I love Eupatorium coelestinum for its showy bloom clusters in early fall. This plant is usually referred to by one of its common names which include Hardy or Perennial Ageratum and Mistflower.
Eupatorium is a deciduous perennial native American plant in the same family as Joe Pye Weed. The bright periwinkle blue blooms of Eupatorium coelestinum attract a lot of attention in September and October, when most flowers have stopped blooming. Mistflower contrasts nicely with fall-blooming asters and the common fall mums.
The plentiful 4-inch flower clusters atop tall stems resemble the shorter annual ageratum, but Hardy Ageratum comes back bigger and better every single year.
With large clumps of bright blooms visible from a distance, Mistflower is a good candidate for a roadside garden.
Hardy Perennial Ageratum prefers full sun, fertile soil, and regular water, but this hardy variety can tolerate periods of drought. Like other varieties of Eupatorium, this one can grow in wet soil too. We have a patch behind the greenhouse that has taken over a shady spot where the sprinkler keeps the soil evenly moist to wet.
This very showy and assertive perennial will reach heights of up to 4 feet tall, but it can be kept mowed to only a few inches. Just to give you a hint of its vigor, Mistflower is in the mint family.
This is a perennial plant that looks best in naturalistic informal gardens. It grows very well in my shade garden, where its blooms seem electrified in September.
Perennial Ageratum emerges late in spring, so be careful not to uproot it when doing your spring weeding.
Cut back once or twice in summer to promote bushiness and more blooms, and to keep it from coming into full bloom too early. I always want mine to wait and bloom in October.