ‘Eco Pink Puff’ and ‘Pink Pincushion’. The preponderance of fertile flowers make their blossoms somewhat resemble those of a joe-pye weed (Eutrochium) or Japanese spiraea (Spiraea japonica). Their stems can be a bit weak, but they draw pollinators better than the mopheads and they blend well in naturalistic gardens. There’s also the lacecap ‘Mary Nell’, which looks similar to the aforementioned ‘Haas Halo’, but with smaller flower heads. It was introduced by Dr. McDaniel, the professor who re-discovered ‘Annabelle’. He named ‘Mary Nell’ for his wife.

The first question one asks about smooth hydrangea is usually does it require sun or shade? Answer: It depends. With regular moisture in the soil, these shrubs can perform well in full sun. This is especially true for the pink-flowered cultivars. Give them half-day sun, preferably morning light, in southern areas where the afternoon sun runs hot, or full sun in cooler zones, provided the soil doesn’t dry out. White-flowered smooth hydrangeas need part sun (morning) or dappled light because in full sun their sterile florets can crisp and brown before their time. 

Although ample water is necessary for smooth hydrangeas in sun, the soil should provide good drainage. In the shade, once established these shrubs can be surprisingly tolerant of dry spells.

The second question surrounding these plants: how should they be pruned? Smooth hydrangeas form flower buds on the year’s new growth. Therefore, cutting the canes down in fall, winter or early spring will not affect the bloom to come. However, fall or winter pruning eliminates the interest provided by the spent flowers and stems’ architecture, so cutting back is best done in early spring. To maintain some structure, remove only several canes each year, or avoid trimming more than two-thirds of every cane. 

In the Mt. Cuba Center trial, specimens of each cultivar were designated for pruning in spring, while others were left to grow. Generally, cutting back resulted in markedly shorter plants at the end of the season, except among those cultivars bred to be compact. For these, pruning made no difference to their eventual size.  

Smooth hydrangeas offer themselves to a number of roles in the garden. They can be massed to fill up space, or they can be planted in a row as a summer hedge. They can participate in a mixed border or bed, but keep in mind their natural tendency to sucker. Unwanted spread can be dug out at the base. 

Good companions include plants with fine foliage, like ferns, which will contrast beautifully with the coarse hydrangea foliage. Low-growing native phloxes (Phlox divaricata or stolonifera) provide floral interest in spring, prior to the hydrangea’s bloom, and make a lovely fine-textured groundcover in summer. So do foamflowers (Tiarella). Sedges (Carex) also lend themselves to skirting the ground around shade-grown smooth hydrangeas. With more sun, hardy geraniums (Geranium) make a good low-growing companion. Asters, such as the compact Aster oblogifolius ‘Bluebird’, can supply a fall-blooming complement.

For woody companions, look to shrubs that share the smooth hydrangea’s native habitats, such as spring-blooming mountain laurel (Kalmia latifolia) and azaleas (Rhododendron). Its cousin the oakleaf hydrangea (H. quercifolia) is another great option. For trees, flowering dogwood (Cornus florida) and eastern redbud (Cercis canadensis) are a natural choice.

Related: Read about native Hydrangea quercifolia, a beautiful four-season garden shrub.

Image credits: 'Haas Halo' courtesy of Plants Nouveau. Incrediball courtesy of Spring Meadow Nursery. All others courtesy of Proven Winners ColorChoice Shrubs.