Hardy Ageratum

Conoclinium coelestinum

Hardy Ageratum looks like the bedding annual ageratum, but it blooms in fall when everything else is winding down, and it comes back year after year. The fluffy blue-violet flowers appear in September and October, sometimes lasting until hard frost, filling the garden with cool color at exactly the moment warm-season annuals give out. The blooms draw late-season skippers, sulphurs, and small native bees that are building reserves for winter. It spreads assertively by both rhizomes and self-seeding. that's a feature, not a bug, if you want it to naturalize along a wet slope or fill gaps in a rain garden. In tighter spaces, simply pull seedlings or divide clumps in spring to keep it in check. The blue color is rare in fall natives and makes it a valuable design element for extending the season.