What does “resiliency” really mean in the context of managed trees in the landscape? For horticulturists like me, situated in the Upper Midwest, you might think our priorities focus on cold hardiness—and you wouldn’t be wrong, we certainly value trees that can survive our frigid winter temperatures. But from my perspective, “resiliency” better aligns with a well-rounded suite of characteristics, especially those that can aid a tree to survive in a world of extremes. In recent years in Minnesota, we have experienced the expected low winter temperatures typical for our region, but we have also endured atypical weather events during the growing season: floods in the spring and severe droughts in the summer. This erratic range of conditions seems to define our climatic expectations for the future: a world of extremes. So, what is a horticulturist to do when they need to select a tree that they want to survive in our changing climate? Instead of screening species suited to one end of a stress-spectrum, we are looking for those “resilient” taxa that offer flexibility in the face of a variety of stressful conditions. In other words, plants that are adaptable, which offer plasticity—the species that can “roll with the punches,” so to speak. But it doesn’t end with just tolerance to weather conditions; also of interest are plants that can diversify our plant palettes with reduced pest and disease vulnerabilities; those with strong branch attachments; and to top it all off, it wouldn’t hurt if they offered some ornamental appeal along the way! This is a tall order for trees adapted to my region (USDA cold hardiness zone 4b) and one of the few groups of plants that can deliver on this bold request is the hickories (Carya spp.).

My Experiences with Hickories

My first interactions with hickories date back to my childhood, growing up in a Northwest suburb of Chicago and visiting my grandparent’s acreage situated just outside of suburbia. The acreage comprised an alfalfa field behind the house and a well-groomed landscape in the front of the property that resembled a golf course, a turfed space complete with an assortment of mature shagbark hickories and bur oaks. This haven of hickory, along with nearby woodlands, were my favorite places to play and explore as a kid. I fondly remember autumn there, when the ground was covered in split husks recently fallen from the canopy and littered with bright-white shagbark hickory nuts, overwhelming busy squirrels attempting to stash them away before winter. These projectiles, along with strips of exfoliating bark and a stray branch or two, became integral components of childhood games. It wasn’t until years later, when taking a plant identification class as an undergraduate at Iowa State University, that I learned these magnificent trees were not widely available in the nursery trade. Our class was told that in the green industry, claims of protuberant taproots, which impede their ability to be successfully transplanted, and a general consensus that they grow too slowly to turn a profit, were mainly to blame. The thought that hickories could not be sought out and planted to grace a home landscape was disturbing. My childhood memories of playing in the shade of old shagbark hickories were experiences that could not be replicated elsewhere. Surely, this was a mistake! I took to the internet and the library seeking contrary evidence, only to confirm that my instructor’s perspective seemed to be shared widely. But what about other trees which have taproots and grow slowly? Are all the hickories limited by these issues? Is there anything to be done to resolve these problems? These questions led to further discussion with my instructor, an undergraduate research project, a master’s thesis, and later a doctoral dissertation, each focused on the central theme of supporting hickories as nursery crops and landscape options. Through these projects, I found evidence that not all the rumors about hickories were accurate, and I explored alternative strategies for propagating, cultivating, and transplanting hickories. With the help of my mentors, we have made some strong cases that hickories should be given more attention in the green industry—because as it turns out, these stately trees could be valuable tools for developing the resilient landscapes of the future.

The Genus Carya

The genus Carya comprises nineteen species worldwide, a number which varies slightly based on who serves as your favorite botanical authority. The genus is further subdivided into three sections: Apocarya, which hosts the “pecan hickories,” Carya, encompassing the “true hickories,” and Sinocarya, the section containing species originating from Asia. Each section is defined by the morphological traits that unify the several species within that section. For example, all the pecan hickories offer “naked” (valvate) buds whose edges touch but do not overlap, while those in the true hickory group all boast overlapping (imbricate) scales on their terminal buds. These traits not only help taxonomists to group hickories, but they are quite useful to the passive observer in narrowing down a species-level identification. Combined, the two sections previously mentioned yield fourteen species endemic to North America, with all but one, Carya palmeri (Mexican hickory), occurring naturally in the eastern half of the contiguous United States. With species spanning such a vast range, it should come as no surprise that these hickories yield an awe-inspiring diversity of ornamental traits and site-specific adaptations that should be embraced and utilized by horticulturists.

Hickories in Horticulture

The genus Carya is a group of plants near and dear to my heart. Having studied their intricacies throughout my graduate career while characterizing their horticultural potential and exploring workarounds for their undesirable production-related challenges, I can safely admit my bias for these stately trees as landscape options. But really, what’s not to like? Arguably, hickories are some of the most ornamental trees for northern gardens in the temperate zone. They are stately, long-lived, woody perennials, all becoming upright trees at maturity except for one taxon, Carya floridana (scrub hickory), which remains a shrubby, multi-stem plant. Among the multitude of traits that define the potential value of hickories for adorning landscapes, the foremost is their bark. Particularly valued by us northern gardeners, bark is a characteristic to be appreciated year-round, especially during the long winter months. For this reason, the shagbark hickory (Carya ovata) is one of the most iconic of this genus. At maturity, shagbark hickory exhibits long, exfoliating strips of bark, which peel away from the trunk to create a dramatic effect in the landscape. However, not all hickories display exfoliating bark. Some, like Carya tomentosa (mockernut hickory), exhibit a strongly ridged pattern while others, like Carya illinoinensis (pecan), reveal a scaly texture. Bark traits vary significantly both within and across taxa, but each can appeal to the appetites of ornamental horticulturists. The second characteristic for which hickories should be known is their autumnal display. Hickories provide some of the most intense and vibrant yellow-to-orange fall foliage displays among northern-adapted trees. Most, if not all, of the hickories can display bright, brilliant yellows, but my personal favorite is the awe-inspiring amber color of Carya glabra (pignut hickory) leaves in autumn. It is hard to convey the vivid color of pignut hickory leaves in the fall other than to say the rich amber color is, in my opinion, superior to the display of any other tree we grow in the Upper Midwest.

Bark variety in Carya: the peeling strips of shagbark hickory (C. ovata, 22868*A)
The scaly texture of pecan (C. illinoinensis, 12913*A)
The ridges of mockernut hickory (C. tomentosa, 20100*B)

In addition to their superb ornamental capacity, species belonging to the genus Carya should also be praised for their lack of susceptibility to pests and diseases, as well as their cold hardiness. The former attribute varies slightly across regions where hickories and pecan are cultivated because no group of trees is completely immune to pests and diseases, but the overwhelming trend across taxa is a lack of a notable “Achilles’ heel.” Cold hardiness is a trait that has never been formally screened across the genus but based on efforts to cultivate Carya spp. in cold climates, multiple taxa are particularly tough. In Minnesota, both bitternut and shagbark hickories are native to the state, with the former occurring naturally throughout regions characterized as USDA zone 4. The latter naturally occurs only in the southeast portion of the state, but is suited to cultivation throughout USDA zone 4 (average annual extreme minimum temperatures ranging from -34.4°C to -28.9°C). In 1957, at the University of Minnesota Landscape Arboretum, a specimen of Carya laciniosa (kingnut hickory) was the first tree to be formally planted on the site and later accessioned into the collections. The tree still exists today, appearing healthy and vibrant, serving as a testament to the amenability of this taxon to expanded planting outside of its native range. Lastly, our collections also include a variety of northern pecans (those which originate from the northern part of the taxon’s native range) which take on a small-statured form of less than forty meters tall in our climate.

Studies in form and color: bitternut hickory (C. cordiformis).
Another view of the kingnut hickory (pictured above), both in the Minnesota Landscape Arboretum.

Hickories adorning a landscape can provide some outstanding coarse structure to a design. When I observe hickories—and only after I have finished admiring their bark—I am usually drawn to their branch architecture. With few exceptions, most hickories display a clear dominant leader, and they rarely break apart—a testament to their strong branch attachments. Mature trees tend to be taller than they are wide, even when grown without nearby competitive vegetation. Aside from their visual appeal, I appreciate how often when I view a hickory, the sight is accompanied with some form of wildlife. Species belonging to the genus Carya offer immense ecological value, whether in a wild setting or in a managed landscape. Hickories have strong ecological connections to a vast array of wildlife—and further, as long-lived, durable plants, they provide many ecosystem services to the sites where they are grown.

One might wonder why a genus that can offer so many unique qualities, and even comprises an agriculturally significant taxon like pecan (Carya illinoinensis), seems to have been largely ignored by commercial horticulture. The fact of the matter is that hickories have been overlooked because they are slow to produce a saleable tree when cultivated from seed, asexual propagation techniques suited to developing trees for ornamental landscape applications are lacking, and they are almost exclusively defined by their development of a taproot—which is often said to limit their ability to be transplanted. In their current state, they serve as the “poster child” for a challenging crop in the nursery.

Pignut hickory (C. glabra, 7873*A) in the Arnold Arboretum.
Shagbark hickory (C. ovata, 22868*K) in the Arnold Arboretum.

Hickories in the Landscape

To qualify as a resilient landscape option, at least as I have previously defined, a tree should be tolerant of various soil-moisture conditions. North American hickories include species adapted to dry upland sites, like Carya tomentosa (mockernut hickory), species found in seasonally inundated swamps, including Carya aquatica (water hickory), and those such as Carya ovata (shagbark hickory) that often are observed somewhere in the middle of that spectrum. These species can be sited well in landscapes with well-defined soil-moisture characteristics (i.e. frequently dry, often wet, or “average” moisture), but the truly distinctive taxa that demonstrate a capacity to tolerate both flooding and drought are likely the resilient trees capable of thriving in the future. On a grand scale, one might wonder if this diversity of adaptations within the genus Carya could be drawn upon for the long-term endeavor of selecting and breeding stress-tolerance traits into a resilient selection. This idea is plausible because many of the hickories, including those in different sections, are known to be capable of hybridization across species lines. In fact, several documented interspecific hybrids are known to occur naturally in the wild where the ranges of their parent species overlap. An unfortunate reality, however, of idealizing a hickory breeding program to provide superior specimens for landscape use is the long maturation period of these stately trees, which would almost certainly require multiple generations of plant breeders before yielding major progress. So, where should a horticulturist with finite time begin with this process? Luckily, the timeline to obtaining superior selections can be greatly reduced because the genus Carya already offers species such as Carya cordiformis (bitternut hickory) and Carya laciniosa (kingnut hickory) which—combined with observations of these trees growing naturally in inundated riparian habitats—recent research suggests are also quite adaptable in the face of drought.

The author in the Arnold’s plant propagation facility with a tray of seedlings he gifted to the Arboretum (pictured below).

Researchers keen on identifying trees for the future have employed a variety of physiological screening tools to characterize stress tolerance thresholds. In recent years, vapor-pressure osmometry, a technique originally developed for ecological research, has been embraced in the horticultural realm to characterize drought tolerance by predicting a taxon’s leaf water potential at the turgor loss point. While water relations for plants is a complex concept and a multitude of factors are involved in the process, this predictive tool has become a staple for researchers because it can fairly easily and quickly be employed, and it measures and predicts a metric that many plant scientists agree can be equated to a tree’s tolerance threshold for drought. Essentially, the leaf water potential at the turgor loss point can be thought of as the minimum water potential threshold that a tree can endure while simultaneously not incurring permanent injury to the tree. This “sweet spot” for drought tolerance can be used as a metric to rationalize the characterization of this trait with a number; it can help inform site specifications in landscapes to match the right plants in the right places; and it can be used to directly compare tolerances across taxa, thus generating relative tolerance rankings and providing resources for those designing a landscape.

While studying at Cornell University in 2019, I led an experiment to replicate this technique with five species of hickory: C. cordiformis, C. glabra, C. illinoinensis, C. laciniosa, and C. ovata. This span of taxa encompasses species that occur naturally in a variety of landscapes—growing on upland sites and ranging down to bottomland riparian habitats. The goal was to capture this diversity to determine where, on a relative scale, the species fell on the drought-tolerance spectrum. Old forestry literature suggested that drought tolerance would increase with species adapted to upland conditions and would decrease with those often found in bottomland habitats. However, to our surprise, our data suggested the exact opposite. Instead, we observed a trend wherein the species which occur naturally in bottomland habitats (C. cordiformis and C. laciniosa) were predicted to be more drought tolerant than their upland counterparts. The upland counterparts were still fairly drought tolerant, but interestingly their bottomland cousins were exceeding expectations. At first, we wondered why this might be. Did we do something wrong? We repeated the study in 2020, only to yield the same results. We looked to the literature and discovered a pattern with some other bottomland taxa that, contrary to expectations based solely on their site association in the wild, were incredibly adaptable to droughty conditions. What appears to be the trend across these species is their localization in bottomland habitats which are characterized not only by seasonal flooding but also periodic droughts. Is it possible some riparian habitats are resulting in water conditions that ebb and flow between drought and flooding, thus creating a natural selection process for resilient plants? We thought so. A classic example that many horticulturists can connect with is Taxodium distichum (bald cypress). In the wild it can be found in swampy bottomland conditions, but when cultivated the plant is adaptable to a range of landscape scenarios, including droughty situations. So, what does this tell us? For one, we don’t think this trend suggests that all bottomland plants are necessarily drought tolerant. Rather, it suggests that there is more than meets the eye for some plants found on riparian sites, and some of those locations could yield incredibly adaptable plants that will perform in conditions we wouldn’t necessarily expect after viewing them in the wild. Second, it supports further screening of a multitude of taxa not only to evaluate stress-tolerance factors on one end of a stress spectrum, but to look for the species that may exhibit plasticity, which ultimately might serve as resilient icons in the landscapes of the future. Lastly, this trend reminds us not to use broad-brush generalizations when it comes to any genus. In this case, hickories should be considered for their resiliency, adoption in the nursery, and their application in the landscape on a case-by-case basis to match the right plant with the right conditions, situation, or place.

At the end of the day, hickories are a highly underutilized source of ornamental characteristics and stress-tolerance traits for managed landscapes. They offer a range of attributes and are adapted to a multitude of site conditions. Horticultural improvement efforts could expand opportunities for embracing hickories in the landscape, and their inclusion in managed settings could mean improved species diversity and landscape-level resiliency in the greenspaces of the future.

I fondly recall my memories of growing up with hickories, conducting research, and appreciating them in situ. I hope that my experiences can be shared by the next generation of horticulturists and those that follow for many years to come.

Brandon Miller is an assistant professor in the Department of Horticultural Science at the University of Minnesota and the curator of plant collections at the Minnesota Landscape Arboretum. He leads extension and research programs focused on resilient landscapes.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

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