When I bought my house five years ago in Chester County, Pennsylvania, I inherited a landscape with a garden tradition dating back to the 1940s. Mature specimens of choice woody plants are bountiful and include a grove of mature dawn redwoods (Metasequoia glyptostroboides), three massive franklinia (Franklinia alatamaha), and one large and aged smooth hydrangea (Hydrangea arborescens). The identity of this smooth hydrangea cultivar is not known, but due to the age and the substantial sprawling nature of the planting I suspect it to be Hydrangea arborescens ‘Grandiflora’, formerly sold under the name ‘Hills of Snow’. This cultivar was originally selected from a wild population of Hydrangea arborescens in the early twentieth century, and is still available from specialty growers today.
Hydrangea arborescens ‘Grandiflora,’ along with the similar and more popular H. arborescens ‘Annabelle’, are staples of American horticulture that are readily recognized by expert and novice gardeners alike. Their oversized, dome-shaped inflorescences borne in June and July have classic ornamental appeal, but these cultivars are not without their shortcomings. Both ‘Grandiflora’ and ‘Annabelle’ have floppy habits, a flaw that is exacerbated by summer storms right when the blooms are at their peak. For this reason, I had always considered these cultivars to be horticultural relics and, admittedly, gave little consideration to the use of the species in modern landscapes. My insular outlook was quickly challenged, however, thanks to my inheritance of a comprehensive trial of smooth hydrangea when I was hired for my current role of Manager of Horticultural Research at Mt. Cuba Center in Hockessin, Delaware, in March of 2019.
Mt. Cuba Center, situated in the rolling hills of New Castle County, Delaware, is the former estate of Pamela and Lammot du Pont Copeland. The Copelands envisioned that Mt. Cuba could be a place to inspire an appreciation for the beauty and value of native plants and spark a commitment to protect the habitats that sustain them. This original intention lives on as Mt. Cuba Center’s core mission, and continues to guide the organization to this day. After Mrs. Copeland passed away in 2001, the transition from private estate to public garden began in earnest. In 2002, the Copelands’ cut-flower plot was repurposed to serve as a trial garden. In this space, native species and cultivars were grown side by side to determine their horticultural qualities in a low-maintenance common garden setting. The first research report was published in 2005, describing findings from a three-year trial on asters, including floral display, habit, cultural adaptability, winter hardiness in Zone 7a, and disease resistance. This publication highlighted and recommended the top-performing plants to gardeners and nurseries in the mid-Atlantic region, and served as a template for future research reports. A report on Echinacea was released in 2009, and in 2011, the trial garden was redesigned with a new perimeter fence, brick walkways, and a shade structure. The shade structure proved to be an invaluable tool, not just for trialing shade-loving species, but also for comparing the performance and adaptability of trialed plants when they are cultivated in both sun and shade.
In 2013, Mt. Cuba Center opened its doors to regular public visitation for the first time, and the trial garden welcomed visitors to observe and interact with trials in progress. Under the deft management of George Coombs, publication of the trial garden’s research reports became a highly anticipated resource valued by the public and nursery industry. Trials were established in a staggered rotation so that four trials run concurrently, with one trial concluding each year. In addition to promoting plants with excellent garden qualities, the Coreopsis, Monarda, and Phlox reports aspired to address the ecological services that these species and cultivars provide by collecting comparative pollinator data on the various species and cultivars in each trial. Pollinator studies quickly became an important component of Mt. Cuba’s trial program, and have allowed consumers to make informed decisions about which plants have the most capacity for attracting and supporting pollinators in their home landscapes.
Quantitative and qualitative horticultural data are collected in the trial garden on a weekly basis between April and September. Scores are assigned that reflect habit, form, foliage, and other ornamental qualities that the plant may exhibit. For example, a weak plant with a floppy habit might receive a low score, while a vigorous plant with a robust and non-floppy habit would receive a high score. A separate rating is assigned to the floral display when the plants are in bloom, and an additional rating assesses disease resistance, which becomes especially important when observing plants that have a known susceptibility to specific pathogens. At the end of each season an overall score is generated for each plant. At the conclusion of the three-to-five-year evaluation, the scores from each year are averaged to determine the final horticultural score. In the most recent trials of hydrangea and Echinacea, supplemental points were then awarded to plants that attracted the most pollinators. This supports our initiative of promoting plants that are both beautiful and also have wildlife value. The top performing plants are then featured in subsequent research reports. For some gardeners, selecting a native plant for its ornamental qualities is enough, but for others this is only part of the greater story of providing habitat and food sources for wildlife in their home landscape.
Pollinator data are collected by a cadre of Mt. Cuba Center’s citizen scientists, called the Pollinator Watch Team. On a near-daily basis, these volunteers observe plants in bloom for 60 seconds and collect information on insect visitors. Counts are made on one plant per accession, or single inflorescences if blooms are numerous or plants are prohibitively large. The number of pollinator visits is recorded, along with the time, weather data, and additional observations. In some trials, all pollinators that visit the plant or inflorescence are counted, while in others, only targeted groups such as butterflies or hummingbirds are recorded. The results of these studies help homeowners make informed decisions about which plants in our evaluations have the most capacity to attract and support insect pollinators. These data become particularly relevant as natural habitats face increasing threat of destruction. Home gardens are uniquely poised to temper those losses, and can help provide small oases for wildlife in the midst of food deserts comprised of the turfgrass and non-native foundation plantings that make up the majority of home and commercial landscapes today. By pointing people in the direction of beautiful and beneficial native plants, Mt. Cuba Center can help people become conservators.
By pointing people in the direction of beautiful and beneficial native plants, Mt. Cuba Center can help people become conservators.
When I was hired as Manager of Horticultural Research in the early spring of 2019, I was greeted with four in-progress trials: Helenium, Echinacea (the second trial of this genus), smooth hydrangea (or “wild” hydrangea, as they are called at Mt. Cuba Center), and Carex. Smooth or “wild” hydrangea represented the first woody plant trial at Mt. Cuba and had just begun the third year of a five-year evaluation. While the study of woody plant material represented a step into new territory for the trial garden, it was a logical one. There is an increasing demand from homeowners for reliable information on alternatives to non-native woody landscape plants that are low maintenance, beautiful, and providers of wildlife value whenever possible. Included in the smooth hydrangea evaluation were three species of hydrangea native to the eastern United States: Hydrangea arborescens, Hydrangea cinerea, and Hydrangea radiata. Hydrangea arborescens was the primary species evaluated, and is the wild version of most of the twenty-six cultivars in the trial. Hydrangea arborescens, or smooth hydrangea, has the largest native range of the three species and can be found in much of the eastern and central United States, often on shaded moist slopes. Hydrangea cinerea, or ashy hydrangea, is exceedingly rare in cultivation and can be found primarily in the central and southeastern United States. The best identifying feature of ashy hydrangea is the concentration of white to grey pubescence on the stems and the backs of their leaves, which differs from the relatively hairless foliage of Hydrangea arborescens. Hydrangea radiata, or silver-leaf hydrangea, has the smallest native range compared to Hydrangea arborescens and Hydrangea cinerea and can only be found in a handful of states in the southern Appalachian Mountains. The most striking feature of Hydrangea radiata is the bright white undersides of the leaves making this perhaps the most easily identified and inherently ornamental species of the three in our trial. Silver-leaf hydrangea was also the first to bloom in our trial in mid-June while the other two species started blooming in early July each year. While the three species vary slightly in size, habit, and overall appearance, each flowers on new wood and produces superficially similar inflorescences. The three species, including two distinct accessions of Hydrangea radiata, were all propagated from wild collected stock from the collection at Mt. Cuba Center, and these plants acted as the controls against which all of the cultivars would be compared. Twenty-five cultivars were then sourced from various arboreta and nurseries, in most cases representing material available to homeowners at that time. All twenty-nine hydrangea accessions were grown in full sun and nineteen cultivars were also grown in shade for comparison. Since these shrubs bloom on new wood and flower buds are not produced until the start of the growing season, smooth hydrangea can be heavily pruned in late spring and flowers are still reliably produced in June and July. To assess response to pruning, one example of each hydrangea grown in full sun was cut back to approximately six to eight inches from the ground in late March 2019, 2020, and 2021. The remaining examples of each hydrangea served as the control group and were left unpruned for the duration of the evaluation.
That first spring at Mt. Cuba, before the hydrangea had even leafed out, I was struck by the diversity of form, texture, and even stem color. Subtle variations between similar cultivars and species were readily apparent when the plants were grown side by side. It became obvious to me how short-sighted I had been when I had written off smooth hydrangea for my own garden use. There was clearly so much more to this group of plants than the old standbys, like Hydrangea arborescens ‘Grandiflora’ and ‘Annabelle’.
As spring progressed and data collection began, I looked forward to what I knew would be a spectacular floral display in June and July. When the time came, the flowers were indeed breathtaking, and included a concert of white and pink inflorescences that fell into two distinct categories: lacecaps and mopheads. Lacecap smooth hydrangea species and cultivars produce flat-topped, corymbose inflorescences that are primarily composed of masses of fertile flowers that number anywhere from 800 to 2,000 individual flowers. Around the perimeter of many lacecap flowerheads are a small number of sterile flowers that give the inflorescences their trademark lacy appearance. Lacecap inflorescences are the predominant flower form found in wild populations of Hydrangea arborescens, and offer visiting pollinators highly accessible nectar and pollen rewards. Hydrangea arborescens and Hydrangea radiata both produced lacecap inflorescences that averaged around 4.5 inches in diameter, although the sterile flowers of the silverleaf hydrangea were noticeably larger than those of the smooth hydrangeas. The fertile flowers of Hydrangea radiata were also less tightly clustered than those of Hydrangea arborescens. Hydrangea cinerea, on the other hand, produced the smallest diameter flower heads in the trial at around 3.5 inches on average, and also some of the smallest sterile flowers. Despite the smaller diameter, the inflorescences of the ashy hydrangeas were tightly packed with fertile flowers and were heavily visited by pollinators.
In contrast, mophead hydrangea blooms are often more dome-shaped and produce a much higher number of sterile flowers per inflorescence, giving them their iconic billowy cloud-like appearance. In most cases, fertile flowers are still produced, but in much lower numbers than their lacecap counterparts. In addition, these fertile flowers are often sequestered within the inflorescences, reducing their accessibility for some insects. Some mophead smooth hydrangeas such as Hydrangea arborescens ‘Annabelle’ are selections made from naturally occurring anomalies found in nature. However, not all mophead smooth hydrangeas in cultivation are the products of simple selection. Wild-origin mutations form the genetic building blocks utilized by modern plant breeders to cross and further select smooth hydrangeas. The progeny of these breeding programs feature ornamental and horticultural improvements as well as novel garden traits.
The trial at Mt. Cuba was planted with mophead cultivars concentrated on one end of the planting bed and lacecap cultivars and species planted on the other. As I walked through the blooming mophead cultivars, the inflorescences were beautiful, but the surroundings were quiet and relatively devoid of insect pollinators. As I continued down the path to the lacecap hydrangeas, this changed, and I was greeted with an immersive audible and visual experience composed of hundreds if not thousands of insects. Bumblebees, wasps, beetles, true bugs, and flies busily collected pollen and nectar from the fertile flowers of the lacecap inflorescences in a frenzy of activity. I was in awe, not just because of the huge number of insects I was witnessing, but also by the stark contrast between the two flower forms and the obvious preference that the various pollinators displayed for the lacecap smooth hydrangeas. One lacecap cultivar in particular, Hydrangea arborescens ‘Haas’ Halo’, completely captivated me. Not only was it covered with pollinators but here was a robust plant with an incredible floral display that rivaled any of the mophead cultivars in the trial. This was an example of a plant that encapsulated Mt. Cuba’s mission. This ornamentally superior and garden-worthy native plant inspired an appreciation for its beauty and for the ecological value that this plant was clearly providing to scores of insect pollinators. I was so taken by Hydrangea arborescens ‘Haas’ Halo’ that I ordered one for my home garden that same day.
As I continued down the path to the lacecap hydrangeas, I was greeted with an immersive audible and visual experience composed of hundreds if not thousands of insects.
This single experience in the trial garden constituted a personal paradigm shift in the way I looked at horticulture and my own gardening practice. There was room for beauty and ornamental interest, which had always been a priority, but ecological value did not have to be sacrificed. In fact, gardening for insects and other wildlife quickly became a focus for me, and has been a deciding factor for the vast majority of my home-garden additions ever since. This one plant has inspired me to take conservation action by planting a native shrub with high ecological value in my home landscape. And I was not alone. Over the next three years, I fielded countless questions about Hydrangea arborescens ‘Haas’ Halo’ from visitors who had experienced this plant for the first time in the trial garden just as I had. They all wanted to know where they could buy one. This is exactly the kind of inspiration and conservation action that I imagine the Copelands envisioned when they first conceived of the idea to one day welcome the public into their home and gardens.
In 2021, the five-year trial of smooth hydrangea was complete and the results were in. The top-performing hydrangeas were identified, and pollinator counts were tallied. The pollinator data confirmed what could be readily observed when the plants were in bloom each year. Insects overwhelmingly preferred lacecap hydrangeas over mopheads. That is not to say that all mopheads are completely devoid of pollinator value. In fact, some attracted a reasonable number of insects. The mophead that attracted the most insects in the trial was Hydrangea arborescens ‘NCHA2’ (Invincibelle® Spirit II) which, maybe not surprisingly, produced a greater proportion of accessible fertile flowers than other mopheads. While some compromises exist where you can have both the mophead aesthetic as well as some pollinator value no mophead hydrangea received enough pollinator visits to be directly compared to any of the lacecaps. Interestingly, the hydrangea with the lowest number of pollinator visits was Hydrangea arborescens ‘Hayes Starburst’. This cultivar exhibits mophead inflorescences but is an extreme case of genetic variation within the species. While most mophead smooth hydrangeas produce a reduced number of fertile flowers, Hydrangea arborescens ‘Haye’s Starburst’ produced no observable fertile flowers. No fertile flowers mean no pollen or nectar and therefore no pollinator benefits. This lack of pollinator value was directly reflected in the low pollinator count. Interestingly, there were three exceptions that broke the rule that lacecaps are better than mopheads for pollinators: Hydrangea arborescens ‘Riven Lace’, Hydrangea arborescens ‘Emerald Lace’, and Hydrangea arborescens ‘Green Dragon’. These are relatively compact plants with lacecap inflorescences and distinctive dissected foliage. These three cultivars are so similar that it is widely believed that they are actually the same, genetically identical plant that was named and introduced into the horticultural trade on three separate occasions. Their inability to attract insect pollinators remains a mystery, but could be attributed to several factors, including the possibility that these cultivars do not offer the same quality or abundance of nectar or pollen as other lacecaps hydrangeas in the trial.
From a purely ornamental perspective, most of the nineteen smooth hydrangea cultivars grown in shade performed better than the specimens in full sun. The primary reason for this is that shade-grown plants avoided issues such as foliar and floral burn as well as premature defoliation. Inflorescences of the hydrangeas grown in shade retained their form for months after the fertile flowers had finished blooming in July, and aged from white to attractive shades of lime green. Hydrangea radiata and its cultivars struggled the most when grown in full sun, particularly during the heat of the summer. These were always the first to show signs of heat and drought stress and were the first to start dropping leaves. Thisspecies is best reserved for use in woodland edges and shade plantings in the mid-Atlantic region. At Mt. Cuba Center, Hydrangea radiata is grown to perfection in the shady naturalistic gardens where the silvery undersides of the leaves add movement and interest to the landscape and their lacecap inflorescences attract insect pollinators in droves.
To my surprise, a handful of pink flowering hydrangeas performed exceptionally well in full sun, even better than in shade, including Hydrangea arborescens ‘NCHA2’ (Invincibelle® Spirit II) and Hydrangea arborescens ‘NCHA4’ (Incrediball® Blush). These two cultivars can be successfully sited in full sun locations (six or more hours of sun a day) in the region surrounding Mt. Cuba Center if they are planted in moisture-retentive but well-drained soils. Good soil moisture remains an essential component for successful cultivation of any of the smooth hydrangeas planted in full sun or near full sun conditions.
The cutback comparison conducted over three seasons revealed some interesting trends. There was a slight delay in bloom (generally one to two weeks) in cutback plants and fewer but larger inflorescences. This increased diameter of the inflorescences can be attributed to a return to a more vigorous juvenile state in the cutback shrubs. On average the inflorescence increase was around thirty percent, while some examples increased between fifty to eighty percent in diameter. For some plants this effect was particularly noticeable especially if the inflorescences produced on a non-cutback plant were already of substantial size. For example, the cut back version of Hydrangea arborescens ‘Haas’ Halo’, which produced the largest inflorescences (8”) of the trial on the control plants, produced a dramatic display of flower heads that were nearly a foot across. Cutback hydrangeas also formed a more compact habit in many cases. Some exceptions to this rule were compact cultivars such as Hydrangea arborescens ‘NCHA5’ (Invincibelle® Wee White) and Hydrangea arborescens ‘NCHA3’ (Invincibelle® Ruby) where there were few discernable differences between cutback and control plants. One unexpected effect of the cutback study was that cutback plants had improved sun tolerance. Not only might the cutback plants have had more efficient water-transporting stems, but their established root systems were supporting fewer water-demanding leaves compared to the controls. However, I would not recommend annual heavy pruning to increase sun tolerance over the long run or to make larger plants permanently more accessible to smaller planting spaces. The best practice is to select the right cultivar for the right garden location based on the ultimate size of the unpruned plants and the goals of the gardener. I recommend a more restrained approach to pruning, where only a handful of the oldest stems are removed on a yearly basis, to maintain some juvenile vigor in your hydrangeas.
One unexpected but welcome benefit of hydrangea pruning in the trial garden was the observation of square-headed wasps (Crabronidae) using the pithy stems as a nesting site. On several occasions we observed adult wasps excavating the spongy, pith-filled cores of recently pruned stems. Once the cavity had been established, the adult wasp could be seen placing paralyzed flies in the hollowed-out stems for its larvae to feed on. The chambers were then sealed, protecting the developing larvae within. The larva eventually pupate, and emerge the following spring as adult wasps. Perhaps this observation would inspire people to tolerate and even embrace some dead wood in their shrubs if they knew what a benefit they could potentially be to wildlife. However, if spring pruning is required, it is still feasible to prune out full stems even if they have been colonized without interrupting wasp’s life cycle. The discarded stems can be placed intact somewhere to allow emergence to occur later that year. In addition, there is an opportunity to further utilize the pruned stems by simply cutting them to lengths of eight inches to a foot and the bundling the stems together. These makeshift bee nesting sites will only further amplify the bee and wasp habitat in your landscape by using raw materials provided by your hydrangeas. In addition, Hydrangea arborescens is the larval host plant for the hydrangea sphinx moth and the hydrangea leaftier moth, although neither species was observed in the trial.
We observed adult wasps excavating the spongy, pith-filled cores of recently pruned stems.
Nine cultivars of smooth hydrangea were identified as top performers, and the highest rated plant of all was indeed Hydrangea arborescens ‘Haas’ Halo’. This cultivar, besides being my personal favorite smooth hydrangea, offers the perfect combination of horticultural merit and ecological value. This exemplary form of smooth hydrangea was originally selected by Frederick Ray in 2008, plantsman and former Delaware Valley College horticulture professor, from the Pennsylvania garden of Joan Haas. It was chosen for its upright, vigorous growth, and for its lacecap flower heads, upwards of eight inches in diameter, that display a greater number of sterile flowers around the perimeter than those of typical Hydrangea arborescens. A comparable, but decidedly more obscure, selection called Hydrangea arborescens ‘Mary Nell’ made the top performer list as well. It too produces abnormally large lacecap inflorescences, distinguished by a double ring of sterile flowers around the perimeter of each flower head. Hydrangea arborescens ‘Mary Nell’ was selected and named by Joseph McDaniel, who also introduced Hydrangea arborescens ‘Annabelle’ in the 1960s. Unfortunately, Hydrangea arborescens ‘Mary Nell’ is rarely commercially available despite its substantial garden and wildlife value. Two more lacecap cultivars made the top performer list. Hydrangea arborescens ‘Dardom’ (White Dome®) is an older cultivar originally selected in Belgium in 1997 and introduced by Proven Winners® in the early 2000s. It boasts large lacecap inflorescences and attracted the highest numbers of pollinators of any hydrangea in the trial. Hydrangea arborescens ‘Total Eclipse’ is a vigorous, slightly more compact, and earlier-blooming selection of the species made by Jim Pyler of Natural Landscapes Nursery in West Grove, Pennsylvania. This cultivar also attracted the third most pollinators of any hydrangea in the evaluation.
The remaining five top-performing hydrangeas from the trial produce mophead inflorescences. While these plants may not have the pollinator value offered by the lacecap top performers, they are still highly recommended from an ornamental perspective for gardens of the mid-Atlantic. Hydrangea arborescens ‘Abetwo’ (Incrediball®) and Hydrangea arborescens ‘Bounty’ are two white-blooming mopheads that offer excellent alternatives to Hydrangea arborescens ‘Annabelle’ and Hydrangea arborescens ‘Grandiflora’. Each features the classic hydrangea aesthetic that gardeners have valued for more than a century, but improve on their predecessors with sturdy stems that are resistant to flopping. Hydrangea ‘SMNHALR’ (Lime Rickey®), a unique mophead hydrangea from Spring Meadow Nursery, displays undeniably attractive, lime-green inflorescences in mid-June that are quickly followed by raspberry-colored fertile flowers later in the month. The sterile flowers rapidly fade to alabaster before reverting to their former green color as the fertile flowers complete their bloom. Last but not least are Hydrangea arborescens ‘NCHA4’ (Incrediball® Blush) and Hydrangea arborescens ‘NCHA2’ (Invincibelle® Spirit II). Both cultivars are some of the finest examples of the cutting-edge plant breeding that continues to push the boundaries of what is thought possible with hydrangea, redbuds, and dogwoods amongst many other woody plant genera coming from North Carolina State University and Thomas Ranney. These two cultivars feature attractive pink mophead inflorescences that are well supported by sturdy stems. Hydrangea arborescens ‘NCHA4’ (Incrediball® Blush) is a great option for smaller landscapes thanks to its semi-compact habit of four to five feet in height and width, while Hydrangea arborescens ‘NCHA2’ (Invincibelle® Spirit II) eventually forms a much larger plant reaching six feet in height and eight feet in width. For more information on the top performing smooth hydrangeas from Mt. Cuba Center’s latest trial, visit mtcubacenter.org/research/trial-garden/.
For me, the smooth hydrangea trial represents a turning point in my personal gardening journey which, just like the garden itself, is constantly evolving and is never truly complete. I went from largely looking at plants as a decoration for my home landscape, an admittedly narrow view, to choosing plants not just for me but for wildlife that I can support and provide refuge for in my garden. Seeing the surge in insects and birds and other wildlife at my home, a trend which I would like to think directly correlates to my native plant choices, has been an incredibly rewarding experience, and is so much more than just cultivating a place of objective beauty. It provides me with a purpose, the same purpose that the Copelands had when they envisioned the future of their property that would eventually become Mt. Cuba Center. Thanks to this trial, I have a newfound appreciation for the Hydrangea arborescens ‘Grandiflora’ that still resides at my home in Kennett Square, although my future hydrangea selections will be made for the benefit of pollinators. My personal journey into the world on native hydrangeas has not concluded with this trial, and in fact will only be expanded upon with Mt. Cuba Center’s newly planted evaluation of oakleaf hydrangeas. This trial will be unique in that, for the first time, we are trialing a single species and comparing its many cultivated forms. Hydrangea quercifolia has many seasons of interest: it is well known for the oak-shaped leaves for which it is named, white to pink panicle inflorescences in summer, incredible burgundy fall color, and exfoliating cinnamon-colored bark that can be admired during winter months. The trial will assess cultivars that are tried-and-true garden classics as well as a handful of newcomers to the horticultural market. The oakleaf hydrangea trial will be sited in full sun; however, most of the subjects will also be grown under our shade structure for comparison. This will be an interesting trial for observing pollinator preference, because the percentage of fertile flowers per inflorescence in cultivated forms and selections of this species is quite variable. Oakleaf hydrangeas are one of my all-time favorite shrubs and I know I am in for more horticultural and ecological inspiration in the trial garden at Mt. Cuba Center before the results are of this latest trial are published in 2027.
Sam Hoadley is the Manager of Horticultural Research at Mr. Cuba Center where he evaluates native plant species and cultivars for their horticultural qualities and ecological value, Sam earned a degree in Sustainable Landscape Horticulture from the University of Vermont.
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