Thornless Honeylocust or Honeylocust

(Fabaceae [also known as Leguminosae] - Pea Family)

Form

• large shade tree
• maturing at about 60' tall by 40' wide, although it can get much larger when grown in open, favorable situations
• thornless cultivars have an upright oval or rounded growth habit
• the thorny species form, when found in open areas, has a growth habit that varies from rounded to narrow upright oval, but usually develops a characteristically flat-topped crown with age; species forms in bottomland woods often have an irregular shape as they search for clearings of light
• rapid growth rate, often with a second flush of growth in Summer

Culture

• full sun to partial sun (tolerant of partial shade in youth)
• prefers moist, deep soils in full sun, but is very adaptable to many adverse conditions and is very urban tolerant (especially to heat, drought, poor soils, soils of various pH, soil compaction, flooding, and Winter salt spray)
• cultivars are propagated primarily by cuttings grafted onto seedling rootstock; the species form is easily propagated by seeds
• Pea Family, with several cosmetic foliage problems (including leaf spot and spider mites), but also subject to some very serious diseases (trunk, bark, and twig cankers) and pests (foliage webworms and trunk borers)
• abundant availability (including numerous cultivars) in ball and burlap form, almost always with a single leader, but rarely sold in multi-trunked (clump) form
• Thornless Honeylocust is somewhat sensitive to being transplanted in Autumn, and care should be taken to amend the soil, fertilize, water thoroughly, mulch adequately, and avoid Winter salt spray, to enhance survival chances during the first Winter

Foliage

• alternate, with pinnately and bipinnately compound leaves often occurring on the same tree, to 10" long
• each rachis base is swollen, attaching to the young zig-zag stems and leaving a proportionately large scar
• the numerous leaflets are miniature, resulting in the tree casting a light shade (also known as filtered shade or dappled shade) that permits shade-tolerant turfgrass and partial shade perennials to grow underneath the tree
• fall color is normally chartreuse, but is a beautiful golden-yellow in good years

Flowers

• polygamo-dioecious (male and female flowers are found on separate trees, but all trees also have perfect [that is, male plus female] flowers as well)
• pendulous male catkins and smaller female inflorescences are not particularly showy, and are often lost in the expanding foliage
• golden-green, occurring in late May and early June

Fruits

• most thornless cultivars have little fruit set; however, some occasionally have tremendous fruit set, being twisting pods up to 1.5' long, changing from yellow green to dark brown-red to purplish at maturity, containing many hard seeds
• fruits abscise in late Autumn or throughout the Winter (the common name of Honeylocust comes from the sweetness of the fleshy pod) and cause quite a litter problem when borne in abundance
• the thorny native species usually has moderate to abundant fruit set

Twigs

• zig-zag and brown-red, somewhat streaked, with no terminal bud
• the species form has single 1.5" long thorns found mostly on twigs located on the lowermost or interior branches (produced when the tree is in its juvenile growth phase); with maturity comes flowering, fruiting, and twigs that are essentially thornless, in the middle and upper portions of the canopy

Trunk

• often relatively straight for 5' to 15' in height, then dividing into several main trunks
• young bark is smooth, tan-olive to gray-olive, heavily lenticeled, and rather thin, but soon becomes fissured, forming distinctly flared gray plates with an orange interior bark, eventually becoming somewhat scaly with great age
• the species form has branched thorns on its trunk (large and intimidating aggregate thorns) and lower branches (tripartite thorns)

ID Summary

• alternate foliage is pinnately to bipinnately compound, composed of many small leaflets of fine texture that cast a dappled shade, and often having chartreuse fall color (but golden in good years)
• stems are zig-zag, sparsely branched in youth but becoming dense with age, from olive-brown branches that are smooth and lenticeled when young, but becoming gray, heavily flared, and ridged with age with an orange interior bark, eventually forming flaky plates on very old trunks
• canopy is upright oval to rounded on most cultivars and without a central leader, but is either irregular or flat-topped on the species form, the latter of which also has long compound thorns on the trunk, tripartite thorns on the young branches, and single thorns in the young canopy, and often has heavy fruit set as long, twisting, prominent pods

USAGE

Function

• shade or specimen tree; in either case, its extremely fine-textured, filtered shade is almost unique amongst commercially available large trees, allowing plants that prefer partial shade conditions (certain turfgrasses, shrubs, and perennials) to be planted underneath that normally could not grow beneath other shade trees
• often used, along with Green Ash (Fraxinus pennsylvanica), as a shade tree for extreme urban stress areas (such as parking lot islands and sidewalk tree squares); in these areas, the stresses encountered include restricted root zones, poor rocky clay soils, alkaline soils, intense heat, drought, high light reflectance, Winter salt spray, and trunk/branch wounding)

Texture

• fine texture when in foliage but bold texture when bare
• when young, open density in foliage and when bare
• when mature, average to thick density in foliage and when bare

Assets

• fine-textured foliage
• filtered or dappled shade
• very urban tolerant
• rapid growth and establishment
• miniature leaflets and rachises of the disintegrating leaves minimize fall cleanup (assuming that fruit pods are not present)
• wet site or dry site tolerant
• moderately ornamental bark

Liabilities

• surface roots can buckle pavement or sidewalks and also cause a serious mowing problem, even at a fairly young age (about 15 years minimum)
• significant diseases (including cankers) and pests (including borers and webworms) can be cosmetic and/or life-threatening
• fruit pods (if present) present a fruit litter problem in late Autumn and throughout the Winter as they abscise
• thorns (for the species form) on the trunk and lower branches present a personal safety hazard; rarely, basal sprouts from cultivars grafted onto the species rootstock may emerge with prominent thorns on their vigorous stems

Habitat

• zones 4 to 9
• native to floodplains of the Eastern, Midwestern, and Southern United States

SELECTIONS

Alternates

• urban-tolerant shade trees with rapid growth and establishment (Acer x freemanii, Fraxinus pennsylvanica, Platanus occidentalis, Populus deltoides, Quercus shumardii, etc.)
• trees with filtered shade (Amelanchier, Betula species, Gymnocladus dioicus [to some extent], Malus, Robinia pseudoacacia, etc.)
• shade trees for wet or dry sites (Alnus glutinosa, Fraxinus pennsylvanica, Nyssa sylvatica, Platanus occidentalis, Salix alba, etc.)

Variants

• many cultivars exist, all of which are thornless, and differing thereafter by differences in mature height and spread, growth habit, medium or dark green Summer foliage color, dependability of golden-yellow Autumn foliage color, and relative fruitlessness
• Gleditsia triacanthos inermis 'Halka' - upright but with many horizontal branches, for a rounded mature habit of 45' tall by 45' wide; vigorous growth in youth for rapid establishment
• Gleditsia triacanthos inermis 'Imperial' - compact cultivar, maturing at 30' tall by 30' wide, with very dense shade for a Honeylocust, perhaps the best to use as a street tree
• Gleditsia triacanthos inermis 'Moraine' - original thornless cultivar, maturing at 45' tall by 45' wide, having greater resistance to webworms
• Gleditsia triacanthos inermis 'Shademaster' - vigorous, vertical growth, vase-shaped in youth and middle age, maturing at 45' tall by 35' wide
• Gleditsia triacanthos inermis 'Skyline' - vertical growth with a central leader throughout most of its life, pyramidal in shape, maturing at 45' tall by 40' wide; perhaps the most common cultivar
• Gleditsia triacanthos inermis 'Sunburst' - new growth emerges lemon yellow in Spring, gradually changing to medium green, with occasional flushes of colorful new growth throughout the Summer, maturing at 35' tall by 35' wide, but especially prone to canker and borers

NOTES

Translation

• Gleditsia is named after Gottlieb Gleditsch, a German botanist of the 18th century.
• triacanthos translates as "three-spined", referring to the native Thorny Honeylocust, which has aggregate thorns on its trunk, three-spined thorns on its branches, and single thorns on its stems.
• Many times cultivars of Thornless Honeylocust are listed under the scientific classification of Gleditsia triacanthos inermis (as listed throughout this text; the variety inermis translates as "thornless"). However, most of these cultivars are actually derived from buds or stem cuttings taken from the upper canopy of physiologically mature trees (that is, they have reached the point in their life cycle where they can flower and fruit in a reproductive growth phase). Cuttings taken from the upper, thornless portions of otherwise thorny-at-the-base mature trees that demonstrate a desirable trait are then grafted onto seedling rootstocks. In actuality, however, they are usually the physiologically mature forms of Thorny Honeylocust, Gleditsia triacanthos.

Purpose

• Thornless Honeylocust is an extremely urban tolerant, rapidly growing, fine-textured, filtered-shade tree in Summer and bold-textured specimen tree in Winter.

Summary

• Gleditsia triacanthos inermis is an urban tolerant tree, excellent for its filtered Summer shade and majestic Winter silhouette coupled with its bold texture.