Thinleaf Alder
Alnus incana subsp. tenuifolia
Betulaceae · broadleaf deciduous shrub · native
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
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
Phenological Calendar
As of April 23, 2026, Puget Sound stations range from 1434.4 to 1592.7 GDD₃₂. Thinleaf Alder has passed 'full bloom' (708 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 | 'Full bloom' | — | — |
| Seattle / UW | 1,554 | 'Full bloom' | — | — |
| Kent / Auburn | 1,537 | 'Full bloom' | — | — |
| Olympia / Tumwater | 1,505 | 'Full bloom' | — | — |
| Bellingham / Whatcom | 1,483 | 'Full bloom' | — | — |
| Tacoma / Puyallup | 1,472 | 'Full bloom' | — | — |
| Sequim / Rain Shadow | 1,434 | 'Full bloom' | — | — |
| Stage | GDD32 | Typical Window |
|---|---|---|
| 'First bloom' BBCH '61' | 380 | '' |
| ● 'Full bloom' BBCH '65' NOW | 708 | '' |
GDD = Growing Degree Days (base 32°F, Jan 1 start). Why base 32? Hover over GDD values for source details. Season tracker for Kent / Auburn as of Apr 23, 2026. Predicted dates use 16-day weather forecast through May 10, 2026, then climate normals.