Announcing the 2020 SMA Urban Tree of the Year: Hackberry
Compiled by Michelle Sutton, Editor, City Trees, Magazine of the Society of Municipal Arborists
Each fall, SMA members nominate and vote for the Urban Tree of the Year. For 2020, members chose hackberry (Celtis occidentalis), a rugged tree with a wide native range, from New England through the Mid-Atlantic and west to Wyoming. Hardy to USDA Zone 2, it can be used much farther afield.
City of Hamilton, Ontario Urban Forestry Supervisor Tami Sadonoja reports planting 252 hackberry trees last year, consisting of common hackberry (Celtis occidentalis) and the columnar C. occidentalis ‘Prairie Sentinel’, which can be used for smaller spaces. “Hackberry trees perform well in our parks, open spaces, and streetscapes,” she says. “They are tolerant of dry conditions, and though the leaves will wilt under sustained drought, we find they recover quickly once watered.”
“I admire the adaptability and toughness of this so-called common hackberry tree, able to tolerate dry or wet and sandy or clay environs,” says Oak Creek, Wisconsin Urban Forester Rebecca Lane. “This versatility should be considered when selecting trees capable of enduring climate changes.”
“In our region (greater Columbus, Ohio), hackberry is an unsung workhorse,” says Upper Arlington, Ohio Parks and Forestry Superintendent Steve Cothrel. “It is native here, and in some landscapes, very common as a volunteer or as a mature tree that survived nearby development as neighborhoods rose from fields and forests. Many of these sites feature shallow, alkaline soils over limestone bedrock, and hackberry tolerates the high soil pH better than almost any other species.”
Wyoming State Community Resource Forester Tara Costanzo echoes Cothrel. “Hackberry is a tough tree that can tolerate the Wyoming wind, the drier and higher pH soils, and the harsh winters. I would like to see it utilized more in new developments and used as replacements in the overly mature, declining canopy in many of our communities. It's a longer-lived species than the commonly planted cottonwoods and poplars that make up a large percentage of our community trees.”
New York Tree Trust Development Director James Kaechele praises hackberry’s numerous ecosystem benefits. “Hackberry provides a home and food to many a native creature,” he says. “Songbirds snack on the ripe, small, dark blue, berry-like drupes that are high in protein and somewhat sweet. The leaves often support the life cycle of numerous gall-producing insects. The resulting gall-ridden leaves are more of an unsightly irritation than they are detrimental to the tree’s health. Perhaps this informs where to plant hackberry: anywhere folks are not looking too closely at the leaves.” The foliage also feeds the caterpillars of mourning cloak, question mark, hackberry emperor, and other butterflies.
Hackberry’s mature size varies correspondingly with planting location, commonly growing to 40 to 60 feet (12 to 18 m) with exceptional specimens approaching 100 feet (30 m). According to Kaechele, the most handsome hackberry in New York City grows along the Mosholu Parkway. “It’s nearly 80 feet (24 m) in height, with remarkable American elm-like branching structure,” he says.
In addition to being noteworthy for its small but showy blue fruit, hackberry is valued for its ornamental bark with its distinctive corky vertical ridges. The fall foliage color can be a pleasant soft-yellow-meets-chartreuse. The grand overall fountain-shape of maturing hackberry trees can be a beautiful ornamental trait.
Research by Cornell’s Urban Horticulture Institute (UHI) found that hackberry can be successfully transplanted in fall or spring, B&B or bare root. However, the study found that fall-planted BR hackberry had a slight edge over spring-planted bare root hackberry, and that when it came to spring planting, B&B hackberry trees grew somewhat better than bare root hackberry trees. Researchers concluded that if a community wants to try planting bare root hackberry trees in order to save resources, preserve fine root systems, and allow more public participation, they should do it in the fall. This and all UHI bare root recommendations assume that bare root trees of any species will be handled according to UHI’s guidelines in Creating the Urban Forest: The Bare Root Method.
Based on molecular evidence, hackberry has been reclassified by plant taxonomists away from the elm family (Ulmaceae) and into the hemp family (Cannabaceae). This appears to have been initiated by the Angiosperm Phylogeny Group in 2009, and in the years since, most major arboreta have accepted this reclassification. Oak Creek, Wisconsin Urban Forester Rebecca Lane says, “As a member of the hemp family, hackberry, too, just might have a thousand medicinal compounds—and have more ethnobotanical significance than once thought.”