The Common Hackberry
By Sophie Nitoslawski, 2020-09-21
Common hackberry (Celtis occidentalis L.), also known as nettle tree, is a deciduous tree native to the Carolinian forest of Canada, which spans the southernmost parts of Ontario. Its ideal conditions are the Mississippi Valley region, with moist rich soils, where it can grow to 40 metres (130 ft) tall. The Celtis genus, which includes 60-70 species, is found around the globe including parts of Europe, North and South America, southern Asia, and Africa. We also know, based on the fossil record, that this tree existed in North America as far back as 50 million years ago, surviving a major climatic event in the Paleocene era characterized by warming global temperatures. Considered a cousin of elm, and often confused with this species given similar leaf shape, Celtis occidentalis was once classified as an Ulmaceae. More recently, hackberry has been placed in the Cannabaceae, or hemp family, along the likes of Cannabis and Humulus – the latter of which may be familiar to you as hops, a key brewing ingredient.
Hackberry has bright green elm-like leaves, alternately arranged on its branchlets, with an oblique (e.g. asymmetrical) base. The hackberry has unique bark – light brown to grey, it has a deeply furrowed appearance with a “cork-like” feeling (much like the cork of a wine or beer bottle) if you run your fingers along the irregularly-spaced ridges. The small, distinctive blue- purple berries are a favourite of robins and cedar waxwings, and the tree itself plays host to the formidable-sounding hackberry emperor, Asterocampa celtis, a species of butterfly. When taking a closer look at the leaves of the hackberry, you may occasionally notice small, bulbous growths. These are called galls and are a response to an infestation of a plant-eating insect called Pachypsylla celtidisumbilicus – thankfully, these do not cause serious damage.
Hackberry leaves, photo by Paul Wray, Iowa State University https://naturalresources.extension.iastate.edu/forestry/iowa_trees/trees/hackberry.html
Hackberry bark, photos by Paul Wray, Iowa State University https://naturalresources.extension.iastate.edu/forestry/iowa_trees/trees/hackberry.html
In cities across Canada, hackberry is planted as an ornamental tree along streets and in public parks. In its more northern planted range, such as Halifax, hackberry grows to about half the size of trees in the Carolinian range of the United States. This smaller stature often comes in handy when dealing with cramped urban growing conditions and competition with other infrastructure, such as driveways, sidewalks, and overhead utility lines.
Young hackberry on Charles St., Halifax
Hackberry under utility lines on Young St., Halifax
Although not considered particularly noteworthy – it does not have dramatic flowering nor autumn colours – it is hardy and tolerant to the stresses common to urban life. Indeed, it seems to exhibit tolerance to the more difficult planting sites. In his book North American Landscape Trees, Arthur Jacobson states about hackberry: “Lacking flamboyance and romantic associations, hackberries are like cinder blocks: eminently useful but stigmatized by default.” We hope that you will not stigmatize but rather appreciate the contributions and usefulness of hackberry to the urban landscape as a shade giver, food grower, and wildlife habitat provider.