Black Locust
Robinia pseudoacacia
Fabaceae · broadleaf deciduous tree · 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.
Quick Facts
Phenological Calendar
As of April 3, 2026, Puget Sound stations range from 1070 to 1180.6 GDD₃₂. Black Locust typically reaches bloom start at 1566 GDD₃₂, predicted around Apr 26.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 3, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,181 | Pre-season | Bloom start | 385 |
| Seattle / UW | 1,171 | Pre-season | Bloom start | 395 |
| Kent / Auburn | 1,111 | Pre-season | Bloom start | 455 |
| Olympia / Tumwater | 1,106 | Pre-season | Bloom start | 460 |
| Bellingham / Whatcom | 1,101 | Pre-season | Bloom start | 465 |
| Tacoma / Puyallup | 1,075 | Pre-season | Bloom start | 491 |
| Sequim / Rain Shadow | 1,070 | Pre-season | Bloom start | 496 |
| Stage | GDD32 | Typical Window |
|---|---|---|
| ○ Bloom start BBCH 61 NEXT | 1566 | Mar 15-May 15 est. Apr 26 (avg) |
| Full bloom BBCH 65 | 1835 | — est. May 9 (avg) |
| 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 3, 2026. Predicted dates use 16-day weather forecast through Apr 19, 2026, then climate normals.