Hackberry/Sugarberry (Celtis laevigata, C. occidentalis)
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama
The genus Celtis is a member of the Cannabaceae (hemp) family. This is a small family of flowering plants with about eleven genera and 170 species. The family has members that grow as trees, shrubs, and erect and trailing herbs. Members of the family are distributed throughout temperate regions of the Northern Hemisphere. Celtis was traditionally placed in the family Ulmaceae (elm family) but in the 1990’s was moved to the family Cannabaceae. Members of this family produce apetalous flowers (flowers that lack petals) and produce dry, one-seeded fruits. The best-known genera in this family are Cannabis (hemp), Humulus (hops), and Celtis (hackberry). Economically, the important genera are Cannabis and Humulus. Members of the genus Celtis have some economic importance and are ecologically important for numerous species of insects and wildlife. Celtis species grow as shrubs and trees and this genus has the largest number of species in the Cannabaceae family. The genus Celtis is native to the southeastern U. S.
Members of Cannabaceae are typically dioecious, but some members are monoecious or hermaphroditic. The flowers are radially symmetrical, small, greenish, and rather inconspicuous. Flowers lack petals and can be bisexual or unisexual. Fruit types seen in this family are drupe and achene.
Our native trees (Celtis) have leaves that are alternately arranged on the branchlets. The leaves are simple, with edges that are serrate (saw-like edge) or entire (no teeth on edge). The leaves are typically oblique (unequal sided or lopsided) at the base and have stipules. Each leaf has three major veins which are particularly notable on the underside of the leaf. The leaves are thin, bright green above and paler green below. The upper surface of the leaf has a rough texture. In the fall of the year the leaves turn a light, yellow color. The trunks of mature trees are usually gray and possess numerous corky ridges; described as warty protuberances. This feature makes mature hackberry trees easy to recognize.
Our native trees are monoecious or hermaphroditic and have flowers that are small, greenish and rather inconspicuous, and lack petals. The flowers may be staminate, pistillate, or perfect and are borne on slender drooping pedicels. Flowers are described as hypogynous, having a superior ovary. The calyx is five-lobed, and no corolla is present. There are five stamens. Pistillate flowers have a two-lobed style and a one-celled ovary. The fruit is a globose to oblong, dry drupe about ¼ to 3/8 inches long.
Celtis laevigata is commonly referred to as sugarberry or hackberry and is found throughout the southeastern U. S. Its range extends from Texas to the east coast and from the Gulf of Mexico to as far north as Tennessee and Missouri. The other common species of Celtis found in the eastern U. S. is C. occidentalis, common hackberry. This species of Celtis is found from southern Ontario into parts of New England and south to North Carolina and Tennessee, and west into portions of Oklahoma and South Dakota. The range of these two species thus overlaps in some regions of the southern U. S. Where the ranges overlap, C. laevigata occurs primarily in bottomland areas while C. occidentalis is primarily found in upland areas.
Lumber from hackberry trees is used in small amounts for construction of containers, dimension stock, and veneer. Most of the sugarberry lumber is used in furniture construction. Hackberry/sugarberry trees are also planted as shade trees in areas in their natural range. They do well in urban areas and its shape and interesting bark make it an attractive landscape tree.
Ecologically, hackberry/sugarberry leaves are important as a food source for various insects, for example caterpillars of the Io moth. They also serve as the home for jumping plant lice called hackberry psyllids. These tiny, cicada look-alikes lay eggs on new leaves in the spring and the nymphs which hatch become enveloped by leaf tissue forming galls that are nipple-shaped and light green in color. The nymphs feed on the new growth and emerge as adults from the galls during the fall. The trees appear to be undamaged by the galls. The pea-sized fruits of hackberry are edible and are remarkably nutritious, but the large seed inside makes eating them unprocessed rather difficult. Some Native Americans pounded the fruits into a fine meal and used them for flavoring meat or mixed them with parched corn. The fruits are used as a food source for birds who help to disperse the seeds.
Hackberry/sugarberry pollen is a mild to moderate allergen. The pollen is not known to act as a major contributor in pollinosis. The pollen is most likely to cause allergic reactions in individuals who are sensitized because of close proximity and continued exposure. The pollen grains are spheroid with aspidate pores. Grains typically are triporate or tetraporate but can have up to 10 pores depending on the size of the pollen grain. Pollen grains are markedly annulate and the pores typically show a “collar” effect due to the presence of obvious onci (thickened portion of intine below aperture). The texture of the grain is very granular and resembles grass pollen. Grains range in size from 26-50, but most often are 28-32 microns. Pollen production is most prevalent in the Southeast during March and April.
Note: This is one of a series of blogs that describes some of the common species of plants that produce pollen in our area that are known to serve as allergens. It is a good idea to know what the plants look like that produce pollen that act as allergens and try to avoid having them around where you live if possible, and to know when they are producing pollen in the area where you live.