Scientific Name
Celtis occidentalis
Hackberry trees go about their utilitarian business with an understated efficiency. Their serrated leaves are somewhat reminiscent of elms. The flowers are not showy, but they are followed by small dark orange fruits, about ½ inch long or less, that turn dark purple by early fall and are readily eaten by birds. The fruits are also edible by humans, though they contain little flesh.
Hackberry trees have a unique bark usually covered with very hard, crusty longitudinal ridges, almost giving the impression of cast concrete. Native to the U.S. Midwest, hackberry performs well in Denver as it is quite tolerant of cold, heat, drought and alkaline soil. It is a relatively fast-growing tree and can eventually reach 75 feet in height (the state champion is about that tall), though 60 feet is more typical. Hackberry can be one of the earlier trees to leaf out in Denver, but its foliage is not especially resistant to subfreezing temperatures. As a result, a late spring freeze can sometimes kill the young leaves. Fortunately, the trees usually produce a second set within a couple of weeks, with little apparent damage.
Hackberry also provides an example of how a tree species can sometimes be identified by the insects that attack it. By midsummer in Denver, many leaves will be affected by the hackberry nipple gall insect, which lays eggs that cause blister-like bumps on the undersides of the leaves. The developing larvae live within these galls and emerge as adults in the fall. Because the insects are so small, they are seldom noticed, and fortunately the galls do not cause significant harm to the tree.
Hackberry is also susceptible to a condition known as “witch’s broom,” which forms bushy clusters of chaotic shoots that look almost nest-like in the trees’ upper canopies. These are revealed along with the trees’ otherwise fine branching pattern after the leaves fall. Witch’s broom on hackberry — and on some other tree species — may result from disruptions in the tree’s hormonal regulation or from injuries caused by insects, fungi, viruses or even improper pruning. These growths generally do not pose a serious threat to the tree’s health.