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Honey locust

The honey locust (Gleditsia triacanthos), also known as the thorny locust or thorny honeylocust, is a deciduous tree in the family Fabaceae, native to central North America where it is mostly found in the moist soil of river valleys. Honey locust trees are highly adaptable to different environments, and the species has been introduced worldwide. Outside its natural range it can be an aggressive, damaging invasive species.

Description

The honey locust, Gleditsia triacanthos, can reach a height of 20–30 m (65–100 ft). They exhibit fast growth, but live a medium life span, as long as 125 years. The leaves are pinnately compound on older trees but bipinnately compound on vigorous young trees. The leaflets are 1.4–3.6 cm (1⁄2–1+1⁄2 in) long when compound and very slightly smaller when bipinnate. The leaves are green in summer and turn yellow in autumn in shades ranging from cream and tan to golden yellow. Honey locusts leaf out relatively late in spring, but generally slightly earlier than the black locust (Robinia pseudoacacia).

The strongly scented flowers appear in late spring. Each cluster is a raceme 3–7 centimeters long with many tiny greenish-yellow to greenish-white flowers. The trees are polygamous-dioecious: many trees have only pollen producing flowers or seed producing flowers (strictly dioecious), but some will have both types of flowers in separate clusters, though usually one type will predominate.

The fruit of the honey locust is a flat pod (a legume) that matures in early autumn and is often twisted or curved. The average size of the pods is 7–35 cm (3–14 in) long and 1.5–3 cm (1⁄2–1+1⁄4 in) wide. Once ripe the pod will contain as many as twenty dark brown oval seeds, each about 2 cm (1 in) long. Surrounding the seeds is a soft, gooey pulp with a slightly sweet flavor. Pods may be produced from mid-September through mid-October in its native habitat.

Honey locusts commonly have thorns 6–10 cm (2+1⁄2–4 in) long growing out of the branches and trunk, some reaching lengths of 20 cm (8 in); these may be single, or branched into several points, and commonly form dense clusters. The thorns are modified branches and occasionally sprout leaves. Thornless forms are occasionally found growing wild and are commonly available as nursery plants.

Taxonomy

Honey locust was given its scientific name of Gleditsia triacanthos by Carl Linnaeus in 1753 in his book Species Plantarum. Its taxonomic history is quite complex with a total of 41 names that are taxonomic synonyms as of 2024 including five botanical forms, fourteen varieties, and twenty-two species. Included in its synonyms by Plants of the World Online, World Flora Online, and the USDA Natural Resources Conservation Service PLANTS database is the subordinate taxon Gleditsia triacanthos var. inermis. However, this scientific name is still found in gardening websites and books to distinguish thornless trees.

Hybridization of honey locust with water locust (Gleditsia aquatica) has been reported.

Names

The genus Gleditsia is named in honor of Johann Gottlieb Gleditsch, the director of what has become the Berlin Botanical Garden and Botanical Museum. The species name is derived from Greek and means "three thorns". The common name most often used for the species is "honey locust". This name comes from the slightly sweet pulp that surrounds the seeds in the tree's pods and their resemblance to the pods of the carob or "locust tree" from the middle east. Honey locust is also used for the genus as a whole or for other species in it. The first recorded use of the name was in 1709 by John Lawson in his account A New Voyage to Carolina. In the late 1800s honey locust was sometimes used as an alternate name in localized areas for other species such as for mesquit (Prosopis juliflora) in Texas and New Mexico, for black locust (Robinia pseudoacacia) in Minnesota, and for clammy locust (Robinia viscosa) in New York and New Jersey. Varients on this name include "common honey locust", "honeylocust", and "thornless honey locust".

Though honey locust is the most commonly used name for the species in English, it has a variety of other names. Some of these many names include "honey shucks locust", "sweet bean tree", "sweet locust", "thorny locust", and "green locust tree". In South Africa it is called "driedoringboom", driedoring-gleditsia", soetpeulboom", "springkaanboom", or "leoka".

In the late 1800s it was known by additional local names including "black locust" (MS, TX, AR, KS, NB), "three-thorned acacia" (MA, RI, LA, TX, MI, NB, Onterio), "thorntree" (NY, IN, LA), "thorny acacia" (TN), and "piquant amourette" (LA).

Range and habitat

The native range of the honey locust is widely agreed to be from northern Mexico through the Gulf Coast of the United States, northwards into the Midwest, parts of the US east coast, and the southernmost parts of Canada. In Canada it is a rare forest species found in southern Ontario near Lake Huron, Ontario, or Erie. However, Plants of the World Online (POWO) lists it as introduced to Ontario while NatureServe and World Plants list it as native. It is listed as native to Nova Scotia by World Plants, but as introduced by POWO and not recorded by NatureServe. It is also recorded by NatureServe as growing as an introduced plant on Prince Edward Island.

In the midwest it grows in very widely in Illinois, Indiana, Ohio, and Missouri and is much rarer and scattered in Wisconsin and Minnesota, North Dakota. In Kansas it grows naturally in the eastern half of the state, and at the eastern and northern edge of Nebraska along the Missouri River, a small area of South Dakota, and the southern portion of Michigan.

In the eastern United States honey locust trees are regarded as native to Connecticut, Massachusetts, New Hampshire, Rhode Island, and New York. Though the botanist Elbert Luther Little showed the range as extending naturally into Pennsylvania, NatureServe list it as introduced to that state. They likewise list it as introduced to Maine, Vermont, and New Jersey, while both POWO and World Plants list it as native to all of them.

In the American South, only NatureServe lists it as introduced to Deleware and the District of Columbia, while World Plants lists it as native to both. Plants of the World Online does not list it as growing wild in Washington, D.C. It is listed as growing natively through all the rest of the south, though Elbert Luther Little's range map does not show it growing east of the Appalachian Mountains. Also, in Florida it is only shown as growing in a few isolated areas of the Florida Panhandle.

To the west it grows throughout eastern Texas and Oklahoma, while becoming scattered and isolated to the west. It is listed as introduced to New Mexico by POWO, but native in Wyoming, Utah, and Nevada. Contradicting this, NatureServe list it as native to New Mexico, while introduced in Colorado, Wyoming, Utah, and Nevada. Only World Plants lists it as native to many western states including Arizona, California, Colorado, Idaho, Montana, New Mexico, Nevada, Utah, and Wyoming.

In Mexico it grows in four states; Coahuila, Nuevo León, Sonora, and Tamaulipas. Trees in Mexico are much smaller than those in the center of its range, reaching a maximum size of just 12 m (39 ft) and less than 6 m (20 ft) in some populations. They are also found in isolated locations growing on south facing slopes at elevations of 1170 to 1400 meters.

World wide it has become established outside of cultivation in Europe, southern Asia, Australia, southern Africa, and in Argentina and Uruguay.

In its native range the honey locust grows in humid or subhumid climates. It grows best in soils that are organically rich and moist, but well-drained. However, it is tolerant of a wide range of soil conditions. It is intollerant of shade and is a minor componant of forests.

Ecology

The sweet pulp in honey locust seed pods is attractive as a food for many animals including cattle, deer, rabbits, squirrels, and hares. White-tailed deer are known to disperse the seeds of honey locust by consuming the pods and passing the seeds in their dung. It is likely that deer move the seeds over one kilometer from where they are eaten, though probably not more than three killometers and aid the tree in reaching new habitats and maintaining its wild population even in fragmented habitats. The seeds themselves are consumed by crows in the winter.

The size and number of thorns on the honey locust are thought to have evolved to protect the trees from browsing Pleistocene megafauna, including mastodons, which may also have been involved in seed dispersal.

The seeds of the honey locust are resistant to sprouting without damage to the seed coat. In controlled experiments only 5% of the seeds sprout without treatment. In comparison seeds soaked in concentrated sulfuric acid for one and a half hours increased germination to 68% and two and a half hours increases it to 98%.

The honey locust moth (Syssphinx bicolor) feeds on honey locust and Kentucky coffee trees while a caterpillar. The first brood of the moths emerge from hibernation in the ground in the late spring. The green larvae have several horns on the backs and reach full size in about three weeks. When they reach full size they pupate in the soil. There may be two or three broods in a year.

Honey locust trees are a frequent host for the parasitic plant American mistletoe (Phoradendron leucarpum), but usually is not infected by large numbers of them and without suffering obvious damage.

Cultivation

Due to the honey locust's tolerance of urban problems such as salt spray, compacted soils, poor aeration, constrained planting areas, and pollution, it has been widely planted in cities. In addition it will adapt to relatively dry conditions and either alkaline or acidic soils. Once established it is also drought tollerant, though it grows best with good moisture.

However, due to its wide planting many problems have been discovered. Like maples and oaks, honey locust is particularly vulnerable to Ganoderma root rots, a fungal infection of the roots and lower trunk of the tree. Thornless cultivars are also particularly vulnerable to the asian mimosa webworm. Though healthy trees are able to withstand one or two years of complete defoliation, stressed trees may be killed.

Cultivars with narrow upright growth habit, such as Northern Sentinel, are especially prized as street trees. It is resistant to spongy moths. Spider mites, cankers, and galls are a problem with some trees. Many cultivated varieties do not have thorns.

Agriculture

The species is a major invasive environmental and economic weed in agricultural regions of Australia. The plant forms thickets and destroys the pasture required for livestock to survive. The thickets choke waterways and prevent both domestic and native animals from drinking and also harbour vermin. The spines cause damage to both people and domestic and native wildlife and puncture vehicle tires. In much of the Midwest of the United States the honey locust is also considered a weed tree and a pest that establishes itself in farm fields. In other regions of the world, ranchers and farmers who employ monocropping deem honey locust a nuisance weed; its fast growth allows it to out-compete grasses and other crops.

Uses

Food

The pulp on the inside of the pods is edible (unlike the black locust, which is toxic) and consumed by wildlife and livestock.

Despite its name, the honey locust is not a significant honey plant. The name derives from the sweet taste of the legume pulp, which was used for food and traditional medicine by Native American people, and can also be used to make tea. The long pods, which eventually dry and ripen to brown or maroon, are surrounded in a tough, leathery skin that adheres strongly to the pulp within. The pulp—bright green in unripe pods—is strongly sweet, crisp and succulent in ripe pods. Dark brown tannin-rich beans are found in slots within the pulp. Likewise, its edible seed has nutritional potential, and the flour made from its cotyledons constitutes a food source with various potential uses for pastry and bakery, among other gastronomic uses.

Timber

Honey locusts produce a high quality, durable wood that polishes well, but the tree does not grow in sufficient numbers to support a bulk industry. However, a niche market exists for honey locust furniture. It is also used for posts and rails because of the dense, rot-resistant nature of the wood. The heartwood of honey locust is reddish brown while the sapwood is pale yellow. It is strong, but has a coarse grain texture.

Nitrogen fixation

The ability of Gleditsia to fix nitrogen is disputed. Many scientific sources state that Gleditsia does not fix nitrogen. Some support this statement with the fact that Gleditsia does not form root nodules with symbiotic bacteria, the assumption being that without nodulation, no nitrogen fixation can occur. In contrast, many popular sources, permaculture publications in particular, claim that Gleditsia does fix nitrogen but by some other mechanism.

There are anatomical, ecological, and taxonomic indications of nitrogen fixation in non-nodulating legumes. Both nodulating and non-nodulating species have been observed to grow well in nitrogen-poor soil with non-nodulating legumes even dominating some sites. The litter and seeds of non-nodulating species contain levels of nitrogen higher than non-legumes and sometimes even higher than nodulating legumes growing on the same site. How this happens is not yet well understood but there have been some observations of nitrogenase activity in non-nodulating leguminous plants, including honey locust. Electron microscopy indicates the presence of clusters around the inner cortex of roots, just outside the xylem, that resemble colonies of rhizobial bacterioids. These may well constitute the evolutionary precursors in legumes for nitrogen fixation through nodulation. It is not known whether the non-nodulating nitrogen fixation, if it exists, benefits neighboring plants as is said to be the case with nodulating legumes.

Research

In research using databases, more than 60 phytochemicals were identified from honey locust, including polyphenols, triterpenes, sterols, and saponins, with in vitro studies assessing for possible biological activity.