When I moved into my first home, I begged for a start of “Perennial Ageratum” from my mother’s garden. She warned me to “be careful what you ask for” and said that the plants spread “like wildfire.” I assumed that the plant could not be THAT bad, since she had acquired her start from her own mother’s yard. Mistaken assumption.
Perennial Ageratum or Blue Mist Flower (Eupatorium colestinum or Conoclinium colestinum) resembles Ageratum, a well-behaved annual with fuzzy blue powder-puff blooms. Blue Mist is a hardy perennial that spreads aggressively via underground rhizomes and self-seeds. I. When driving through the countryside in September, I often spot it growing in ditches and near old home sites. It can be considered a weed because of its spreading tendencies.
The 24” tall plants will thrive in sun or shade (more blooms in sun), wet or dry, and any type of soil. They will tolerate either drought and soggy wet soils. They are not bothered by insects, disease or deer. Pollinators love them.
In late summer or early autumn, they erupt into a haze of soft blue flowers that look like asters from a distance. I cope with their invasive nature by planting them with other aggressive growers and allowing them to duke it out for ground space. A great combination is Blue Mist and Goldenrod (Solidago). These two bloom at the same time. Add ‘Autumn Joy’ sedum and accent with Mexican Salvia (Salvia leucantha) for a pretty fall combination.
Control the root spread of both Blue Mist and Goldenrod by planting them in plastic nursery pots with the bottoms removed, then sinking those pots into flower beds. Be sure that the top rim of the pots extend slightly above soil surface. This will prevent spreading via roots. Deadheading as soon a blooms fade will help control reseeding.
The photo shows that a tiny seedling can have an impressive root network underground.