SMA Announces Hackberry as 2020 Urban Tree of the Year

Young hackberry (Celtis occidentalis) used in extended road median. Photo by Michelle Sutton

Each fall, members of the Society of Municipal Arborists (SMA) nominate and vote for the SMA Urban Tree of the Year. You can see a list of winners going back to 1996 here.

Here’s a reflection on the 2020 SMA Urban Tree of the Year, hackberry (Celtis occidentalis), from New York Tree Trust Development Director and NYSUFC Board Member James Kaechele. Following that is a word about transplanting hackberry from Urban Horticulture Institute (UHI) Director and NYSUFC Board Member Nina Bassuk and former UHI graduate student Michelle Sutton.

James Kaechele with hackberry street tree in Syracuse. Courtesy James Kaechele

James Kaechele, New York Tree Trust Development Director, NYC Parks: 
This fine native tree is an underestimated contributor to many an urban forest. With a wide native range from New England through the Mid-Atlantic and west to Wyoming, hackberry grows in rocky, alkaline sites where other trees may struggle. It weathers cold and windy winters through USDA Hardiness Zone 2 and sweats the hot summers of Zone 9; it seems equally happy curbside in Toronto as it does in a Washington, D.C. neighborhood park.

Characteristic corky-warty vertical bark ridges of hackberry. Photo by Michelle Sutton

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). The most handsome hackberry in New York City grows along the Mosholu Parkway; it approaches 80 feet (24 m) in height, with remarkable American elm-like branching structure.

Hackberry foliage and fruit. Photo by Kathy Zuzek, from extension.umn.edu

Alongside measurable ecobenefits like cleaning air and water, hackberry provides a home and food to many a native creature. Songbirds (and humans) snack on the ripe, small, berry-like drupes that are high in protein and somewhat sweet. Hackberry leaves 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.

Winter silhouette of a mature hackberry. Photo by Jamie King

There have been a number of cultivars that claim improved resistance to galls and witches’ brooms; others are selected for form and vigor. The upright form of ‘Prairie Sentinel’ lets you sneak a tree into a spot that may be otherwise too skinny for a full-width tree. The increased vigor of ‘Magnifica’ is nice but comes with a reduced cold hardiness owing to its mixed parentage from C. laevigata, known through the South as sugarberry. For me, planting out seedling grown hackberry is a fine choice if that’s what you have available. 🔹

Herbarium specimen showing leaves and nondescript flowers of hackberry. Photo by W.D. Brush, hosted by the USDA-NRCS PLANTS Database

UHI Director Nina Bassuk/Former Graduate Student Michelle Sutton:
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.

Native range of hackberry. 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. 

About the straight species of hackberry, Nina Bassuk further adds: “My experience with hackberry over the years is that it is not as drought tolerant as reported. In a droughty situation, the leaves can become somewhat yellow (not in the same way as the yellow presented by interveinal chlorosis). It also suffers from nipple gall and witches’ broom. Overall it is a survivor, but folks should be aware of some of hackberry’s limitations.”🔹

A hackberry “fairy ring” in a Wisconsin residential yard. The mother tree in the center sent out root suckers that were probably selectively favored by a former homeowner to create a ring of clonal trees. Photo by Rebecca Lane

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