Black Locust

Full bloom

Robinia pseudoacacia

Fabaceae · broadleaf deciduous tree · native-native

Last updated

Black locust (Robinia pseudoacacia) is a fast-growing deciduous tree in the Fabaceae family native to eastern North America from Pennsylvania to Georgia, west to Iowa, Missouri, and Oklahoma. It reaches 60 ft tall with a 49 ft spread (40 ft at 20 years) and has an erect, multiple-stem form with dark furrowed bark. White drooping flower clusters to 10 inches long are fragrant, and the foliage turns gold-yellow in fall.

Black locust is hardy in zones 4a-8b and prefers full sun with well-drained soil (pH 4.0-8.0) and moderate water, requiring a minimum 36-inch root depth. It is nitrogen-fixing and CaCO3-tolerant with four cultivars in the trade. All parts are toxic except the flowers. Two diseases and three pests are documented; the species has weed and invasive potential.

Plant Profile

Size & Form

Height
30–80 ft
Spread
20–35 ft
Growth Rate
Fast
Size at 20 yr
~40 ft
Lifespan
60–100 years

Site Requirements

Light
Full Sun
Soil Drainage
Well Drained
Soil pH
4.0–8.0
Water
Low to moderate
Drought Tolerance
High
Hardiness
Zones 3a–8b

Ornamental Interest

Bloom Time
May to June
Fall Color
Yellow (insignificant)
Origin
Pacific Northwest native

Field Observations

Black Locust full bloom
Black Locust: Full bloom
May 23, 2026 · Kent
Watch for this season
AphidHigh

Peak Population & Dispersal

CarpenterwormModerate

Pupation & Adult Emergence

Diseases: Regionally Documented (2)

Pests: Regionally Documented (3)

Phenological Calendar

As of June 4, 2026, Puget Sound stations range from 2458.5 to 2700.8 GDD₃₂. Black Locust has passed full bloom (2082 GDD₃₂).

Regional Season Tracker

GDD₃₂ accumulation across 7 Puget Sound stations · as of Jun 4, 2026
Station GDD₃₂ Current Stage Next To Go
Issaquah / East King 2,701 Full bloom — —
Kent / Auburn 2,695 Full bloom — —
Seattle / UW 2,637 Full bloom — —
Olympia / Tumwater 2,598 Full bloom — —
Tacoma / Puyallup 2,564 Full bloom — —
Bellingham / Whatcom 2,560 Full bloom — —
Sequim / Rain Shadow 2,459 Full bloom — —
View full calendar (2 stages)
Stage GDD32 Typical Window
First bloom BBCH 61 1775 —
● Full bloom BBCH 65 NOW 2082 Observed —
Regional estimate
Range: 1329–1539 GDD₃₂ (6yr)

Sources: OSU Phenology Calendar (Herms, weather.cfaes.osu.edu) ; HortGuide field observation 2026-05-23 About GDD₃₂ →

Season tracker for Kent / Auburn as of Jun 4, 2026. Predicted dates use 16-day weather forecast through Jun 20, 2026, then climate normals.

Cultivars (4)
'Frisia'
Bright golden-yellow new foliage, maturing to greenish-yellow by summer. Full-sized tree to 40 ft. Retains thorns. Less vigorous than species. Selected in the Netherlands (W.J. Oosterbeek nursery, 1935). One of the most popular ornamental selections for foliage color.
Hardy to USDA Zone 4. Full sun required for best foliage color; shade produces green leaves. Still suckers, though less aggressively than species. Retains significant invasive potential: produces viable seed and root suckers. Not recommended where invasiveness is a concern.
'Twisty Baby'
Compact multi-stemmed shrub or small tree, 8-10 ft. Distinctive zigzag branching pattern especially prominent on new growth. Also sold as 'Lace Lady.' Much smaller than species - suitable for small gardens.
Hardy to USDA Zone 4. Grafted onto species rootstock; watch for vigorous rootstock suckers below graft union. Own-root suckering minimal due to compact habit. Rarely produces flowers or seed pods, reducing seed-based spread. Among the least invasive cultivars, but rootstock management still required.
'Umbraculifera'
Mushroom- or globe-shaped, thornless, 6-8 ft. Dense rounded crown. Rarely flowers. Non-suckering. Commonly used as a lollipop standard in European street plantings.
Hardy to USDA Zone 4. The most contained cultivar: thornless, rarely flowers, produces no viable seed. Grafted onto species rootstock, so rootstock suckers below the graft union are still possible and must be monitored. No truly non-suckering black locust exists, but this is the closest. Best choice where invasive potential is a primary concern.
'Purple Robe'
Dark rose-pink to purplish-pink flowers; bronze-red new foliage maturing to dark green. Thornless. Hybrid origin (R. pseudoacacia × R. ambigua group). 30-40 ft. One of the showiest flowering locusts.
Hardy to USDA Zone 4. Grafted onto species rootstock; monitor for rootstock suckers below graft union. Less vigorous suckering than species but still produces some root suckers. Moderate invasive risk - reduced but not eliminated.
Data Maturity
Structured Multiple sources. Expert review underway.