Swamp milkweed

Asclepias incarnata

Apocynaceae · [VERIFY] · introduced

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

Height
3–4 ft
Spread
2–3 ft
Growth Rate
Medium
Light
[Verify]
Soil
[Verify]
Water
[verify]
Hardiness
Zone [VERIFY]
Bloom Time
july to august
Origin
North America

Phenological Calendar

As of May 24, 2026, Puget Sound stations range from 2172.5 to 2394 GDD₃₂. Swamp milkweed typically reaches beginning of flowering at 4228 GDD₃₂, predicted around Jul 24.

Regional Season Tracker

GDD₃₂ accumulation across 7 Puget Sound stations · as of May 24, 2026
Station GDD₃₂ Current Stage Next To Go
Issaquah / East King 2,394 Pre-season Beginning of flowering 1,834
Kent / Auburn 2,373 Pre-season Beginning of flowering 1,855
Seattle / UW 2,332 Pre-season Beginning of flowering 1,896
Olympia / Tumwater 2,295 Pre-season Beginning of flowering 1,933
Tacoma / Puyallup 2,259 Pre-season Beginning of flowering 1,969
Bellingham / Whatcom 2,256 Pre-season Beginning of flowering 1,973
Sequim / Rain Shadow 2,173 Pre-season Beginning of flowering 2,056
View full calendar (1 stages)
Stage GDD32 Typical Window
○ Beginning of flowering BBCH 61 NEXT 4228 — est. Jul 24 (avg)
Regional estimate
Range: 2989–3410 GDD₃₂ (6yr)

Source: UMD phenology catalog (UMD: extension.umd.edu) About GDD₃₂ →

Season tracker for Kent / Auburn as of May 24, 2026. Predicted dates use 16-day weather forecast through Jun 9, 2026, then climate normals.

Data Maturity
Baseline Extension data. Expert review underway.