Tree · Fabaceae
Black Locust
Robinia pseudoacacia
Fragrant white flowers, paired thorns, and famously brittle wood.
Full sun
Every 14–30 days
2190–2920 days
USDA 3–8
Overview
Black Locust (Robinia pseudoacacia) carries its leaves the way the whole pea family does, in long pinnate sprays of small oval leaflets that hang in two neat rows along a thin central stalk. The leaflets are soft and blue-green to fresh green, rounded at the tip, and they fold and droop in dim light, so the whole crown reads as airy and feathery rather than dense.
Where the leaves join the twigs you find the tree's signature defense: sharp paired spines sitting at the nodes, short stiff thorns that flank each leaf base on young growth. The bark thickens with age into deep furrows and rough interlacing ridges, and the wood beneath it is hard yet brittle, prone to cracking at the limb unions.
In late spring the tree drips with bloom. Long drooping clusters of white pea-like flowers open in chains, each blossom built on the classic legume plan with a broad upper petal and folded keel, and the scent carries on warm air. The flowers give way to flat brown seed pods that rattle dry and hang into winter.
Care
The short version: Black Locust is a fast, tough tree for full sun and poor, dry ground, fixing its own nitrogen so it asks for almost nothing. Expect feathery pinnate leaves, paired thorns at the nodes, fragrant white flower chains, and brittle wood you will need to prune and site with care.
Light
Give it full sun for the densest crown and the heaviest flush of white flower chains; it will hold on in partial shade, but the airy pinnate foliage thins and the bloom drops off as light fades.
Water
Once rooted in, this tree is extremely drought-tolerant and pulls through dry, sandy, eroded ground without help, fixing its own nitrogen along the way. Water only young transplants, soaking deeply every 14 to 30 days, then let the soil dry out completely between drinks.
Soil
It takes well-drained poor to average soil and shrugs off dry, sandy, rocky, and infertile sites because its roots fix nitrogen. The roots run happily across a wide range of 4.6 to 8.2 on the pH scale, from acidic to alkaline.
Temperature
Cold-hardy and heat-tolerant, the tree handles -40°F to 104°F (-40°C to 40°C) across zones 3 through 8, with the bare gray winter framework of thorned twigs leafing out again each spring.
Humidity
Average outdoor humidity is fine; this is a tree of open, exposed ground, and its pinnate leaves and thorned twigs are untroubled by dry or moist air alike.
Feeding
As a nitrogen-fixing legume it makes much of its own food, so feed it rarely if at all and skip high-nitrogen products that only push soft, weak, break-prone growth.
Pruning
Prune in late winter or early spring while the branches are bare and dormant, taking out dead, crossing, weak, or storm-cracked limbs and lifting the crown. Cut root suckers back often to hold its spread, and wear gloves against the sharp paired spines.
Repotting
Not a container plant in the long run; only young potted or balled-and-burlapped stock gets moved, ideally while dormant. Watch for roots circling the pot, roots pushing out the drainage holes, or water running straight through without soaking in.
Size & spacing
This is a big, fast tree reaching 30 to 50 feet (9 to 15 m) tall and spreading 20 to 35 feet (6 to 11 m) wide, so give it generous room well clear of buildings and walkways where the brittle limbs cannot do harm.
When to plant
The northern-hemisphere rhythm runs from late-winter sowing to a spring planting, then those hanging white flower chains in late spring before the long seed pods form.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter; scarify seed (hot water or acid) before sowing to break hard seed coat.
- Plant: Spring after the last frost, or fall, while dormant.
- Harvest: Fragrant white pea-like flowers in drooping clusters in late spring (April to June).
Hardiness
USDA hardiness zones: 3–8
Temperature: down to -40°C
Troubleshooting
Most trouble shows up in the wood and the leaflets: borers hollowing the trunk, leafminers lacing the foliage, and the naturally brittle limbs that crack and drop. Here is how to read each sign on the plant itself.
Borers tunneling the trunk
The trunk pocks with round holes and swollen knots while pale sawdust collects at the base, and the wood inside grows hollow and snaps in a stiff breeze.
Fix:
- Maintain vigorous tree health to resist attack
- Prune out and destroy heavily infested, dead, or hazardous limbs
- Remove severely declining hazard trees near targets
Brittle limbs shattering
The hard, brittle wood cracks at the unions and overlong branches break clean off in wind or under ice, littering the ground below.
Fix:
- Prune out weak, dead, and hazardous limbs while dormant
- Reduce long, heavy limbs over walkways or structures
- Remove hazardous trees near high-use areas
Leaflets mined and scorched
By high summer the green pinnate leaflets go papery and brown, mined into a lace of veins so the whole crown looks singed.
Fix:
- Tolerate damage as it rarely kills established trees
- Rake and remove heavy leaf litter
- Maintain tree vigor with adequate moisture for young trees
Toxicity
Every part but the flowers is poisonous, with the inner bark and seeds the most dangerous, carrying toxalbumins that hit horses hardest. Keep bark, pods, prunings, and sprouts well away from animals and children.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Moderate
Vomiting, low appetite, and weakness if a cat chews bark, leaves, or seeds.
Dogs Moderate
Vomiting, depression, poor appetite, and weakness after gnawing bark, seed pods, or sprouts.
Horses Severe
The most vulnerable species; chewed bark brings on weakness, colic, labored breathing, and sometimes fatal collapse.
Rabbits Toxic
Treat all bark, seeds, and green sprouts as unsafe to nibble.
Livestock Severe
Bark, seeds, and young shoots can cause weakness, bloody diarrhea, and death.
Birds Toxic
Keep seed pods and bark out of reach as a poisoning risk.
People Caution
Eating seeds, inner bark, or sprouts can cause vomiting and bloody diarrhea; the flowers are the only safe part.
Wear gloves when pruning to avoid the sharp paired thorns, and keep bark, seed pods, prunings, and sprouts away from horses, livestock, pets, and children.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Selected forms shift the look of the species, recoloring the flowers and foliage or twisting the branches, but most keep the same brittle wood underneath.
Purple Robe
Hangs heavy drooping clusters of rose-pink to purple pea flowers above bronzy new growth, on the same weak wood as the species.
Frisia
Carries golden-yellow pinnate foliage from spring through fall on wood every bit as brittle and break-prone as the wild tree's.
Twisty Baby (Lace Lady)
A dwarf form whose branches twist and contort into corkscrew tangles; it rarely flowers and keeps the same brittle wood.
Umbraculifera
Thornless, with a tight rounded mop-head crown of branches that rarely sets flowers.
How to propagate
You can grow it from scarified seed, lift its eager root suckers, or graft the named forms that refuse to breed true.
SeedModerate
Spring · 1-2 seasons
Sow in spring after nicking the stony seed coat open with near-boiling water for roughly a minute or an acid soak. Treated seed sprouts and needs a season or two to settle in, and stored seed still wakes up after 10 or more years.
DivisionEasy
Dormant season · 1 growing season
The tree throws root suckers on its own, the simplest route, lifted in the dormant season. Suckers and dormant root cuttings from a quarter inch to an inch thick strike roots fast and fill a gap within one growing season, which is exactly why it resists containment.
GraftingDemanding
Dormant season or summer budding · 1-2 seasons
Named forms such as Purple Robe and Frisia will not breed true from seed, so they get budded or grafted onto seedling rootstock. Work the union in the dormant season or bud it in summer; the join needs a season or two to knit.
Myths
Black locust must be harmless because it is a native legume like edible beans and people eat its flowers.
Only the flowers turn up in food, and the tree does fix its own nitrogen, but everything else is poisonous. Bark, leaves, seeds, and sprouts hold toxalbumins that can kill, horses above all.
Black locust is a neat, well-mannered shade tree.
It pushes up root suckers and scatters seed into dense thickets, turning invasive beyond its home range. The wood is brittle and breakage-prone, and locust borers eat away at it from inside.