›

Black locust

Przemysław Malkowski / Wikimedia Commons - CC BY 3.0

Black locust

Robinia pseudoacacia

A fast-growing, thorny, suckering deciduous tree of the pea family, native to the Appalachians and, by the US Forest Service account, the Ozark and Ouachita mountains, that has escaped planting across much of North America and Europe. The Forest Service Fire Effects Information System records it crowding out native plants in the Northeast, the mid-Atlantic states, the Midwest, California and parts of Europe, worst in pine barrens, sand prairies and oak savannas, and the Missouri Botanical Garden says not to plant it in the Midwest, where one or more states outside Missouri list it as a noxious weed. It fixes nitrogen and grows on poor, dry ground, and in late spring it hangs out long clusters of fragrant white pea flowers that honey bees work hard. The bark, leaves and seeds are poisonous to people, pets and livestock.

Native: 14 US states

Climate fit: moderate (67/100)

Structure

Pollinator

Light

Full sun / Part sun

Water

Moderate water

Mature size

30-50 ft tall · 20 ft apart

Hardy in zones

3a-8b

brutally cold to frosty winters

Life cycle

Perennial

Bloom period

Spring / Summer

AHS heat range

3-9

Plant range authored in AHS heat-zone terms.

Native in Illinois

Related products

Cold hardiness

These values are location-based: this location's current hardiness is the baseline, and the 2050 value is a projected future climate for this same location.

Now

Zone 6b

Plotwright

USDA Zone 6b

-5°F to 0°F

Well-suited

2050

2100

Zone 7a

Plotwright

0°F to 5°F

Well-suited

In plain terms: This location has cold winters. Its winters are projected to keep warming through 2050.

✓

Well-suited today and still thriving in 2050.

Heat tolerance

Heat tolerance values are location-based too: heat days today are observed at this site, and the 2050 value projects this same location under a future climate.

Loading AHS heat-zone data for this location...

Where this plant fits

Suitable across 42 ecoregions - 33 climate-resilient through 2070 · 6 suited today · 3 newly possible by 2070. Best matches first.

Appalachian mixed mesophytic forests

›

Appalachian-Blue Ridge forests

›

Arizona Mountains forests

›

Atlantic coastal pine barrens

›

Blue Mountains forests

›

Central Pacific Northwest coastal forests

›

Central Tallgrass prairie

›

Central-Southern Cascades Forests

What it needs

Conditions

Light

Full sun / Part sun

Water

Moderate water

Soil pH

Acid / Neutral / Alkaline

Acid is below pH 6.0, neutral 6.0 to 8.0, alkaline above 8.0.

Spacing

20 ft

Habit

Upright, fast-growing deciduous tree with an open crown and forking, often crooked limbs, armed with short paired spines when young; its long, shallow lateral roots send up suckers that grow into clonal thickets

Care tips

•

Check your state and regional invasive and noxious weed lists before planting: the Missouri Botanical Garden says it should not be grown where it could spread, and FEIS documents it invading pine barrens, sand prairies and oak savannas.

•

Pull or cut suckers as soon as they appear, as the Missouri Botanical Garden advises; the long, shallow roots send them up well away from the trunk.

•

To get rid of a tree, do not just cut or girdle it: FEIS says that alone does not generally control it and sprouting follows, and that cutting the stems and treating the stumps with herbicide is widely suggested, with every stem in the clone treated and follow-up often needed.

Lifecycle

Planting

Think hard before planting it at all: NC State Extension does not recommend it for the home landscape, the Wildflower Center says to reject it for most landscape uses, and the Missouri Botanical Garden says it should not be grown where it could spread. Where it is native, or on a hard reclamation site, plant it in full sun; the Missouri Botanical Garden and Plants For A Future both say it will not take shade. It grows in sand, loam or clay and in poor, dry ground, and NC State Extension lists tolerance of drought, salt, pollution and black walnut. Silvics of North America says it does best on moist, rich, loamy or limestone soils and is very sensitive to poorly drained or compacted ones. Allow 20 to 35 feet of spread, and keep it well away from beds and lawns, which its root suckers invade.

Transplanting

NC State Extension says it transplants well. Plants For A Future and the Wildflower Center both move rooted suckers while they are dormant.

Early growth

Fast from the start where competition is light: Silvics of North America calls it very intolerant of shade and of grass and weed competition, so keep the ground clear around a young tree. Expect paired spines on young stems, and root suckers from about the fourth or fifth year; the Missouri Botanical Garden advises removing suckers as soon as they appear unless you want a thicket.

Maturity

A short-lived tree: NC State Extension and FEIS give a life span of about 90 years, and Silvics of North America says growth slows rapidly after 30. Forest trees run larger than the garden size on this record: Silvics and FEIS give 40 to 60 feet as usual, and Silvics up to 100 feet on better sites. Silvics also calls it damaged by insects and disease probably more than any other eastern hardwood. The locust borer, often fatal according to NC State Extension and the Missouri Botanical Garden, tunnels through the wood and opens it to heart rot, and the locust leafminer, which Silvics says breaks out almost every year, browns or strips the leaves. The branches are brittle and break in wind, ice and storms, as NC State Extension, the Wildflower Center and FEIS all note.

Propagation

The Wildflower Center gives seed, spring root cuttings or dormant sucker divisions. The hard seed coat needs scarifying or a hot-water soak first; Plants For A Future soaks seed for 48 hours in warm water and sows it in late winter in a cold frame. Seed keeps a long time: FEIS cites a Michigan flora saying it can survive more than 88 years in the soil, so a removed tree can come back from the seed bank.

Pollination

Pollination not applicable

Grown for its flowers, its tough wood and its ability to cover poor ground, not for a crop, so a garden needs no pollination plan. Silvics of North America says the flowers are pollinated by insects, primarily bees; the Wildflower Center calls them a favourite of honey bees and hummingbirds, FEIS records hummingbird visits and says bees visit for nectar throughout its native and introduced ranges, and FEIS cites a review crediting black locust plantings with supporting a commercial honey industry in Hungary. FEIS also notes a concern in New England that its abundant nectar may draw pollinators away from native plants.

Plants don't grow alone

Larval host

Butterfly

Silver-spotted skipper (Epargyreus clarus)

FEIS records black locust as a host of the silver-spotted skipper in the southeastern United States, and NC State Extension lists it as a larval host plant for the species.

Nectar forage

Bee

European honeybee (Apis mellifera)

The Wildflower Center calls the flowers a favourite of honey bees, and FEIS records bees visiting for nectar throughout its native and introduced ranges, with plantings that support a commercial honey industry in Hungary.

Nectar forage

Bird

Ruby-throated hummingbird (Archilochus colubris)

FEIS and the Wildflower Center both record hummingbirds at the flowers without naming a species. In the eastern United States, where black locust is native, the ruby-throated hummingbird is the one likely to visit.

Seed forage

Mammal

Eastern gray squirrel (Sciurus carolinensis)

FEIS and Silvics of North America both record squirrels eating the seeds without naming a species; Silvics rates the seeds low as wildlife food. Across the eastern range of the tree, the eastern gray squirrel is the likely one.

Foliage forage

Mammal

White-tailed deer (Odocoileus virginianus)

FEIS records white-tailed deer heavily browsing black locust in North Carolina, Georgia and Arkansas, and Silvics of North America cites a southern Appalachian study in which 92 percent of the sprouts were browsed.

Shelter / habitat

Bird

Downy woodpecker (Dryobates pubescens)

FEIS and Silvics of North America both record downy woodpeckers nesting in cavities in black locust, which heart rot makes common in older trees.

For people

Toxic - do not consume

Poisonous. NC State Extension rates the poison severity as high: the bark, leaves and seeds contain robin, robitin and robinine, and eating them can cause vomiting, bloody diarrhea, weakness, a weak pulse, dilated pupils, paleness, shock and, in some cases, death. The US Forest Service FEIS review likewise records it as poisonous to humans, sometimes fatally, and to horses, cattle, sheep and mules. For pets, the ASPCA lists locust as toxic to dogs, cats and horses from the bark, leaves and seeds; its entry covers the genus Robinia rather than this species alone. The flowers are the exception: NC State Extension lists them as edible, and FEIS records that people eat them fried or cooked and make tea from them. Plants For A Future also lists cooked seeds and young pods as eaten, but NC State, FEIS and the ASPCA all name the seeds as poisonous, so this record does not treat them as food. FEIS notes that the wood can cause dermatitis, though NC State lists no contact rash.

Native range

Native

Naturalized

Native across 10 US states; naturalized in 4 US states.

Worldwide native range.

AHS heat range

3-9

Native ecoregions

Naturally belongs in 13 ecoregions - its native range crosses a state or province each one spans (not necessarily this exact habitat).

Appalachian mixed mesophytic forests

›

Appalachian-Blue Ridge forests

›

Central Tallgrass prairie

›

Cross-Timbers savanna-woodland

›

Eastern Great Lakes lowland forests

›

Flint Hills tallgrass prairie

›

Mississippi lowland forests

›

Northeastern Coastal Forests

›

Northeastern Hardwoods

›

Piedmont

›

Climate notes

•

NC State Extension and the Missouri Botanical Garden both list Robinia pseudoacacia for USDA hardiness zones 3 to 8 (3a-8b). Plants For A Future gives 4 to 9, which is not used here. FEIS and Silvics of North America both warn that it is highly susceptible to frost damage, and FEIS records hard winter frosts killing new growth on trees planted in Ontario.

•

Plants For A Future gives AHS heat zones 9 to 3, the source of the heat range here, and NC State Extension tags it heat and drought tolerant.

•

The native range here is taken from Kew's World Checklist of Vascular Plants (WCVP), the dataset POWO publishes: the central and southern Appalachians of the eastern United States, in Alabama, Georgia, Kentucky, Maryland, North Carolina, Pennsylvania, South Carolina, Tennessee, Virginia and West Virginia.

•

The US Forest Service draws the native range wider. Its FEIS review (Stone, 2009) and its Silvics of North America chapter (Huntley, 1990) describe two regions: the Appalachians from central Pennsylvania and southern Ohio to northeastern Alabama, northern Georgia and northwestern South Carolina, and the Ozark Plateau and Ouachita Mountains of southern Missouri, Arkansas and eastern Oklahoma, and say outlying native stands are thought to exist in southern Indiana and Illinois. WCVP records Ohio, Missouri, Arkansas, Oklahoma, Indiana and Illinois as introduced. FEIS adds that the original extent is not accurately known, and the Missouri Botanical Garden and the Wildflower Center side with the Forest Service in treating it as native to the Ozarks.

•

WCVP records it as introduced in 128 areas: most of the rest of the United States; southern Canada from British Columbia to Nova Scotia; nearly all of Europe, from Spain, France, Great Britain and Ireland east to Ukraine and European Russia; North Africa, Türkiye and the Middle East; Central Asia, China, Korea, Japan and Taiwan; South Africa and Lesotho; Australia and New Zealand; and central Chile, northeastern Argentina and northwestern Mexico.

•

FEIS documents where it escapes and does harm: pitch pine and scrub oak barrens on Long Island, at the Albany Pine Bush and on Cape Cod; beech-maple and aspen woods, dunes and riverbanks in Michigan; the black oak savannas of the Indiana Dunes, sand prairie in Illinois and oak savanna in Minnesota; old fields in the sagebrush steppe of eastern Washington; plant communities throughout California; and, in Europe, chestnut forest in northern Italy and Switzerland, old fields in southern Poland and gravel pits in the Czech Republic. Its nitrogen-rich litter changes the soil, which stays altered after the trees are removed, and FEIS records it altering habitat of the endangered Karner blue butterfly. NC State Extension says it has become invasive in the Midwestern United States.

•

The state list on this record keeps the states WCVP records as native and adds Ohio, Missouri, Arkansas and Oklahoma, which both Forest Service accounts place in the native range; Indiana and Illinois are left out because the Forest Service only thinks the stands there are native. The Wildflower Center lists nearly every state and several Canadian provinces because its list records where black locust grows, not only where it is native, and USDA PLANTS records it as native to the lower 48 states as a whole and introduced in Canada.

•

Sources differ on when it flowers: FEIS gives April to June in the native range and mostly May to June elsewhere, NC State Extension and the Wildflower Center April to June, the Missouri Botanical Garden May to June, Silvics of North America May or June after the leaves emerge, and Plants For A Future June.