Hackberry (Celtis occidentalis)

Common hackberry (Celtis occidentalis) is a medium-sized deciduous tree, native to North America. It looks similar to elms and some other hackberry trees at first glance, but you can distinguish it by looking at the bark – if it is verrucous and has corky texture, then it is probably a common hackberry.

I. Appearance and Characteristics

Celtis occidentalis, commonly known as the common hackberry, is a large deciduous tree native to North America. It is also known as the nettletree, sugarberry, beaverwood, northern hackberry, and American hackberry. It is a moderately long-lived hardwood with a light-colored wood, yellowish gray to light brown with yellow streaks.

The common hackberry is a medium-sized tree, 9 to 15 metres (30 to 50 ft) in height, with a slender trunk. In the best conditions in the southern Mississippi Valley area, it can grow to 40 metres (130 ft). It has a handsome round-topped head and pendulous branches. It prefers rich moist soil, but will grow on gravelly or rocky hillsides. The roots are fibrous and it grows rapidly. In the western part of its range, trees may still grow up to 29 m (95 ft). The maximum age attained by hackberry is probably between 150 and 200 years in ideal conditions.

The bark is light brown or silvery gray, broken on the surface into thick appressed scales and sometimes roughened with excrescences; the pattern is very distinctive. The remarkable bark pattern is even more pronounced in younger trees, with the irregularly-spaced ridges resembling long geologic palisades of sedimentary [layered] rock formations when viewed edge-wise [cross-section]. Coins as large as U.S. quarters can easily be laid flat against the valleys, which may be as deep as an adult human finger.

The branchlets are slender, and their color transitions from light green to red brown and finally to dark red-brown. The winter buds are axillary, ovate, acute, somewhat flattened, one-fourth of an inch long, light brown. The bud scales enlarge with the growing shoot, and the innermost become stipules. No terminal bud is formed.

The leaves are alternately arranged on the branchlets, ovate to ovate-lanceolate, often slightly falcate, 5–12 cm (2–4+3⁄4 in) long by 3–9 cm (1+1⁄4–3+1⁄2 in), very oblique at the base, with a pointed tip. The margin is serrate (toothed), except at the base which is mostly entire (smooth). The leaf has three nerves, the midrib and primary veins prominent. The leaves come out of the bud conducive with slightly involute margins, pale yellow green, downy; when full grown are thin, bright green, rough above, paler green beneath. In autumn they turn to a light yellow. Petioles slender, slightly grooved, hairy. Stipules varying in form, caducous.

The flowers are greenish and appear in May, soon after the leaves. They are polygamo-monœcious, meaning that there are three kinds: staminate (male), pistillate (female), perfect (both female and male). They are born on slender drooping pedicels.

The calyx is light yellow green, five-lobed, divided nearly to the base; lobes linear, acute, more or less cut at the apex, often tipped with hairs, imbricate in bud. There is no corolla.

There are five stamens, which are hypogynous; the filaments are white, smooth, slightly flattened and gradually narrowed from base to apex; in the bud incurved, bringing the anthers face to face, as flower opens they abruptly straighten; anthers extrorse, oblong, two-celled; cells opening longitudinally.

The pistil has a two-lobed style and one-celled superior ovary containing solitary ovules.

The fruit is a fleshy, oblong drupe, 1⁄4 to 3⁄8 in (0.64 to 0.95 cm) long, tipped with the remnants of style, dark purple when ripe. It is borne on a slender stem and ripens in September and October. It remains on the branches during winter. The endocarp contains significant amounts of biogenic carbonate that is nearly pure aragonite.

II. How to Grow and Care

Sunlight

While hackberry can also grow in partial shade, it should be planted in a location where it gets at least 6 hours of full sun every day.

Temperature and Humidity

Hackberry tolerates both hot, windy conditions and cold winters in northern locations. It is adaptable to pollution and salt, making this plant suitable for urban and suburban yards and streets. It is not known to be negatively affected by high humidity levels.

Soil and Water

Hackberry grows best in moist, well-drained organically rich soil with a pH between 5.0 and 8.0 but it is quite adaptable. The tree also tolerates a wide range of wet and dry soil conditions, including clay and limestone—in fact, the tree often naturally occurs in limestone soil with a high pH.

Watering is only required during the first year. The established tree is considered drought-tolerant.

Fertilizing

An annual springtime application of a granular balanced fertilizer is only required if the soil is poor in nutrients. For the amount to use, follow product label directions. Otherwise, hackberry does not need fertilizer.

Propagation

While the hackberry can be grafted or grown from cuttings, your best chance at success is to grow it from seed, or transplant a small sapling.

• From Seed

When propagating C. occidentalis indoors, gather fallen berries in the autumn and either allow them to air dry, or soak them and scrape the pulp off the seeds.

In the winter, place the seeds in a clear plastic bag containing soil that is moist, but not wet. Label the bag with the date.

Keep the bag in the refrigerator for 60 to 90 days. Once this period is complete and the seeds have started to split in the spring, plant the seeds in their permanent location about half an inch deep.

Cover them in a layer of mulch to prevent squirrels or other animals from digging them up.

If you live in a location that gets at least 60 to 90 days of cold weather with temperatures of 38°F or lower, you can direct sow hackberry seeds in their permanent location outdoors.

Keep the soil moist during germination and water the seedlings every week for the first year or two.

• Transplanting

Hackberry saplings and seedlings respond well to transplanting. You can find healthy specimens both as bare root transplants, which are usually smaller saplings, or larger transplants with a root ball and the surrounding soil wrapped in burlap.

C. occidentalis can spread 25 to 45 feet at maturity, so be sure to keep appropriate spacing in mind when you plant. Consider the distance from other trees, fences, buildings, and power lines, as well as other plants and structures.

For saplings, dig a hole as deep as the root ball and twice as wide, stand the tree up straight in the hole, then fill the hole in with soil, tamping down with your feet every so often to make sure the soil settles in well to support it.

The more tender seedlings will need to be hardened off before planting. Bring them outside in mid- to late summer, then bring them back inside before winter.

Early the next spring, repot them in bigger containers, and repeat the process for one more year. The next spring, your hackberry seedlings can be planted outside in their permanent location.

Water your hackberry transplant every week to help it establish a healthy root system. Once your tree is strong and established, it will rarely need supplemental water.

Pests and Diseases

Most of the pests and diseases that may plague the hackberry are usually pretty harmless, but keeping an eye out for them can prevent issues down the road.

Common Pests

While there are a few pests that may affect this species, infestations aren’t usually life-threatening and can often be remedied by spraying with neem oil or water from the hose.

• Aphids

The Asian wooly hackberry aphid, also known as Shivaphis celti, is an insect that sucks sap. It produces large amounts of a sticky honeydew, which can lead to sooty mold growth on leaves as well as surfaces and other plants growing beneath affected trees.

This is an issue if the tree is growing over sidewalks or parking lots in particular, as it makes a mess and causes mold issues. For this reason, the hackberry has been placed on some cities’ “do not plant” lists.

In the event of an aphid infestation, spraying neem oil on the leaves can help remedy the situation.

• Mites

Eriophyid mites can cause galls, blisters, and curled and warty leaves, and infestations can lead to the formation of witches’-broom.

Many of these problems will not harm your hackberry, but they do affect the tree’s appearance. Mite infestations can be helped by spraying the leaves with a hose nozzle set to high power.

Mourning Cloak Butterflies

The mourning cloak butterfly, Nymphalis antiopa, will lay eggs in the hackberry tree, and the caterpillars that hatch out eat the leaves.

If they are consuming enough leaves that it negatively affects the look of your tree, you might consider applying a pesticide to combat large numbers of caterpillars.

• Scale

Scale insects also produce honeydew, which may result in messy mold growth. Scale infestations can significantly weaken trees, leaving them more susceptible to other diseases and stressors as well.

To combat scale, horticultural oil can be sprayed on the affected areas.

Common Disease

Most diseases that affect hackberry trees do not cause harm, and are mainly just aesthetic concerns.

• Galls

Hackberry nipple gall can be caused by a few different types of insects, including psyllids, midges, and other gallmakers. Galls are bumps found on the underside of leaves.

These are usually just an aesthetic issue, a physiological ailment as opposed to a sign of infectious disease, though they can cause leaves to drop and interfere with photosynthesis in severe cases.

Insecticides can help to prevent pests from forming galls, but once formed, the effect on the leaves cannot be reversed.

• Leaf Spot

Leaf spot is a fungal disease that presents as discolored spots on leaves. This can be prevented with a fungicidal foliar spray.

If your hackberries are affected, make sure the leaves are raked away at the end of the season.

The following spring, fungicidal leaf spray should be applied as the buds are beginning to break, and then again two or three more times through the spring in seven- to 14-day intervals.

• Powdery Mildew

Powdery mildew, a fungal disease caused by Sphaerotheca phytophylla, can lead to the formation of witches’-broom.

Though not normally incredibly harmful to your tree, the fungus can be prevented and/or cured by pruning the canopy a bit to allow for better air circulation.

If your hackberry is affected by powdery mildew, hold off on adding any fertilizer, thin out some branches and remove any affected by the fungus, and make sure the infected branches are burned or tossed in the trash – not put on your compost pile.

• Rot

Ganoderma rot is caused by a fungus that enters through wounds on the tree. This can happen, for example, if it’s cut by a weed-whacker or nicked by a lawnmower.

Adding mulch around the base of your hackberry can help protect it from damage, as well as taking care not to damage the trunk when maintaining your lawn.

In the instance of rot, the tree will begin to decline rapidly. Mushrooms may start to grow near the base, and these fruiting bodies of Ganoderma fungi are a sure sign that your tree is rotting.

If the hackberry has rotted, there’s a significant risk that it will not be able to withstand a wind storm and will need to be removed in the name of safety.

• Sooty Mold

Sooty mold is a fungal disease that commonly crops up when pests leave honeydew behind. It covers the leaves and stems and can be quite messy.

This fungus can be cleaned up by thoroughly spraying insecticidal soap on the affected leaves and branches. Repeat this process as often as needed, until the sooty mold has been washed away and your hackberry leaves look clean once more.

An occasional spray of soapy water can also prevent sooty mold from developing.

• Witches’-Broom

Witches’-broom is a common cosmetic problem, another physiological ailment that is usually the result of a powdery mildew infection or mite infestations. It is a deformity where a dense cluster of branches grows from a single point.

These do not harm the hackberry, but if you don’t like their appearance, pruning any affected branches will solve the problem. Be sure to address the root cause to prevent further spread and future issues.

Pruning and Maintenance

In addition to raking up leaves and cleaning up dropped berries as needed, pruning is really the only maintenance your hackberry will need.

You can cut out small branches year round, but you should hold off on pruning larger branches until the dormant period in the late fall or winter, after the leaves have fallen.

Remove low-hanging, weak, or broken branches, and branches that touch or intersect with others.

When pruning, make a clean cut outside the branch collar. For large branches, under-cut the branch several inches from the trunk. Then overcut, undercut, and overcut, until the stub is at the branch collar.

This method will prevent you from accidentally tearing the tree past the branch collar, exposing your hackberry to infection.

III. Uses and Benefits

Hackberry’s wood is light yellow; heavy, soft, coarse-grained, not strong. It rots easily, making the wood undesirable commercially, although it is occasionally used for fencing and cheap furniture.

Hackberry is only occasionally used as a street or landscape tree, although its tolerance for urban conditions makes it well suited to this role. Sombor in Serbia and Bratislava, the capital of Slovakia, are known for the extensive use of hackberry (in the latter case along with closely related but Eurasian Celtis australis) as a street tree.

The tree’s pea-sized berries are edible, ripening in early September. Unlike most fruits, the berries are remarkably high in calories from fat, carbohydrate, and protein, and these calories are easily digestible without cooking or preparation. Omaha Native Americans ate the berries casually, while the Dakota used them as a flavor for meat, pounding them fine, seeds and all. The Pawnee also pounded the berries fine, added a little fat, and mixed them with parched corn.