Swamp milkweed
Asclepias incarnata
Apocynaceae · [VERIFY] · native
Last updated
Erect, clump-forming perennial growing 3 to 4 feet tall with narrow, lance-shaped leaves and fragrant pink to mauve flowers in tight clusters at stem ends in summer. Stems exude a toxic milky sap when cut.
Surprisingly tolerant of average garden soils despite native preference for wet sites; prefers full sun to partial shade and medium to wet soils. A reliable choice for low spots, stream and pond banks, and butterfly gardens; deep taproots mean plants are best left undisturbed once established.
Quick Facts
Phenological Calendar
As of April 19, 2026, Puget Sound stations range from 1342.9 to 1500.8 GDD₃₂. Swamp milkweed typically reaches beginning of flowering at 4228 GDD₃₂, predicted around Jul 28.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 19, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,501 | Pre-season | Beginning of flowering | 2,727 |
| Seattle / UW | 1,465 | Pre-season | Beginning of flowering | 2,763 |
| Kent / Auburn | 1,441 | Pre-season | Beginning of flowering | 2,787 |
| Olympia / Tumwater | 1,415 | Pre-season | Beginning of flowering | 2,813 |
| Bellingham / Whatcom | 1,390 | Pre-season | Beginning of flowering | 2,838 |
| Tacoma / Puyallup | 1,381 | Pre-season | Beginning of flowering | 2,848 |
| Sequim / Rain Shadow | 1,343 | Pre-season | Beginning of flowering | 2,885 |
| Stage | GDD32 | Typical Window |
|---|---|---|
| ○ Beginning of flowering BBCH 61 NEXT | 4228 | — est. Jul 28 (avg) |
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 19, 2026. Predicted dates use 16-day weather forecast through May 5, 2026, then climate normals.