// SPECIES PROFILE · TREE · NATIVE · POLLINATOR

Black Locust

Robinia pseudoacacia

Black Locust is a fast-growing, nitrogen-fixing native tree of the central and eastern US — one that arrives in May hung with fragrant, pendulous clusters of white pea-like flowers that perfume the air with a sweet, grape-like scent and make it one of the most important honey plants in North America. Robinia pseudoacacia produces some of the hardest, most rot-resistant wood of any temperate tree — fence posts of black locust can last a century in the ground without treatment — and as a legume it enriches depleted soils with atmospheric nitrogen. In NE Oklahoma, it is a premier pioneer species for reclaiming degraded land, an outstanding windbreak component, and a key nectar source for bees in the critical spring buildup period.

// QUICK FACTS

Family: Fabaceae (pea / legume family)
Life cycle: Deciduous tree
Native range: Appalachians & Ozarks, from PA to GA, west to IA, MO, and eastern OK; widely naturalized
USDA hardiness: Zones 4–8 (Tulsa = 7a/7b — well within range)
Mature size: 30–50 ft tall typical; 20–35 ft wide; can reach 80 ft on good sites
Bloom: May – early June (NE OK); peak bloom lasts ~10 days
Flower: White, fragrant, pea-like, in drooping racemes 4–8 in long
Sun: Full sun (intolerant of shade)
Soil: Tolerates almost any well-drained soil, including poor, dry, sandy, or clay soils; pH 5.5–8.0
Water: Low to medium; very drought-tolerant once established
Wildlife: Major bee forage (black locust honey) · silver-spotted skipper host · seeds eaten by birds
Notable: Wood rated Janka 1,700 lbf; ground-contact durability 50+ yrs untreated

Black Locust (Robinia pseudoacacia) with drooping racemes of fragrant white pea-like flowers and compound leaves — *Robinia pseudoacacia* in bloom — the sweet fragrance of the pendant white flower clusters signals the peak of spring bee season. Photo: Rooted Revival.

Identification

[ field key — stem · leaf · flower · fruit · habit ]

Habit & Stem

A medium to large deciduous tree with an open, irregular, often leaning crown. The trunk is typically straight on good sites, with deeply furrowed, dark gray-brown bark forming thick, rope-like ridges that are unmistakable in mature specimens. Young twigs are armed with paired stipular thorns at each node — sharp, stout spines up to 1 in long that are modified stipules. The tree spreads aggressively by both root suckers and seed, particularly on disturbed ground, and will form dense, thorny colonies if unmanaged.

Leaves

Alternate, pinnately compound, 8–14 in long with 7–19 oval to oblong leaflets, each 1–2 in long. Leaflets are entire (smooth-margined), medium green above and paler beneath, with a small, soft bristle at the tip. The foliage emerges relatively late in spring (after the flowers) and turns a clear, unremarkable yellow in fall before dropping. Leaves are nyctinastic — the leaflets fold upward and together at night and during heavy rain. The foliage contains toxic proteins and is not browsed by deer.

Flowers

The flowers are the tree's glory: white (rarely pink-tinged), intensely fragrant, pea-like blooms arranged in pendulous racemes 4–8 in long that hang from the branch tips like clusters of butterflies. Individual flowers are about 3/4 in across with the classic papilionaceous structure of a broad banner petal, two lateral wings, and two fused keel petals. The fragrance is sweet and penetrating, reminiscent of orange blossoms and vanilla, and can carry for hundreds of feet on warm May days. Peak bloom lasts roughly 10 days in NE Oklahoma, typically in early to mid-May.

Fruit & Seed

Following the flowers, the tree produces flat, dark brown legume pods 2–4 in long and about 1/2 in wide, each containing 4–8 kidney-shaped, dark brown seeds. The pods mature by late summer and persist on the tree through winter, splitting open in early spring to release the seeds. The seeds have an extremely hard, impermeable seed coat and can remain viable in the soil seed bank for decades. Like other parts of the plant, the seeds and pods contain toxic compounds (robinine, a toxalbumin) and should not be eaten.

Habitat & Range in NE Oklahoma

Robinia pseudoacacia is native to a relatively restricted area of the central and southern Appalachians and the Ozark-Ouachita highlands, but it has been planted and naturalized across all 48 contiguous states and much of temperate Europe and Asia. In NE Oklahoma, it is native to the easternmost counties (the Ozark foothills and Arkansas River valley) but is now ubiquitous in fencerows, abandoned fields, woodland edges, and disturbed sites throughout the Tulsa region. It colonizes roadsides, railroad embankments, strip-mined land, old pastures, and any sunny, well-drained site where soil has been exposed.

Its aggressive pioneer habit and tendency to sucker make it a plant that requires thoughtful placement in the managed landscape. In a permaculture context, that same vigor becomes an asset: black locust is an unsurpassed nurse tree for establishing oak, hickory, and other slower-growing hardwoods, sheltering and nitrogen-enriching the soil for decades before being gradually shaded out and dying, leaving behind improved soil and a longer-lived forest.

Ecology & Wildlife Value

[ honey bee forage · native pollinators · lepidoptera host · soil building ]

Bee Forage & Honey

Black locust blossoms produce copious, high-quality nectar during the short but intense spring bloom, making it one of the most important honey plants in eastern North America. Black locust honey is prized by beekeepers: it is water-white to pale amber, resists granulation for years, and has a delicate, floral flavor. In years with favorable weather during the bloom window (warm, not rainy, not windy), a single mature tree can yield pounds of honey. The flowers are also heavily visited by bumble bees, carpenter bees, mining bees, and various solitary bees during the critical spring colony-building period.

Lepidoptera Hosts

Black locust is the primary larval host for the silver-spotted skipper (Epargyreus clarus), one of the largest and most recognizable skippers in North America. The caterpillars are distinctive: they construct leaf shelters by cutting a flap of leaf tissue and folding it over with silk, emerging to feed at night. Several moth species also use black locust foliage, including the locust underwing (Catocala robinsoni) and the orange- patched smoky moth (Pyromorpha dimidiata).

Birds & Mammals

The seeds are eaten by Northern bobwhite quail, wild turkey, and mourning doves, as well as by squirrels and chipmunks. The dense, thorny thickets formed by suckering colonies provide superb cover and nesting sites for songbirds such as Northern mockingbirds, brown thrashers, and cardinals. The sharp thorns deter mammalian predators, making locust thickets a favored nesting location for several bird species. White-tailed deer browse the foliage sparingly (it is mildly toxic).

Soil Building & Nitrogen Fixation

As a legume, black locust forms symbiotic relationships with nitrogen-fixing rhizobia, specifically Rhizobium species that nodulate the roots and convert atmospheric nitrogen into plant-available ammonium. A stand of black locust can add 50–100 lbs of nitrogen per acre per year to the soil. The deep, extensive root system also stabilizes eroding slopes and mines minerals from subsoil layers, bringing them to the surface in leaf litter. This nutrient-pumping and nitrogen-fixing ability makes black locust an unparalleled pioneer for reclaiming degraded, compacted, or nutrient-depleted soils.

Horticulture & Care

[ site selection · establishment · coppice management · permaculture uses ]

Site selection & planting

Black locust is among the easiest trees to establish in NE Oklahoma. It demands only full sun and well-drained soil — it will grow, and grow well, on sites where almost nothing else will: eroded clay subsoil, gravelly slopes, sandy old fields, and compacted urban fill. The one thing it will not tolerate is shade; even partial shade slows growth dramatically and leads to spindly, unhealthy form.

Planting stock: Bare-root seedlings (1–2 ft) establish quickly and are the most economical option. Plant in late winter or early spring.
Spacing: 10–15 ft for a windbreak or nurse crop; 20–30 ft for specimen trees.
Water: Water deeply at planting and once weekly through the first summer. After the first year, irrigation is unnecessary.
Fertilizer: Do not fertilize — the tree produces its own nitrogen, and added fertility encourages weak, leggy growth.

Coppicing & pollarding

Black locust responds exceptionally well to coppicing (cutting to the ground) and pollarding (cutting at head height). Cut on a 5–10 year rotation, a coppiced locust stool produces straight, durable poles for fence posts, trellises, and garden stakes. The regrowth is vigorous, and the harvested wood requires no treatment for ground contact. This management style also prevents the tree from reaching a size where it becomes hazardous — mature black locust branches are brittle and can drop without warning in wind or ice storms.

Maintenance & cautions

Suckering: Root suckers will appear, especially after root disturbance. In a maintained landscape, mow or prune suckers regularly; in a wilder setting, embrace the thicket.
Thorns: The paired stipular thorns are sharp and unforgiving. Do not plant black locust near paths, play areas, or anywhere people will brush against it bare-skinned.
Brittleness: Branches are brittle and prone to storm damage. Do not plant near structures, vehicles, or power lines.
Invasiveness: Black locust is listed as invasive in several states outside its native range. In eastern Oklahoma, where it is native or long-naturalized, thoughtful management (not planting near natural areas where it could displace native vegetation) is appropriate.

Permaculture roles

Black locust fills multiple niches in a NE Oklahoma permaculture system: nitrogen-fixing nurse tree for establishing a food forest canopy; windbreak component for sheltering fruit trees and vegetable gardens; coppice source for rot-resistant fence posts, stakes, and firewood; spring bee forage for honey production; and erosion control on steep, degraded slopes. It is less a specimen tree for the front lawn and more a workhorse species for the productive edges of the property.

Edible & Cultural Uses

The flowers are edible and are the sweetest-tasting part of the plant. They can be gathered in May (when fully open but before browning) and used fresh. However, all other parts of black locust are toxic to humans and livestock, particularly the bark, seeds, and inner root bark, which contain the toxalbumin robinine and related compounds that disrupt protein synthesis. Consumption of seeds or bark by children or pets is a medical emergency.

Flower fritters: The fragrant flower clusters, dipped whole in a light batter and fried, make a delicate spring fritter with a sweet, slightly grape-like flavor. Collect only fully open, unblemished racemes.
Flower syrup & cordial: The blossoms steeped in simple syrup or vodka yield a fragrant cordial that captures the essence of May in NE Oklahoma.
Black locust honey: As noted, one of the finest monofloral honeys in the world — water-white, slow to crystallize, with a delicate floral nose. A black locust tree within foraging range of a hive is a serious asset to a beekeeper.
Timber: The wood is extraordinarily durable. Fence posts of black locust heartwood have been known to last 50–100+ years in ground contact without chemical treatment, outlasting pressure-treated pine and cedar. The wood is also dense (specific gravity ~0.69), burns hot, and makes excellent firewood. Historically, black locust was used for shipbuilding (trunnels, or "tree nails"), and it remains prized for decking, outdoor furniture, and tool handles.

Toxicity warning: The inner bark, seeds, leaves, and roots of black locust contain toxalbumins (robinine and robin) that are poisonous to humans, horses, cattle, and poultry. Symptoms include nausea, vomiting, dilated pupils, weak pulse, and in severe cases respiratory failure. Do not ingest bark, seeds, or any plant part other than the cooked flowers, and never feed black locust trimmings to livestock. Children should be supervised around the tree, as the seeds resemble small beans.

Sources & Further Reading

USDA NRCS PLANTS Database — Robinia pseudoacacia: plants.usda.gov/plant-profile/ROPS
Lady Bird Johnson Wildflower Center — Native Plant Database: wildflower.org — ROPS
Missouri Botanical Garden Plant Finder — Robinia pseudoacacia: missouribotanicalgarden.org
USDA Forest Service — Silvics of North America, Vol. 2: Hardwoods. Robinia pseudoacacia: srs.fs.usda.gov
Buchanan, B. (2018). A Handbook of Native Trees for the Central U.S. — detailed silvicultural and landscape notes for black locust.
Wikipedia — Robinia pseudoacacia: en.wikipedia.org/wiki/Robinia_pseudoacacia (CC BY-SA 4.0; portions of the description summarize Wikipedia content).
Oklahoma State University Extension — tree selection and management for NE Oklahoma landscapes.

Hero photo: Rooted Revival.