Wild Hydrangeas

To the human eye, the minuscule florets of a wild hydrangea, Hydrangea arborescens, appear as scraps of white fuzz flung slapdash on a shrub. Those tiny fertile florets, clustered into a single inflorescence, make up a lacecap. Though not particularly showy, a lacecap is rich with nectar, its anthers covered in pollen. Lacecaps are perfectly suited in size to the wide variety of bees and wasps that typically visit them. To spy pollinators swarming across the flat surface of a lacecap is not unlike watching guests at an outdoor wedding reception nibble cake and sip champagne. The lacecap offers a feast.

In a typical lacecap, the florets are ringed with circular arrays of larger sepals that look like bunches of little flowers, but they are not. Unlike flowers, sepals are not a reproductive structure. They offer little or no nutritional sustenance. Sepals are more like engraved wedding invitations asking bees and their relatives to please attend.

The other type of inflorescence within the many hydrangea species is the mophead. Compared to the lacecap, the mophead is generally more sumptuous in its floral display. Mopheads have flower heads dense with sepals but few fertile florets and little nectar or pollen. Consequently, unlike the lacecap, mopheads tend to skimp on the pollinators' banquet.

The Mt. Cuba Center is a botanic garden and research site in Delaware that promotes the use of native plants and habitat conservation. It completed a native hydrangea trial in 2022, aiming to assess the horticultural and ecological value of three closely related native species along with 26 cultivars. Known as both wild and smooth hydrangea, Hydrangea arborescens was the most common of the three native species in the trial. It can grow seven feet tall and a bit wider. Smooth hydrangea grows along shady, moist slopes throughout the eastern and central United States. Ashy hydrangea, H. cinerea, and silver-leaf hydrangea, H. radiata, both with variations in leaf color but a smaller stature,were also included in the trial. All three species produce flower heads that are primarily lacecaps.

A key aspect of the Mt. Cuba study was comparing the inflorescence of a lacecap to a mophead, both for pollinator preference and ornamental value. Each type rests on a similar stem structure called a corymb. Hydrangeas, when found in natural environments, generally produce blooms of lacecaps. That is, lots of fertile florets clumped into inflorescences on relatively flat corymbs. If sepals are part of the bloom, their ratio is low. Conversely, mopheads possess big, showy dome-like corymbs consisting mainly of sepals, sometimes with actual florets tucked in between. They have been favorites of gardeners, landscapers, and florists for generations. The opulence of mopheads, gathered into bouquets or grouped in masses, speaks to celebrations like weddings, coronations, and royalty. In terms of ornamental value, mopheads have always taken the cake.

Overwhelming data from the study, with a few outliers, concluded that pollinators visited the lacecaps much more frequently than the mopheads. The Mt. Cuba hydrangea trial underscores a dilemma frequently faced by many gardeners. Should they choose a plant for its aesthetic appeal or select a species based on its contribution to the ecosystem, supporting insects and birds? The problem of highly cultivated plants that offer little to pollinators is ubiquitous.

Considered more of a phenomenon of cultivation, mopheads do occur in nature. They are natural anomalies that produce a spattering of fertile florets tucked among the sepals. For more than a century, native hydrangeas with this floral mutation were selectively bred and widely distributed because of their beautiful blooms, which plant growers sometimes call "floral pop." Mopheads were also grown from cuttings and swapped with neighbors. Heirlooms of native mophead hydrangea are common in many home gardens and landscapes, especially in the eastern United States. Hydrangea arborescens 'Grandiflora' has been on the horticultural scene for decades. Hydrangea arborescens 'Annabelle' became available in the 1960s. Both were included in the trial and are still widely sold in nurseries today. Due to weak stems, their flower heads tend to flop, hence the name mophead. In the trial, visits by pollinators were relatively few.

Sam Hoadley, manager of horticultural research at Mt. Cuba, suspects that the smooth hydrangea shrub growing in his home landscape might be 'Hills of Snow,' a progenitor of 'Grandiflora.' He noted the shrub's floppy habit. Over time, plant breeders have produced newer cultivars, creating stronger stems to keep the mopheads from flopping over.

Recently, horticulturalists have addressed the ecological value of plants as the relationship between insects and native plants has become better understood. Mt. Cuba has conducted native plant trials since 2002. However, only in the last decade did it begin to consider pollinator preference criteria. Only in the last five years did Mt. Cuba collect data from pollinator watch teams.

Hydrangeas, native and non-native combined, are an enormous slice of our overall horticultural market, second only to roses. Plant breeders exploit and develop fascinating and unusual traits, like hydrangeas with changing colors and hydrangeas with seasonally endless blooms. In 2014, Hydrangeas accounted for 13.5 percent of total sales of shrubs in the United States. Over a thousand cultivars and hybrids are grown, mostly from Asian species, including bigleaf hydrangea H. macrophylla, mountain hydrangea H. serrata, and panicle hydrangea H. paniculata. The oakleaf hydrangea, H. quercifolia, native to the southeastern United States, is also popular. Nurseries sell the species and cultivars available, mostly mopheads, both exotic and native, because that's what consumers buy. While beautiful, many hydrangeas that dot the landscape are sterile or nearly so. Also, cultivars of any plant, because they are not true to seed, do not increase genetic diversity within ecosystems or help plants evolve.

In the Mt. Cuba native hydrangea trials, five stars could be awarded to plants with exceptional ornamental value. Important considerations included length of bloom time, disease resistance, the quality of the foliage, how well a plant retains its appeal throughout the seasons, and the plant’s overall shape or habit. The initial scoring did not consider the importance of pollinators. Pollinator preference was awarded only as bonus points. Ornamental value came first.

The following explains how the rating system worked. Two plants tied for the overall ornamental rating, H. arborescens ‘SMNHALR’ (Lime Rickey®) and H. arborescens ‘Haas Halo.’ ‘Lime Rickey®’ is an impressive color-changing mophead. A cross between H. radiata and H. arborescens 'Pink Pincushion' blooms green with sterile flowers, only to be followed by a deep pink bloom of fertile flowers. As the flower heads fade, they revert to their lime-colored hue. Its competitor, 'Haas Halo,' is a lacecap. When grown in full sun, 'Haas Halo' produced flower heads among the largest in the study, lacecaps or mopheads. It performed well in the shade, too. 'Haas Halo' was the only specimen to receive five stars. While tied for ornamental value with 'Lime Rickey®,' 'Haas Halo' received extra points because it drew more pollinators, 183 to Lime Rickey’s 42. Literature from the trial called 'Haas Halo' a "knockout." In another revelation, the straight species of wild hydrangea attracted 209 pollinators, but H. arborescens ‘Dardom’ (White Dome®) tallied the most at 250 visits. This lacecap cultivar was one of several top-ranked selections at four stars. However, some mopheads scored higher on ornamental points.

"It comes down to what you are trying to accomplish in your home landscape," said Hoadley. "The [native] mopheads are adding some ecological value. In most cases, it's not zero." He added that it is better to encourage gardeners to plant a cultivar of a native if it satisfies their taste, even if it has less pollinator value than the straight species, than to select a non-native specimen or, worse, a plant that is known to be invasive. "We're trying to give people alternatives and the information they need to make informed decisions."

During the last 20 years, Mt. Cuba has been conducting trials of many native plants, including bee balm (Monarda), tickseed (Coreopsis), coneflower (Echinacea), and, most recently, sedge (Carex), to name a few. Hoadley plans to assess the greater impact of the Mt. Cuba trials. Is the availability of native plants for consumers increasing? Who is responding to their data? To what degree are they influencing the marketplace and public attitudes? Whether gardeners decide to shift away from plants whose extravagant blooms possess little ecological value to more nature-friendly cultivars, time will tell.

Click the links below to learn more about the Mt. Cuba wild hydrangea trial and their other trials.

Wild Hydrangea for the Mid-Atlantic Region

Mt. Cuba Trial Garden