Blue wood aster
Aster cordifolius

Hardiness: Zones 3 to 8
Plant type: Herbaceous perennial
Height: 2- 5'
Spread: 1.5- 2'
Sun: Part shade to full sun
Water: Dry to medium
Bloom time: September, October
Bloom color: Pale blue
Soil: Loamy, average well-draining soils, neutral (≈7)

Notes: Self-seed to the point that some describe the plant as “weedy”.  This is not a big issue in a natural setting but in smaller more manicured gardens, spent flowers can be removed to prevent seed formation.

More information on this plant here.