October Perennial of the Month-Blue Mist Flower
Late season in the garden usually means most plants are slowing down…unless you are in the Asteraceae family. Asters and their close relatives provide us, and pollinators, with late-season blooms well into first frost. Conoclinium coelestinum, commonly called blue mist flower or wild ageratum, is one such perennial. Native to the southeast and central parts of the U.S. In Missouri, it grows in low, wet woods, on bluff bases, and along streams, ponds, and ditches, though it tolerates clay and short dry spells once established. Growing 1-3’ high, blue mist flower prefers full sun to part shade. The richer your garden soil, the better it grows, and the faster it can spread into a colony or mass.
So, about that “spread” thing… although it’s not considered invasive since it’s a native, it can be “aggressive” in the right conditions, mainly moist, rich soils. You can use this trait to your advantage if you are seeking a natural solution for erosion or late-season blooms in your rain garden. In borders and beds, it can be a problem, but it pulls out easily when it gets out of line, and deadheading right after the blooms fade also helps.
Common Boneset (Eupatorium perfoliatum) is closely related, given that a synonym for Conoclinium coelestinum is Eupatorium coelestinum. With white flowers instead of blue-violet, it’s commonly found along wooded edges.
The bedding plant sold as ageratum or floss flower is Ageratum houstonianum, a tender annual from Mexico and Central America.
What I love about this plant;
-It’s native!
-No serious insect or disease problems
-Deer resistant.
-Great pollinator plant. magnet for late-season butterflies (like migrating Monarchs), native bees, and moths from July through October.
-Gives a softness to perennial borders and can be used to fill in gaps.
-Good cut or dried flower.
What’s not so great;
-A vigorous spreader by seed and rhizomes, but fairly easy to pull out with its shallow root system. Deadheading right after blooms are spent is also helpful.
-Powdery mildew can be an issue. Avoid overhead watering as much as possible and provide good air circulation.
-Can grow leggy and flop in too much shade.
-One of the last perennials to wake up, appearing well into late spring. Mark its location if you’re planting in spring, but if you forget, don’t worry; it spreads quickly enough to fill back in.
-Comments or questions? Email Garden-Lou at gardenloustl@gmail.com
Words and photos by Jo Batzer
© Jo Batzer, garden-lou.com-2026, All rights reserved.
