Thinleaf Alder
'Full bloom'Alnus incana subsp. tenuifolia
Betulaceae · broadleaf deciduous shrub · native
Last updated
Thinleaf alder is a thicket-forming shrub or small tree along streams in western mountains from Alaska to New Mexico. Its thin, smooth green-gray or reddish-brown bark and multiple stems signal its colonizing habit. The leaves are thinner than those of white alder, with more finely toothed margins. It rarely exceeds 40 feet and often remains shrubby, especially on harsh, exposed sites.
Thinleaf alder tolerates cold, harsh mountain sites with short growing seasons. It establishes readily on wet disturbed soils and eroding streambanks where its nitrogen-fixing roots stabilize soils and begin enrichment. Fast-growing (20 feet in 20 years on good sites) and moderate-lived. This species is primarily valuable for riparian and avalanche path restoration in mountain settings.
Quick Facts
Spore Release (Fall & Spring Rains)
Aecial Stage (Alternate Host)
Larval Feeding
Active Below-ground Growth
+ 4 more — see full disease and pest lists below
Phenological Calendar
As of May 14, 2026, Puget Sound stations range from 1926.1 to 2121.1 GDD₃₂. Thinleaf Alder has passed 'full bloom' (708 GDD₃₂).
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of May 14, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 2,121 | 'Full bloom' | — | — |
| Kent / Auburn | 2,113 | 'Full bloom' | — | — |
| Seattle / UW | 2,085 | 'Full bloom' | — | — |
| Olympia / Tumwater | 2,047 | 'Full bloom' | — | — |
| Tacoma / Puyallup | 2,016 | 'Full bloom' | — | — |
| Bellingham / Whatcom | 1,994 | 'Full bloom' | — | — |
| Sequim / Rain Shadow | 1,926 | 'Full bloom' | — | — |
| Stage | GDD32 | Typical Window |
|---|---|---|
| 'First bloom' BBCH '61' | 380 | '' |
| ● 'Full bloom' BBCH '65' NOW | 708 | '' |
Source: 'Master catalog (OSU), GDD50 base' About GDD₃₂ →
Season tracker for Kent / Auburn as of May 14, 2026. Predicted dates use 16-day weather forecast through May 30, 2026, then climate normals.