by Melinda Heigel, EMGV
You know it’s late summer with fall on the way when you see the delicate blue to purple flowers of hardy ageratum (Conoclinium coelestinum) begin to emerge. Hardy ageratum has more common names than you can shake a stick at: blue boneset, blue mistflower, mist flower, and wild ageratum to name a few. Beauty is in the eye of the beholder when it comes to this wildflower, as some might consider this plant to be a mere invasive weed you often see growing along roadsides. But it can be right at home in your garden, offering great fall color and texture until the first frost. This plant also provides late-season nectar for pollinators–especially butterflies, and birds enjoy the seeds, too.
Hardy ageratum is a herbaceous perennial native to the eastern US and one of three native mist flowers in the US overall.1 The flowers are tubular in nature, resemble thistles, and grow in dense clusters called corymbs. These soft, fuzzy and showy flowers offer an etherial old-fashioned look. But make no mistake about this plant– it’s tough, reliable, and get can get assertive in the cultivated landscape. It spreads by rhizomes (underground stems that creep horizontally and give off roots and shoots) and is also self-seeding. Its enthusiastic growth habit makes it a great plant for naturalized areas, cottage gardens, or urban prairies. But don’t let its vigor deter you. It is also great as a back-of-the-border plant. And while that likely means some diligent “management” on your part, it is well worth the effort. You’ll enjoy roughly 2-3 months of continuous flowers. This late-bloomer has a clumping upright habit and typically grows 1.5′-3′ tall and up to 3′ wide.
Growing Conditions and Care
Hardy ageratum prefers full sun to part shade, but full-sun conditions ensure the most abundant blooms and compact habit. These natives grow best in average, medium to wet but well-drained soils and grow all throughout North Carolina. They prefer fertile environments. Propagating these plants is easy; clump division in early spring is best. Also consider a springtime prune as well, again, to encourage a denser plant and reduce the need to stake later in the summer. These plants can get weedy if not cut back.
Hardy ageratum isn’t a plant you easily find these days unless your local nursery places an emphasis on native plants. These fall into the “pass-along plant” category, as these might be plants you get from a neighbor or a plant swap (and there is always that roadside ditch). You can, however, find seeds for this perennial wildflower online or at your local garden center. But don’t confuse hardy ageratum with its cultivated cousin Ageratum houstonianum. You can find these tender annual varieties in abundance for sale each spring and summer. The look is quite similar, but they tend to be shorter, more compact plants (usually 6″- 8″ tall) and grow in tight mounds, though some taller varieties are available. While they share the same preferences for growing conditions as the perennial hardy ageratum and also have a long bloom time, these annuals are often best for containers and front-of-the-border spots in your beds.
Two examples of the many choices of annual non-native varieties of ageratum that bloom from spring to fall. (Image Credits: Cathy DeWitt CC BY 4.0 and Mauricio Mercadante CC BY-NC-SA 2.0 )
Despite its propensity to be overzealous, hardy ageratum can be a solid addition to your landscape. A workhorse of a plant, it supports wildlife like beneficial insects and birds while at the same time managing to be deer and rabbit-resistant. Keep an eye out for it now through November and imagine what it might lend to your garden.
1–There are 3 hardy ageratums native to the US, and Texas is the only state where you can find them all. In addition to the our specimen spotlight Conoclinium coelestinum, there are also the betony-leaf mistflower (C. bentonicifolim) native only to Texas and the palmleaf mistflower (C. dissectum, formerly known as the C. greggii) native to Texas, New Mexico and Arizona. All perform well in Zones 7-11.
Resources and Additional Information
Visit North Carolina State University’s plant toolbox website for more details on hardy and annual ageratum
Native perennial: https://plants.ces.ncsu.edu/plants/conoclinium-coelestinum/
Non-native annual: https://plants.ces.ncsu.edu/plants/ageratum-houstonianum/
Missouri Botanical Garden’s website offers a through profile of hardy ageratum
For a finer look at the taxonomy changes for hardy ageratum formerly known as Eupatorium coelestinum, the University of Arkansas extension site outlines the journey of scientific classification
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