Black Locust

Problem Species

Pacific Northwest

345 cities

Black locust (Robinia pseudoacacia) is a fast-growing deciduous tree with deeply furrowed bark, compound leaves of small paired leaflets, and vicious paired thorns near each bud. In spring it produces hanging clusters of fragrant white flowers that look almost like wisteria. It was native to the Appalachians but has spread aggressively throughout the Pacific Northwest, where it is now classified as invasive. If you see it on your property, treat it as a problem that will get worse, not better, without intervention.

Lifespan

80 to 100 years under normal conditions, though stressed or borer-infested trees can fail structurally well before that.

Mature Size

40 to 70 feet tall with a spread of 20 to 35 feet, though suckering colonies can expand horizontally far beyond the original tree over time.

⚠ Problem Species

Why it's a problem: Invasive suckering, thorny, displaces native understory

Care & Maintenance

Black locust tolerates poor, dry, rocky soils and actually fixes nitrogen through root nodules, so fertilizing it is counterproductive and will accelerate growth and suckering. It prefers full sun and handles drought well once established. Do not irrigate it unless you want a larger, more aggressive tree. This is not a tree that needs your help to thrive.

Common Issues & Threats

Root suckering: Here is what most people get wrong about this tree. Cutting it down makes the problem worse. Black locust responds to injury by sending up dozens of suckers from the root system, turning one tree into a thicket within a few years. Removal requires stump treatment with herbicide immediately after cutting, ideally with triclopyr, applied within minutes of the cut.
Locust borer (Megacyllene robiniae): This native beetle specifically targets black locust and is one of the few things that actually weakens it. Larvae tunnel through the wood, compromising structural integrity. You will see sawdust-like frass at the base or in bark crevices. A heavily infested tree can fail without warning.
Toxicity: The bark, seeds, and leaves contain the toxin robin, which is poisonous to horses, cattle, and children if ingested. The seeds look innocuous but can cause serious illness. If you have livestock near a property with black locust, that is a real and specific risk you need to manage.

Pruning Guide

Avoid pruning black locust if your goal is eventually to remove it, because wounds trigger suckering from the roots. If you must prune for safety reasons, do it in late summer when the tree is focused on storing carbohydrates rather than pushing new growth. Wear heavy gloves and long sleeves regardless of season. The thorns on young growth and root sprouts are sharp enough to go through standard work gloves.

Did You Know?

Black locust wood is harder than white oak and one of the most rot-resistant woods in North America. Fence posts made from it can last 50 years in the ground without treatment. The flowers are also a major honey source, and black locust honey is produced commercially in Europe. None of that makes it less of an invasive problem in the Pacific Northwest, but it explains why it was planted so widely before people understood what it would do here.

Where Black Locust Is Found

Black Locust is common in 345 of the US communities we cover, across 1 climate regions.

Hardiness Zones 6-9

Redmond, WA Zone 8b Marysville, WA Zone 8b South Hill, WA Zone 8b Sammamish, WA Zone 8b Lakewood, WA Zone 8b Corvallis, OR Zone 8b Shoreline, WA Zone 9a Tigard, OR Zone 8b Olympia, WA Zone 8a Aloha, OR Zone Burien, WA Zone 9a Bothell, WA Zone 8b

... and 333 more cities

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