Gleditsia triacanthos (honey locust)

This species is a thorny, woody legume tree that has a tendency to be an aggressive colonizer. Root suckers and abundant seed production gives an ability to rapidly form dense, impenetrable stands. Thornless clones are still widely planted for ornamental purposes. G. triacanthos is an invasive weed in Australia and the USA, notably in the states of Queensland, Califormia and New York. It appears to require warm (temperate or mediterranean) climates with moist (moist semi-arid to sub-humid) conditions to become invasive, though actual requirements are far from clear, and human influences are also probable.

G. triacanthos is one of 14 species in the genus Gleditsia (family Fabacaeae, subfamily Caesalpiniodeae). There are many ornamental cultivars of G. triacanthos. The US common name (honey-locust) derives from the Christian tradition: it is reported that John the Baptist fed on 'locusts' while he was living in the desert. According to one interpretation they could have been carob (Ceratonia siliqua) fruits; therefore, the carob-like American species gained the common name of honey or sweet locust.

G. triacanthos is a medium-sized tree, in natural stands usually growing to 21-25 m, with a dbh of 60-100 cm; as an exotic in India it grows up to 45 m tall, and 180 cm dbh. It has attractively-arranged branches ascending in the lower part of the stem and horizontal in the upper one, resulting in an umbrella-like crown form. The crown is variable in shape, from narrow and plume-like to moderately wide. The stem form is straight, with a round cross section, often short and divided near the ground. The bark of mature trunks is usually 0.6-3.5 cm thick with narrow ridges divided by fissures, peeling in strips. Branches are densely armed with reddish, simple or compound stout thorns, flattened at the base. The roots are fibrous and thick and form a deep, widely spreading and profusely branched root system, with a strong taproot. This generalized root system is adaptable to environmental conditions and enables G. triacanthos to grow on both upland and lowlands sites. For example, young saplings on upland clay soils can produce root systems that are about twice as long as those of older trees growing in lowland alluvial soils with higher water tables (Fowells, 1965). Deep soils can be penetrated as far as 3-6 m. Leaves are pinnate (20-30 leaflets) or bipinnate in the same tree. Leaflets are sessile, acute, ovate, 2-3 cm long, finely crenate and bright green, turning golden-yellow before leaf fall. G. triacanthos is polygamo-dioecious. The flowers are small, greenish, actinomorphic, borne in axillary, racemes of staminate flowers, 5-7(-13) cm long, pubescent, and often clustered. The calyx is campanulate, with five elliptic-lanceolate lobes; there are 4-5 petals, erect, oval, and longer than the calyx lobes; and up to 10 stamens, inserted on the calyx tube. The pistil is rudimentary or absent in the staminate flowers. Pistillate racemes are 5-8 cm long, slender, with few flowers, and usually solitary. The pistils are tomentose, the ovary nearly sessile, and the style short; there may be two ovules or many. The stamens are much smaller and abortive in pistillate flowers. Seeds, borne in long (15-41 cm), flat, indehiscent, sickle-shaped and often twisted pods, 30-40 cm long and glossy dark brown or purplish-brown with sweet, edible pulp. Seeds are oval, dark brown and 7-8 mm long.

The native range of G. triacanthos is North American, extending from central Pennsylvania, USA through southern Ontario, Canada, and southern Michigan, southern Wisconsin, and southeastern Minnesota to southeastern South Dakota; south through eastern Nebraska to eastern Texas; east to Alabama; and northeast along the western slopes of the Appalachians. Isolated populations also occur in northwestern Florida. G. triacanthos is naturalized east of the Appalachian mountains as far north as Nova Scotia, Canada. Scattered relic populations of G. triacanthos have also been found in Mexico (Briones, 1988).

From the 1600s, G. triacanthos was introduced to other states in the USA, and later to South America, Europe, Africa, west and South Asia.

It is likely that G. triacanthos will be further introduced internationally as a valuable ornamental and shelterbelt species. Whereas the commercially available (patented) thornless clones may pose less of a risk, thorny material from rootstocks or contamination of seed or scions may occur. G. triacanthos is a declared noxious weed in some states of Australia.

G. triacanthos is probably spread along water courses during periods of flooding. The plump pods constitute excellent feed for animals, and it is assumed that both livestock and wild animals are able to spread seed widely. Since the outbreak of Dutch elm disease, the artificial spread and planting of G. triacanthos has increased to substitute the role of elms (Ulmus spp.) as roadside trees, in agroforestry and especially as a fodder tree (Gold and Hanover, 1993; Papanastasis, 1994; Foroughbakhch et al., 1995a; Wilson, 1996; Ainalis and Tsiouvaras, 1998; Hauad et al., 1998). The thornless variety G. triacanthos f. inermis is now generally preferred for plantation use and many of the cultivars selected for roadside plantations are derived from this variety and are both thornless and fruitless, or they bear only a small amount of fruit.

G. triacanthos is native to the hardwood forests of eastern, central and southern USA, and is one of the hardiest, most adaptable and most useful tree species known there. It thrives in climates ranging from cold-temperate to subtropical within its native habitat and has been grown successfully in tropical conditions where it has been introduced. It is drought- and frost-tolerant and grows in all types of soil. It is an attractive ornamental or shade tree for hedges, shelterbelts, avenues, etc., and is thus found in urban and roadside situation. G. triacanthos can easily be grown from seed, suckers or cuttings, but has a tendency to form dense thickets and to become invasive.

GeneticsThe population genetic structure and genetic diversity of G. triacanthos has been studied by Schnabel and Hamrick (1990, 1995). In nature, the most common hybrid is Gleditsia x texana, derived from crossing G. triacanthos with G. aquatica. Other thorny cultivars, such as cv. Bujotii (= 'pendula') and cv. elegantissima (= 'columnaris') are also quoted. Many thornless (and usually fruitless) patented cultivars have been derived from G. triacanthos f. inermis, for example, cvs. Imperial, Majestic, Moraine, Rubylace, Shademaster, Skyline and Sunburst. Resistance of G. triacanthos cultivars to fungal cankers is reported by Jacobi (1989), Neely and Himelik (1989), Potter and Hartmann (1993), and Calkins and Swanson (1997). McDaniel (1980) and Scanlon (1980) cover the topic of germplasm in a SERI symposium on Tree Crops for Energy Co-production on Farms, and Foroughbakhch et al. (1997) evaluated 16 clones of G. triacanthos in France.Physiology and PhenologyG. triacanthos is a light-demanding species, colonizing bare soils by seed propagation or suckers. In mixed natural stands it behaves as a dominant tree. When coppiced, G. triacanthos resprouts abundantly and forms dense thickets. Growth is rapid (Geyer, 1989, 1993) and maturity is reached at about 12 years. The following phenological description is based on the species in its native range, taken from Blair (1990). Flowering occurs in late spring, the average date being about 10 May in the southern limit of the range and 25 June in the north. G. triacanthos leaves are nearly full grown when the flowers are produced, which is usually late enough in the year for the seed crop to escape frost damage. The species is polygamo-dioecious; flowers in axillary, dense, green racemes, ripening about mid-September in the southern portion of the range and around mid-October in the north. Soon after fruits mature they begin falling and dissemination often continues into late winter, but sometimes remaining on the tree until February (Vines, 1960; Burns and Honkala, 1990).Reproductive BiologyG. triacanthos is a polygamous species, having male, female and bisexual flowers and giving male, female and bisexual plants following the gender of the prevailing flower. Seed propagation is reliable, as good seed crops occur nearly every year, the fruits falling gradually throughout the whole winter. G. triacanthos bear seed from about 10 until 100 years of age, with optimum production occurring between 25 and 75 years. There are about 8000 seeds/kg, and the viability is high after some pretreatment. For other seed characters refer to Bonner et al. (1992) or Singh et al. (1996).Environmental RequirementsG. triacanthos thrives in its native range in climates ranging from cold-temperate to subtropical. In the western portion of its range, it grows in a subhumid climate, whereas in the middle and eastern portions the climate is humid. Typical annual precipitation is about 510 mm in South Dakota and Texas to >1520 mm in southern Louisiana, Mississippi and Alabama. The mean annual snowfall can be up to 102 cm. In the north and northeast of its range, the length of the growing season is about 150 days, increasing to >300 days in the south. G. triacanthos is tolerant of low temperatures and in the northern parts of its distribution it is hardy at -29° to -34° C, but outside of its native habitat, it has also been successfully planted in tropical climates. It is drought- and frost-tolerant (Roberts and Schnipke, 1994; Calkins and Swanson, 1998). G. triacanthos is commonly found or planted on rich, moist alluvial soils or in soils of limestone origin (alfisols, inceptisols and mollisols), near streams or lakes. Growth is poor on gravelly or heavy clay soils and G. triacanthos often fails on shallow soils. It is fairly tolerant of acid and alkaline soils, but best development is usually on soils having a pH between 6.0 and 8.0. Preliminary tests using artificially salinized soils have determined that young saplings and seed germination are not affected by salinity (for example, 0.2% sodium chloride in soil dry weight), although whether G. triacanthos can tolerate the cumulative effects of salinity over some years is unclear (Burns and Honkala, 1990). It can behave as a very hardy and drought-resistant species when planted elsewhere, but water stress can induce early leaf abscission and interact with parasites (Smitley and Peterson, 1996, 1997). The natural range of G. triacanthos is generally below 760 m elevation, although the upper limit appears to be 2200 m.AssociationsIt is commonly found on rich alluvial soils in mixed forests associated with others broadleaved trees such as oak (Quercus), ash (Fraxinus), elm (Ulmus), hickory (Carya) and maple (Acer) species, but is generally only a minor component of natural forest stands. Mesophytic species commonly associated with G. triacanthos include Acer rubrum (red maple), Diospyros virginiana (persimmon), Nyssa sylvatica (blackgum), Carya illinoensis (pecan), Acer negundo (boxelder), Gymnocladus dioicus (Kentucky coffee tree) and Juglans nigra (black walnut). The plump pods constitute excellent feed for animals. Although classed as a nitrogen-fixing species, G. triacanthos does not appear to have root nodules (Harlow et al., 1979; Allen and Allen, 1981).

Insects

The increased popularity and plantings of G. triacanthos, particularly thornless varieties, has been accompanied by a corresponding increase in numbers and species of attacking insects. In general, attacks by insects or pathogens are not fatal, but they do weaken the tree and slow growth. It is worth noting that G. triacanthos is highly resistant to attack by the polyphagous defoliator and forest pest Lymantria dispar, and was the most resistant of 17 tree species tested by Smitley et al. (1993).

The mimosa webworm, Homadaula anisocentra, is a severe and widely distributed defoliator (Miller and Hart, 1987; Bastian and Hart, 1990, 1991), but identification of a parasitoid, Elasmus albizziae, offers to potential for its use in an integrated pest management programme. Heavy infestations of the spider mite Platytetranychus multidigituli can occur in hot dry weather and severely defoliate trees. Other important defoliators include the white-marked tussock moth, Orgyia leucostigma, the honey-locust plant bug, Diaphnocoris chlorionis, and the very similar Orthotylus robiniae, the walkingstick Diapheromera femorata and the leaf hopper Empoasca pergandei. The gall midge pest Dasineura gleditchiae appears to cause substantive damage in some regions and is undergoing detailed study in California, USA (Thompson et al., 1998), Spain (Estal et al., 1998), Serbia (Simovich-Tosic and Skuhrava, 1995), Hungary, Poland and elsewhere in Europe. Another defoliator, the white flannel moth caterpillar, Norape ovina, is an additional problem on amenity trees as the caterpillars can also 'sting' humans (Hyche, 1997). The aphid Aphis craccivora has also been reported from Argentina (Rossa et al., 1993), and a new species of Tortricidae, Lobesia triancanthis, from Madgascar (Diakonoff, 1992) and a new species of Buprestidae from the USA, Actenodes davidi (Nelson, 1979), though the severity of their effects on G. triacanthos is not reported.

A number of seed-feeding insects are also known to attack G. triacanthos, notably members of the Bruchidae family, common pests of legume seeds. Amblycerus robiniae was collected from seeds of G. triacanthos in New Mexico, USA (Pfaffenberger, 1979). The tropical bruchid Megabruchidius tonkineus was found in Hungary for the first time in 2002 and is considered to be able to over-winter there, at least in urban surroundings (Jermy et al., 2002). Agrilus difficilis (Buprestidae) is a noted pest of G. triacanthos in Ohio, USA (Akers et al., 1986).

Diseases

G. triacanthos does not suffer from any serious diseases. Stem cankers are caused by Nectria cinnabarina (Bedker and Blanchette, 1983, 1984) and Thyronectria austroamericana (Jacobi, 1989; Neely and Himelik, 1989; Jacobi, 1992; Potter and Hartmann, 1993) are generally associated with wounds or at the bases of dying branches but may cause significant damage. A root rot caused by Xylaria polymorpha has been observed in Michigan (Proffer, 1988). A new and potentially damaging disease, honeylocust knot, of unspecified aetiology, has been observed and studied in Ohio, USA (Bonello et al., 2003).

Abiotic factors

Honey-locust is easily injured by fire due to its thin bark and appears to be excluded from prairies or rangelands by frequent fire. Damage to trees by poor air quality has been recorded in southeast Texas, USA. It has been reported to be highly resistant to ice damage and average in resistance to flooding in the USA (Burns and Honkala, 1990).

The increased popularity and planting of G. triacanthos, particularly thornless varieties, has been accompanied by a corresponding increase in numbers and species of attacking insects. In general, insect attacks are not fatal, but they do weaken the tree and slow growth. The mimosa webworm (Homadaula anisocentra) is a severe and widely distributed defoliator (Miller and Hart, 1987; Bastian and Hart, 1990, 1991). Heavy infestations of the spider mite Eotetranychus multidigituli can occur in hot dry weather and severely defoliate trees. Other important defoliators include the whitemarked tussock moth (Orgyia leucostigma), the honey-locust plant bug (Diaphnocoris chlorionis), the walkingstick (Diapheromera femorata) and the leaf hopper Empoasca pergandei. The gall midge pest Dasineura gleditchiae has recently been studied in California, USA (Thompson et al., 1998), Spain (Estal et al., 1998), Serbia (Simovich-Tosic and Skuhrava, 1995) and Hungary. The twig girdler, Oncideres cingulata, prunes small branches and can inflict severe injury on nursery seedlings. Heavy infestations can also severely damage large trees; the larvae of Amblycerus robiniae, a bruchid weevil, feed on G. triacanthos seed, and the female periodical cicada (Magicicada septendecim) can damage G. triacanthos, especially young transplanted trees, by depositing eggs in the twigs (Blair, 1990). G. triacanthos does not suffer from any serious diseases, though stem cankers caused by Nectria cinnabarina (Bedker and Blanchette, 1983, 1984) and Thyronectria austro-americana (Jacobi, 1989; Neely and Himelik, 1989; Jacobi, 1992; Potter and Hartmann, 1993) are generally associated with wounds or at the bases of dying branches. Damage to young honey-locust is caused by rabbits gnawing the bark and by livestock and white-tailed deer browsing.

No detailed studies of the economic impacts of G. triacanthos have been reported.

G. triacanthos has excellent characteristics for using as windbreaks and hedges on plains and rangelands. When trimmed, it resprouts strongly producing thick, thorny and impassable hedges. As a fast growing species with an attractive crown, it is also suitable for ornamental planting. The high levels of adaptation to extreme climatic conditions, rapid growth and early fruit production make G. triacanthos suitable for use in agrosilvopastural systems producing cattle forage, particularly in semiarid regions. It allows sustained and diverse forage production during periods of fodder scarcity, although thornless cultivars are recommended. When pods are still greenish and immature, they contain a sweetish substance which makes them palatable to cattle; but after maturation they become too hard and bitter to be eaten (Foroughbakhch et al., 1995; Ainalis and Tsiouvaras, 1998; Hauad et al., 1998).The sapwood is generally wide and yellowish in contrast to the reddish-brown heartwood, providing an attractive grain. The wood is dense, very heavy (700-800 kg/m³), very hard, strong in bending, stiff, resistant to shock and is durable when in contact with the soil. Although not particularly valued, it is used locally as a timber for pallets, crating and general construction, also for fence posts and as a fuelwood.Indigenous peoples of North America formerly used the thorns for many purposes. Medicinally, it is reported to be anodyne, mydriatic, narcotic and experimentally oxytocic (Duke, 1983). The pods are a folk remedy for dyspepsia and measles among the Cherokee Indians. A tea made from the bark is used to treat whooping cough. Delaware Indians used the bark for blood disorders and coughs, the Fox Indians for colds, fevers, measles and smallpox. A potable or energy alcohol can be made by fermenting the pulp, and the seeds have been roasted and used as a coffee substitute.

G. triacanthos is easily injured by fire due to its thin bark, and appears to be excluded from prairies or rangelands by frequent fire. Damage to trees by poor air quality has been recorded in southeast Texas, USA. It has been reported to be highly resistant to ice damage and average in resistance to flooding in the USA (Burns and Honkala, 1990). G. triacanthos is susceptible to triclopyr and to a mixture of picloram and 2,4,-D which are used to control weedy trees (Melichar et al., 1986). Also, three insects are proposed as biological control agents by USDA (2003), Monarthrum mali (apple wood stainer), Macropsis fumipennis (honey locust leafhopper) and Micrutalus calva (treehopper).

G. triacanthos is a light-demanding species, colonizing bare soils by seed propagation or suckers. Seed propagation is reliable as good seed crops occur nearly every year, the fruits falling gradually throughout the whole winter. Honey -locust bear seed from about 10 until 100 years of age, with optimum production occurring between 25 and 75 years. In mixed natural stands it behaves as a dominant tree. When coppiced, honey-locust resprouts abundantly and forms dense thickets. Growth is rapid (Geyer, 1989, 1993) and maturity is reached at about 120 years.

In natural stands the silvicultural management of G. triacanthos usually follows the same system adopted for the more profitable tree species with which it is growing.
Since the outbreak of Dutch elm disease, the artificial spread and planting of G. triacanthos has increased to substitute the role of elm as roadside trees, in agroforestry and especially as a fodder tree (Gold and Hanover, 1993; Papanastasis, 1994; Foroughbakhch et al., 1995a; Wilson, 1996; Ainalis and Tsiouvaras, 1998; Hauad et al., 1998). It can be profitably pollarded (Burnett and Gilluly, 1988) and when wood products are also required, G. triacanthos can be managed as a short-rotation coppice species (Geyer, 1989). The thornless variety G. triacanthos f. inermis is now generally preferred for plantation use. Many of the cultivars selected for roadside plantations are derived from this variety and are both thornless and fruitless, or they bear only a small amount of fruit.

Growth data

In eastern Nebraska, 18- to 35-year-old honey-locust plantations grew an average of 4.6 cm in diameter each 10 years. The average height growth of G. triacanthos planted in shelterbelts from North Dakota to Texas was 49 cm per year during the first 7 years (Fowells, 1965). Annual diameter growth for young trees of 8-13 mm per year has been recorded. The species is an excellent tree for windbreaks.

G. triacanthos is a polygamous species having male, female and bisexual flowers and giving male, female and bisexual plants following the gender of the prevailing flower. The population genetic structure and genetic diversity of honey locust has been studied by Schnabel and Hamrick (1990, 1995). In nature, the most occurring hybrid is Gleditsia x texana, derived from crossing G. triacanthos with G. aquatica. Other thorny cultivars, such as cv. Bujotii (= 'pendula') and cv. elegantissima (= 'columnaris') are also quoted.
Many thornless (and usually fruitless) patented cultivars have been derived from G. triacanthos f. inermis, for example, cvs. Imperial, Majestic, Moraine, Rubylace, Shademaster, Skyline and Sunburst. Resistance of G. triacanthos cultivars to fungal cankers is reported by Jacobi (1989), Neely and Himelik (1989), Potter and Hartmann (1993), and Calkins and Swanson (1997).

Germplasm

McDaniel (1980) and Scanlon (1980) cover the topic of germplasm in a SERI symposium on Tree Crops for Energy Co-production on Farms. Foroughbakhch et al. (1997) evaluated 16 clones of honey-locust in France.

The thorniness of honey-locust is a disadvantage for some uses, and the early thorniness of young seedlings can slow weeding or transplanting operations. G. triacanthos can sometimes as an aggressive colonizer in suitable conditions (Burton and Bazzaz, 1991; Csurhes and Kriticos, 1994; Ansin and Marlats, 1997).