Horse Chestnut
end of floweringAesculus hippocastanum
Sapindaceae · broadleaf deciduous tree · introduced
Last updated
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
Plant Profile
Size & Form
Site Requirements
Ornamental Interest
Field Observations
Active Conidial Spread
Diseases: Regionally Documented (2)
Phenological Calendar
As of June 3, 2026, Puget Sound stations range from 2435.5 to 2672.8 GDD₃₂. Horse Chestnut has passed end of flowering (2398 GDD₃₂).
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Jun 3, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 2,673 | end of flowering | — | — |
| Kent / Auburn | 2,665 | end of flowering | — | — |
| Seattle / UW | 2,610 | end of flowering | — | — |
| Olympia / Tumwater | 2,570 | end of flowering | — | — |
| Tacoma / Puyallup | 2,535 | end of flowering | — | — |
| Bellingham / Whatcom | 2,533 | end of flowering | — | — |
| Sequim / Rain Shadow | 2,436 | end of flowering | — | — |
View full calendar (2 stages)
| Stage | GDD32 | Typical Window |
|---|---|---|
| Beginning of flowering BBCH 61 | 1130 | — |
| ● end of flowering BBCH 69 NOW | 2398 Observed | — |
Sources: OSU phenology catalog (OSU: weather.cfaes.osu.edu) ; Field observation, SeaTac, WA (Highline SeaTac Botanical Garden). About GDD₃₂ →
Season tracker for Kent / Auburn as of Jun 3, 2026. Predicted dates use 16-day weather forecast through Jun 19, 2026, then climate normals.