Black locust (Robinia pseudoacacia)

Synonyms: Robinia pseudo-acacia, Pseudacacia odorata

Common Names: False acacia, common locust, white locust, yellow locust, locust

Description:

Black locust (Robinia pseudoacacia) is a fast-growing tree in the pea family, native to the Appalachian and Ozark regions of the United States but planted far beyond that range ? and now invasive across much of the Midwest, Northeast, and other continents [1][3]. MISIN tracks it outside its native range because it spreads aggressively by root suckers into dense thickets, crowds out native plants, and adds nitrogen to the soil in ways that can help other weeds take hold [1][2].

Habit:

Black locust is a tall, fast-growing tree, often 40 to 80 feet high, with deeply furrowed gray-brown bark and a giveaway feature: a pair of short, stiff thorns (about ??? inch) at the base of each leaf along the zigzag twigs [1]. Its leaves are made up of 7 to 19 small, oval leaflets arranged along a central stalk [1]. In late spring it is unmistakable ? long, drooping clusters of fragrant, white, pea-like flowers, each with a yellow blotch on the upper petal ? followed by flat brown seed pods 2 to 4 inches long [1].

Look for it from May to June when the white flowers bloom, and year-round by the paired thorns and flat pods that hang on into winter [1]. It is most common in sunny, disturbed places ? old fields, roadsides, prairies, and open or degraded woods ? where it often grows in clumps of connected stems [1].

At-a-glance cues:

1. A pair of short, stiff thorns at the base of each leaf along the twigs [1]
2. Leaves made of 7?19 small, oval leaflets [1]
3. Drooping clusters of fragrant white pea-like flowers (each with a yellow blotch) in late spring [1]
4. Flat brown seed pods 2?4 inches long, often persisting into winter [1]

Leaves:

1. Arrangement: alternate, pinnately compound (leaflets arranged along a central stalk) [1]
2. Leaflets: 7?19 (sometimes up to 21), small and oval with rounded tips [1]
3. Margin: smooth (untoothed) [1]
4. Surface: dark green above, paler blue-green below [1]
5. Size: whole leaf about 8?14 inches long; each leaflet roughly 1?2 inches [1]

Stems:

1. Habit: fast-growing deciduous tree, commonly 40?80 feet (12?25 m) tall, often in clonal clumps connected by roots [1]
2. Thorns: a pair of short, stiff spines (about ??? inch) at the base of each leaf, along zigzag twigs [1]
3. Bark: thick, gray-brown, deeply furrowed into rope-like ridges with age [1]
4. Roots: shallow and wide-spreading, fixing nitrogen and sending up suckers that form thickets [1][2]

Flowers:

1. Color: white, with a yellow blotch on the uppermost petal [1]
2. Shape: fragrant, pea-like (papilionaceous), with both male and female parts (perfect) [1]
3. Arrangement: in long, showy, drooping clusters (racemes) [1]
4. Bloom time: late spring, about May to June [1]

Fruit and seeds:

1. Fruit: flat, smooth, brown seed pods (legumes), about 2?4 inches long [1]
2. Seeds: several small, hard seeds per pod [1]
3. Timing: pods ripen in fall and often hang on the tree into winter [1]
4. Spread: by seed, but mostly vegetatively ? vigorous root suckering and stump sprouting form connected clonal thickets [1][2]

Habitat:

Black locust thrives in sunny, disturbed ground. It is most common in old fields, roadsides, prairies, savannas, degraded woods, and other open or disturbed sites, and it grows fast ? often several feet a year ? on a wide range of soils [1][2]. Its roots host bacteria that fix nitrogen from the air, so it can colonize poor, low-nutrient soils where many trees struggle [2].

The tree is native to the Appalachian and Ozark regions of the eastern United States [1][3]. It has been planted widely for timber, fence posts, erosion control, and its flowers, and has naturalized across the rest of North America and on other continents ? where, outside its native range, it behaves as an invasive [1][3]. It is considered one of the most significant invasive trees worldwide [1].

Reproduction:

Black locust spreads most of all from its own roots. A single tree sends out underground roots that sprout new stems (suckers), and cut or damaged trees resprout vigorously from the stump and roots ? so one tree can grow into a connected thicket of many stems [1][2]. It also produces flat pods full of hard seeds that can build up in the soil, and it spreads when people plant it [1].

The tree flowers in late spring and ripens pods in fall. The most important thing to know for control is that cutting or girdling a black locust on its own usually makes it worse ? it triggers a burst of new suckers and sprouts [1]. Because the stems in a clump are all connected, effective control means dealing with the whole clone, not just one stem, and expecting to return over several seasons (see Management) [1].

Impact and Damage:

For homeowners, black locust is a fast tree that can sucker into a thorny thicket and spread into a neighbor's yard or nearby woods; its bark, leaves, and seeds are also toxic if eaten, and it can poison livestock such as horses [1][3]. Handling the plant is not dangerous beyond its thorns, but keep animals from grazing on it [3].

For natural areas, the bigger problem is in open habitats. Black locust forms dense clonal thickets that shade out and displace native plants, and it is especially damaging in prairies, savannas, and open woodlands that depend on sunlight [1][2]. By fixing nitrogen, it also enriches the soil, which can shift the plant community and favor other non-native, weedy species ? an effect that can persist even after the locust itself is removed [2]. These traits make it one of the harder invasive trees to manage once established [1].

Similar species:

A few trees with compound leaves or thorns can be mistaken for black locust.

Honey locust (Gleditsia triacanthos) shares thorns and compound leaves, but its thorns are long and often branched (growing right out of the trunk), its flowers are small and greenish, and its pods are long and twisted, unlike black locust's paired short thorns, showy white flowers, and short flat pods.

Tree-of-heaven (Ailanthus altissima) also has large compound leaves but has no thorns, gives off a strong unpleasant smell when crushed, has small teeth near the base of each leaflet, and produces papery winged seeds rather than pods.

Bristly locust (Robinia hispida) is a close relative, but it is a smaller shrub with bristly, hairy stems and pink to purple flowers, not the tall tree with white flowers.

Monitoring and rapid response:

Signs to watch for:

1. Fast-growing trees with paired thorns at the leaf bases and leaves made of many small oval leaflets [1]
2. Drooping clusters of fragrant white pea-like flowers in late spring, and flat brown pods that hang on into winter [1]
3. Clumps or thickets of many connected stems spreading into old fields, prairies, or open woods [1][2]

What you can do:

1. Pull or dig out seedlings and small saplings while they are young, removing as much root as possible ? easiest in moist soil; wear gloves and long sleeves because of the thorns [1].
2. Know that simply cutting or girdling a larger tree usually backfires ? it triggers a flush of root suckers and stump sprouts ? so don't cut big stems unless you can commit to repeated follow-up on the regrowth [1].
3. Don't plant black locust, and watch the edges of any existing clump for new suckers spreading outward [1].
4. Natural area crews should treat every stem in a connected clone (not just one), plan for repeat visits over several seasons, and prioritize outlying clumps threatening high-quality prairies, savannas, and open woods [1][2].
5. Report sightings (below) to help track where this tree is spreading outside its native range [1].

When to call a professional:

1. Large trees, established clonal thickets, or any stand that resprouts aggressively after cutting ? black locust is difficult to control, so a land manager or licensed applicator is often the right path [1].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Michigan Department of Natural Resources, Invasive Species Program. Black Locust (Robinia pseudoacacia). michigan.gov/invasives
2. University of Connecticut, Connecticut Invasive Plant Working Group. Black Locust (Robinia pseudoacacia). cipwg.uconn.edu
3. USDA NRCS. Plant Guide: Black Locust (Robinia pseudoacacia L.). plants.usda.gov
4. Minnesota Department of Natural Resources. Black Locust (Robinia pseudoacacia). dnr.state.mn.us

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