Honey Locust Tree: Wild Thorns Reach 8 Inches, and Most Thornless Cultivars Start as Buds From the Top of a Thorny Tree

A wild honey locust (Gleditsia triacanthos) carries thorns that University of Arkansas Extension puts at 3 to 8 inches long, often in clusters on the trunk. The honey locust you can actually buy almost never does. The usual explanation is that someone found a thornless seedling. The USDA plant guide adds a stranger detail: most thornless cultivars are cloned from buds and cuttings taken off the upper, thornless part of a mature tree that is thorny lower down. The thorns are a position on the tree, not a different tree, and that is why you can propagate around them.

This guide covers what the wild tree is really armed with, where thornless cultivars come from and why a thornless parent doesn’t guarantee thornless seedlings, why grass survives under the canopy, how five common cultivars behaved in a 15-year Ohio study, and the one pest worth watching. It also covers one thing the “perfect street tree” sales pitch skips: the USDA notes that overuse in cities has led to calls to plant less of it.

What does a wild honey locust actually look like?

The wild tree grows thorns on the trunk and branches, and the length depends on where you measure. NC State Extension describes stout, three-forked spines 2 to 3 inches long; the University of Arkansas says 3 to 8 inches and often clustered on the trunk; the USDA plant guide gives up to 20 cm (about 8 inches) on the main trunk and lower branches. The sources probably disagree because they measure different places: twig spines are short, trunk clusters are the long ones. None of them describes a gentle tree. Arkansas Extension doesn’t recommend the species for home landscapes “for an obvious reason,” and notes the thorns were once used as nails, fish hooks and sewing needles.

The takeaway: the species and the thornless cultivars differ in thorns and pods, and the nursery trade has settled on the thornless version. Sources disagree on mature size by a factor of two, so treat any single height number with suspicion and read the cultivar section below.

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Where do thornless honey locusts come from?

Most named thornless cultivars are clones of branch tissue that never grew thorns. The USDA plant guide says that although thornless trees (var. inermis) turn up occasionally in the wild, “most” thornless horticultural forms are derived from buds or stem cuttings taken from the upper, thornless portions of mature trees that are thorny in the lower portions, and scions from that area “generally remain thornless.” The Arbor Day Foundation, for its part, calls the thornless tree a “form” of honeylocust: not different enough to be its own species, defined as a seedling variation where a percentage of seedlings show the trait.

That explains the nursery practice. NDSU’s datasheet for the ‘Northern Acclaim’ cultivar lists its propagation as “chip budding onto one-year field established rootstocks”: a bud from the cultivar is joined to a young rootstock plant. What you buy is the cultivar’s top growing on a different plant’s roots.

Two practical points follow, and the second one is my reading of the sources, not something they state outright:

• Don’t plant seed from a thornless tree and expect the same. Arbor Day says offspring from thornless trees “will sometimes have thorns.” You’ll see a flat percentage for this all over the web, often 40 percent. We found no extension, university or arboretum source that gives a number, so treat any figure as unverified. “Sometimes” is the most the evidence supports.
• Remove suckers from below the graft union. The rootstock is a different plant from the cultivar, so anything sprouting beneath the bud union isn’t the clone you paid for, and nothing in the sources promises it is thornless.

“Inermis” simply means unarmed in Latin. The USDA says wild inermis trees are “apparently more as a populational variant than what is generally given formal taxonomic status as a variety.” In the nursery aisle you’ll see a cultivar name instead: Imperial, Skyline, Shademaster, Moraine, Sunburst and others.

Why does grass still grow under a honey locust?

Two things combine: very small leaflets that let light through, and a leaf season that starts late and ends early. The USDA plant guide credits “the small leaflets and open crown” for a light shade that permits shade-tolerant turfgrass and partial-shade perennials to grow underneath. Each leaf is 10 to 20 cm long, but it is divided into leaflets of just 1 to 3 cm, so the canopy is a mesh rather than a roof. Virginia Tech calls the result “loose, open shade ideal for patios and shade loving plants.”

The timing is the part most competitors miss. The UC Statewide IPM Program says honey locusts “leaf out late, go dormant early, and drop their leaves early in the fall,” and NC State puts leaf emergence in late spring. I’d draw the conclusion carefully, since no source we found measured light under the canopy: a lawn under a honey locust probably gets close to full sun in early spring and again in autumn, which are the seasons cool-season grasses grow best. A tree that holds a full canopy for longer leaves the lawn less of that window.

The same leaflets cut your fall chores. Virginia Tech notes the small leaflets “filter into the grass as they fall, requiring little raking.” NC State describes the cleanup as easy. Two limits apply. The grass has to be one that tolerates partial shade, since the USDA says “shade-tolerant.” And the tree itself needs sun: NC State says it “does not grow in full shade,” and the Morton Arboretum recommends six hours of direct light.

If you’re weighing shade trees for a hot climate, our guide to fast-growing shade trees for USDA zones 7 to 11 rates 19 options.

Which thornless cultivar should you buy?

Pick Imperial for a compact, rounded tree, Skyline or Shademaster for a taller pyramid, and avoid Sunburst unless you want its yellow new growth and accept the pest risk. The best comparison data comes from an Arboriculture & Urban Forestry study that measured five cultivars at 15 years old at the Ohio Agricultural Research and Development Center in Wooster, Ohio. It is one site and one climate, so the numbers show relative behavior, not what your tree will do.

In short, Imperial is the smallest tree, Skyline the tallest, and Sunburst the one with the most documented problems. One detail worth noticing: Imperial had the longest annual twig growth (15.2 inches) yet was the shortest tree, while Skyline grew the least twig (11.7 inches) yet was the tallest. That fits their shapes. A rounded tree puts growth into side branches, while a pyramidal tree with a leader puts it into height. So “fast growing” on a plant tag tells you little about how tall the tree will be in 10 years.

Form helps with identification, too. The study’s authors report that an informed observer could identify 3 of the 5 cultivars from branching structure and habit alone, which is a useful check if a nursery’s labels look unreliable. One catch: the study describes Shademaster as pyramidal, while NC State calls it vase-shaped and vertical, so don’t lean on shape alone.

What pests and problems should you watch for?

Mimosa webworm is the one that matters, and the most useful thing to know is when spraying is a waste of time. Iowa State Extension describes two caterpillar generations a year: the first active from mid-June to early July, the second from early to late August. The larvae grow to almost 1 inch and tie leaflets into tight webs, and the leaflets inside brown. Virginia Tech calls webworms “the worst problem” and says they occasionally defoliate trees by August, recommending a spray twice a year.

Iowa State takes a different line. It calls the damage “more aesthetic than serious” and “seldom serious to otherwise healthy, well-established trees.” Its spray advice is to treat at the start of the caterpillar period, before webbing is apparent, so mid-June and early August. After the foliage browns, spraying is “ineffective and a waste of time” and kills the pest’s natural enemies instead. My reading: a mature, healthy tree can usually take a browned August canopy, so skip the late spray. Reserve spraying for a young or already stressed tree, and time it to the early windows only.

The pattern across the table: your best defense is cultivar choice at the nursery. Skipping Sunburst costs nothing and removes the cultivar that both Iowa State and Virginia Tech single out.

Where and how should you plant one?

Give it full sun and a wide range of soils, in USDA zones 4 to 9 for most cultivars. NC State says the thornless variety is rated for zones 4a to 9b, and the species tolerates soil from acid (below pH 6.0) to alkaline (above 8.0). Virginia Tech adds that it takes wet and dry soil and is “extremely salt tolerant,” which is why you see it along roads, though the USDA is more cautious, calling it “somewhat tolerant of salinity.” If you garden on alkaline ground, our list of plants that thrive in alkaline soil shows what else to put under it. In zone 3, look at ‘Northern Acclaim’, which NDSU lists as hardy in zones 3b to 6.

Pets and the mess question come up next. The ASPCA lists honey locust as non-toxic to dogs, cats and horses, and our guide to dog-safe shade trees that grow fast compares six non-toxic picks by zone. One caution: the USDA plant guide says the sweet pod pulp is “reported to be irritating to the throat and somewhat toxic,” even though cattle, deer and rabbits eat the pods. Choosing a fruitless cultivar removes the question. NC State says most thornless cultivars bear no fruit, though some drop a lot, and the USDA says most cultivars are “both thornless and fruitless.”

If low mess is your goal, a honey locust isn’t the only tree sold that way. Our article on the fruitless mulberry, the “no-mess” shade tree, covers what that one traded for its clean driveway: a 10 out of 10 on the Ogren Plant Allergy Scale.

Finally, avoid planting a row of identical trees. The Arbor Day Foundation calls honey locust “one of the toughest, most tolerant urban trees,” which is exactly why cities planted so many. The USDA says that overuse has led to recommendations to discourage its use until adequate biodiversity is restored. Mixing it with two or three other species is the safer way to use its strengths.

Honey locust FAQ

Do all honey locust trees have thorns?

No. Wild Gleditsia triacanthos does, but most trees sold in nurseries are thornless cultivars, per the Morton Arboretum. Seedlings from a thornless tree can still have thorns, according to the Arbor Day Foundation, so buy a named cultivar rather than a seedling.

Are honey locust thorns dangerous?

Yes, on the wild tree. The University of Arkansas describes thorns 3 to 8 inches long, tough and sharp enough to have served as nails and fish hooks, and advises extreme caution. It doesn’t recommend the species for home landscapes.

Is honey locust toxic to dogs?

The ASPCA lists honey locust (Gleditsia triacanthos) as non-toxic to dogs, cats and horses. A fruitless cultivar avoids the question of the pods, whose pulp the USDA says is reported to irritate the throat.

How fast does a honey locust grow?

Fast for a shade tree. NC State lists rapid growth, and in the Ohio study twigs on five cultivars grew 11.7 to 15.2 inches a year, with the tallest (Skyline) reaching 35 feet in 15 years, roughly 2.3 feet a year on average by my arithmetic.

Is honey locust messy?

It depends on the cultivar. The small leaflets filter into the grass and need little raking (Virginia Tech), and most thornless cultivars don’t produce pods, though NC State says a few produce a lot.

Will grass grow under a honey locust?

Shade-tolerant turfgrass will, according to the USDA, because the leaflets are small and the crown is open. The tree also leafs out late and drops its leaves early, so the lawn gets extra light at both ends of the season.

Sources

1. University of Arkansas Cooperative Extension Service. Honey Locust. Reference Desk, Trees
2. NC State Extension. Gleditsia triacanthos (Common Honey Locust). Extension Gardener Plant Toolbox
3. NC State Extension. Gleditsia triacanthos var. inermis (Thornless Honeylocust). Extension Gardener Plant Toolbox
4. USDA Natural Resources Conservation Service. Plant Guide: Honey Locust (Gleditsia triacanthos L.). National Plant Data Center
5. Arbor Day Foundation. Thornless Honeylocust: Nature’s Aberrant. Perspectives
6. Arboriculture & Urban Forestry. Growth Habits of Five Cultivars of Gleditsia triacanthos. International Society of Arboriculture
7. Virginia Cooperative Extension. Thornless Common Honeylocust (Gleditsia triacanthos var. inermis), Publication 2901-1073
8. Iowa State University Extension and Outreach. Mimosa Webworm. Yard and Garden
9. The Morton Arboretum. Honey-Locust (Gleditsia triacanthos). Plant and Protect
10. UC Statewide IPM Program. Honey Locust: Pest Management Guidelines. University of California Agriculture and Natural Resources
11. ASPCA. Honey Locust. Toxic and Non-Toxic Plants
12. North Dakota State University. Northern Acclaim Thornless Honeylocust datasheet. NDSU Agriculture