Black Locust
Robinia pseudoacacia
Fabaceae · broadleaf deciduous tree · native
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
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.
Quick Facts
Phenological Calendar
As of April 24, 2026, Puget Sound stations range from 1455.1 to 1613.4 GDD₃₂. Black Locust typically reaches bloom start at 1566 GDD₃₂, predicted around Apr 25.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 24, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,613 | Bloom start | Full bloom | 222 |
| Seattle / UW | 1,574 | Bloom start | Full bloom | 261 |
| Kent / Auburn | 1,559 | Pre-season | Bloom start | 7 |
| Olympia / Tumwater | 1,524 | Pre-season | Bloom start | 43 |
| Bellingham / Whatcom | 1,504 | Pre-season | Bloom start | 62 |
| Tacoma / Puyallup | 1,492 | Pre-season | Bloom start | 74 |
| Sequim / Rain Shadow | 1,455 | Pre-season | Bloom start | 111 |
| Stage | GDD32 | Typical Window |
|---|---|---|
| ○ Bloom start BBCH 61 NEXT | 1566 | Mar 15-May 15 est. Apr 25 (forecast) |
| Full bloom BBCH 65 | 1835 | — est. May 7 (forecast) |
| Bud break BBCH 07 | — | Feb 15-Mar 15 |
| Leaf emergence BBCH 11 | — | Mar 1-Apr 1 |
| Bloom end / petal fall BBCH 69 | — | Apr 15-May 31 |
| Fruit/seed development BBCH 71 | — | Mar 15-May 31 |
| Fruit/seed maturity BBCH 85 | — | Jun 1-Aug 31 |
| Leaf drop BBCH 93 | — | Oct 15-Nov 30 |
| Dormancy BBCH 97 | — | Nov 15-Feb 28 |
GDD = Growing Degree Days (base 32°F, Jan 1 start). Why base 32? Source GDD₅₀ thresholds from Herms 2004 (OSU, Secrest Arboretum, Ohio) and UMD IPMnet (Gill & Klick, mid-Atlantic), converted to GDD₃₂ via Kent bloom-date mapping. Season tracker for Kent / Auburn as of Apr 24, 2026. Predicted dates use 16-day weather forecast through May 10, 2026, then climate normals.