Horse Chestnut
Aesculus hippocastanum
Sapindaceae · broadleaf deciduous tree · introduced
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
Horse chestnut produces a dramatic spring flower display and then spends the rest of summer looking increasingly rough. Leaf blotch (Guignardia) turns foliage brown from midsummer onward in most years, and leaf scorch adds to the problem in hot Augusts. Neither condition threatens the tree but both make it look terrible from July through leaf drop. The tree needs moisture; our summer drought stresses it. Bloom happens early (251 GDD base 50, typically May), and the flowers are showy. The seeds contain saponins and are toxic to humans and most animals, which is a serious consideration where children play. 'Baumannii' produces double flowers and few seeds, which addresses the toxicity concern. For a large shade tree with spring flowers and minimal pest issues, horse chestnut works if you can tolerate the late-summer foliage decline.
— Chris Welch, ISA Certified Arborist
Quick Facts
Phenological Calendar
As of April 23, 2026, Puget Sound stations range from 1434.4 to 1592.7 GDD₃₂. Horse Chestnut has passed beginning of flowering (1130 GDD₃₂).
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 23, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,593 | Beginning of flowering | — | — |
| Seattle / UW | 1,554 | Beginning of flowering | — | — |
| Kent / Auburn | 1,537 | Beginning of flowering | — | — |
| Olympia / Tumwater | 1,505 | Beginning of flowering | — | — |
| Bellingham / Whatcom | 1,483 | Beginning of flowering | — | — |
| Tacoma / Puyallup | 1,472 | Beginning of flowering | — | — |
| Sequim / Rain Shadow | 1,434 | Beginning of flowering | — | — |
| Stage | GDD32 | Typical Window |
|---|---|---|
| ● Beginning of flowering BBCH 61 NOW | 1130 | — |
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 23, 2026. Predicted dates use 16-day weather forecast through May 10, 2026, then climate normals.