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
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Mar 30, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,116 | Pre-season | Beginning of flowering | 3,112 |
| Seattle / UW | 1,112 | Pre-season | Beginning of flowering | 3,116 |
| Olympia / Tumwater | 1,049 | Pre-season | Beginning of flowering | 3,180 |
| Kent / Auburn | 1,048 | Pre-season | Beginning of flowering | 3,180 |
| Bellingham / Whatcom | 1,041 | Pre-season | Beginning of flowering | 3,187 |
| Tacoma / Puyallup | 1,017 | Pre-season | Beginning of flowering | 3,211 |
| Sequim / Rain Shadow | 1,012 | Pre-season | Beginning of flowering | 3,216 |
| Stage | GDD32 | Typical Window |
|---|---|---|
| ○ Beginning of flowering BBCH 61 NEXT | 4228 | — est. Jul 30 (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 Mar 30, 2026. Predicted dates use 16-day weather forecast through Apr 15, 2026, then climate normals.