Tree · Fabaceae
Black Locust
Robinia pseudoacacia
A fast, hardy tree with a wild streak and a toxic side
Full sun
Every 14–30 days
2190–2920 days
USDA 3–8
Overview
Black locust (Robinia pseudoacacia) is the kind of tree that just keeps going. It's a fast-growing member of the pea family that shrugs off poor, dry, rocky ground, partly because it fixes its own nitrogen and feeds itself. Give it full sun and it'll climb to 30 to 50 feet (9 to 15 m) tall and spread 20 to 35 feet (6 to 11 m) wide, throwing fragrant white flowers in late spring.
So what's the catch? This tree has a wild streak. It sends up root suckers and seeds itself into dense thickets, and outside its native Appalachian and Ozark range it's considered invasive in plenty of places. The wood is naturally brittle too, and locust borers tunneling through it only make limbs more likely to snap in a storm.
There's also a safety angle worth taking seriously. Nearly every part except the flowers is poisonous, with the bark and seeds being the worst, and horses are especially vulnerable. If you've got livestock, pets, or kids around, that's a real reason to think carefully about where this tree goes.
Care
The short version: Black locust is about as low-maintenance as trees get. Plant it in full sun, give it well-drained ground, and water young transplants until they settle, then more or less leave it alone. It barely needs feeding thanks to its nitrogen-fixing roots, and the main jobs are pruning out weak limbs in winter and pulling root suckers so it doesn't take over.
Light
This is a full-sun tree through and through. It'll put up with a bit of partial shade, but you'll get the best growth, flowering, and form when it sits out in the open with plenty of light all day.
Water
Once it's settled in, black locust is seriously drought-tolerant and rarely needs watering, since it does well on poor, dry, sandy ground. Young transplants are the exception, so water those while they root, roughly every 14 to 30 days depending on the weather, then let the ground dry out fully between drinks.
Soil
Easygoing is the word here. It handles dry, sandy, rocky, eroded, even downright infertile sites because it makes its own nitrogen, so well-drained ground is really all it asks for. Anywhere from 4.6 to 8.2 on the pH scale suits it just fine.
Temperature
Cold? Heat? This tree takes both in stride, comfortable from -40°F to 104°F (-40°C to 40°C). That toughness is a big part of why it grows happily across zones 3 through 8.
Humidity
No fuss needed on humidity. Black locust does fine in average outdoor conditions and doesn't ask for anything special on that front.
Feeding
Here's a nice perk: as a nitrogen-fixing legume, it makes its own nutrients and needs little to no fertilizer. If you do feed it, skip the high-nitrogen stuff, since that can do more harm than good.
Pruning
Prune in late winter or early spring while the tree's dormant, taking out dead, crossing, weak, or storm-damaged limbs and lifting the crown if you like. Pull root suckers regularly to keep it from spreading, and wear gloves because those paired spines are sharp.
Repotting
Repotting isn't really a thing for a landscape tree this size. The only time you're moving one is young container or balled-and-burlapped stock, and that's best done while it's dormant. Watch for roots circling the pot, roots poking out the drainage holes, or water running straight through.
Size & spacing
Give it room, because this tree gets big. Expect 30 to 50 feet (9 to 15 m) of height and a spread of 20 to 35 feet (6 to 11 m), and it gets there fast. Plant it well away from buildings, walkways, and parking so brittle limbs aren't dropping onto anything important.
When to plant
Timing is pretty forgiving with this tree, but a few moves make life easier. Here's roughly when to do what.
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 black locust trouble comes down to a handful of usual suspects: borers, leafminers, and brittle wood. Here's how to spot what's going on and what to do about it.
Borers are tunneling through the wood
Those holes, knots, and piles of sawdust usually mean borer larvae are chewing tunnels inside the trunk and limbs, and that's what leaves the wood weak enough to snap.
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 are breaking off
This wood is naturally brittle, and a good wind or ice load is enough to bring branches down, especially when borers have already hollowed things out. Want to keep people and property safe? Stay ahead of it with pruning.
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
Leafminers are chewing up the foliage
When the leaves go brown and lacy by late summer, it's usually leafminers feeding on the foliage. It looks rough, but the tree shrugs it off and leafs out fine the following year.
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
This is where black locust earns its warning label. Almost all of it is toxic, so let's run through who's at risk and what to watch for.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Moderate
A cat that nibbles bark or seeds can end up vomiting, off its food, and weak, sometimes with bloody diarrhea, so keep the curious ones away from prunings.
Dogs Moderate
Dogs who chew on bark or seed pods may get sick to their stomach, lose their appetite, and turn sluggish, and in worse cases you might see bloody diarrhea or trouble breathing.
Horses Severe
Horses are the most at-risk here, and chewing the bark can trigger colic, weakness, brain-related symptoms, and even death, so fence them well clear of the tree.
Rabbits Toxic
The toxalbumins in bark, leaves, and seeds are genuinely dangerous for rabbits, so don't let them anywhere near the foliage or trimmings.
Livestock Severe
Cattle, sheep, and goats can be hit hard if they browse bark, seeds, or sprouts, with weakness and gut upset that can turn fatal.
Birds Toxic
The seeds and bark carry compounds that poison birds, so keep pet birds and the prunings far apart.
People Caution
People have been poisoned by the seeds and young sprouts, so steer kids clear of pods and shoots, and watch out for the sharp thorns while you work.
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
A few named types get planted for looks rather than the wild form. Just remember they share the same brittle wood.
Purple Robe
Planted for its showy rose-pink to purple blooms and bronze-tinted new leaves, though the wood is just as weak as the wild tree's.
Frisia
Holds golden-yellow leaves right through the season, but it's every bit as brittle and break-prone as its wild cousin.
Twisty Baby (Lace Lady)
A dwarf with curling, contorted branches that hardly ever flowers, and yes, the wood is still brittle.
Umbraculifera
Thornless with a tight, rounded mop-head crown, and it rarely puts out flowers.
How to propagate
Want more black locust? There are three ways to go about it, and they range from easy to fiddly.
SeedModerate
Spring · 1-2 seasons
Sow in spring, but first nick that tough seed coat by soaking it in near-boiling water for about a minute or giving it an acid bath. Treated seed sprouts and needs a season or two to settle in, and here's a handy bit: stored seed can still germinate after 10 or more years.
DivisionEasy
Dormant season · 1 growing season
Since the tree already spreads on its own through root suckers, this is the easiest path, and you do it in the dormant season. Suckers and dormant root cuttings from a quarter inch to an inch thick take root quickly and fill in within one growing season, which is exactly why it's so hard to keep in check.
GraftingDemanding
Dormant season or summer budding · 1-2 seasons
Named picks like Purple Robe and Frisia won't grow true from seed, so they get budded or grafted onto seedling rootstock instead. Do the work in the dormant season or bud in summer, and give the union a season or two to knit together.
Myths
Black locust must be safe since it's a native legume like the beans we eat, and people eat the flowers too.
Sure, the flowers turn up in some dishes and the tree feeds itself by fixing nitrogen, but everything else is toxic. The bark, leaves, seeds, and sprouts hold toxalbumins that can poison fatally, and horses are most at risk of all.
Black locust is a neat, well-mannered shade tree.
Not quite. It sends up root suckers and seeds itself into thick stands, and it's invasive beyond its home range. On top of that the wood is brittle and prone to breaking, and locust borers only make it weaker.