Conoclinium coelestinum aka Blue Mistflower

Conoclinium coelestinum belongs to the aster family. It is a herbaceous perennial: a plant with soft stems that returns each year. Gardeners call it blue mistflower or hardy ageratum. It is native to southeastern Canada and Cuba. Its range also covers the central and eastern United States. It looks like annual ageratum but comes back each year. It grows in moist places, such as shady sites and roadside ditches.

Garden plants reach about 1–3 feet [0.30–0.91 metres] tall. Some sources give a narrower range of 1–2 feet [0.30–0.61 metres]. Clumps can spread to 3 feet [0.91 metres] wide. Rhizomes, or stems below ground, let it creep outward. This growth helps it form a cover over the ground. It also spreads by seed and grows fast. Fine hairs cover the stems, which may look purple. Green leaves grow in pairs across from each other. They have coarse teeth along their edges and pointed tips. Their shape ranges from egg-like to a rough triangle. A clear web of veins marks each leaf's upper side. The leaves fall rather than stay green year-round.

The flowers give mistflower its soft, hazy look. Small blue-purple tubes form thick groups with flat tops. These flower heads lack the outer rays of many daisies. Bloom runs from July to October. In some places, the flowers last until frost. Nectar draws butterflies and other pollinators, which carry pollen. Fruit appears from August to November, and birds eat the seeds. The evidence does not show whether humans can eat this plant. Nor does it show whether the plant is toxic.

Grow mistflower in full sun or partial shade. It prefers moist, fertile soil rich in organic matter. It accepts sandy soil and clay with enough moisture. Soil that drains well suits it, but wet sites also work. Keep the soil from drying out while it grows. Most cited guidance places it in USDA zones 5–10. Chicago Botanic Garden gives a broader range of zones 4–10. Divide clumps in early spring to make more plants. Tall stems may lean over and need support. Cutting them back can help reduce this problem.

Mistflower suits meadows, open woods, and pond or stream edges. It also fits native gardens and beds for pollinators. Its creeping growth can cover ground beneath taller plants. Give its spreading roots room so they will not crowd neighbors. Small beds may struggle to contain its growth. Some areas view it as a weed since it spreads so much. Deer cause less damage than they do to some plants. Serious pest and disease problems are not usual. Powdery mildew can occur. Aphids and leaf miners may also visit.