Ageratum flowers usually emerge as a vibrant shade of blue or purple.
Ageratum (Ageratum spp.), also called flossflower or whiteweed, is a genus of several dozen flowering plant species. The Ageratum taxa belong to the Asteraceae family, also called Compositae, which contains more than 20,000 species. This taxonomic family encompasses several genera of common garden plants, including sunflowers and daisies. While Ageratum is native to regions of South and Central America, it has been imported and successfully grown in North American climates. Does this Spark an idea?
While there are more than 50 species of ageratum, only a handful are cultivated and distributed in the United States. Only two of these species, bluemink (Ageratum houstonianum) and tropical whiteweed (A. conyzoides), maintain a significant presence in North America. Bluemink is mostly found in Southeastern states, including Florida and the Carolinas, but also grows in Connecticut and Massachusetts. Tropical whiteweed has a wider geographical spread. It grows in the Southeast as well as in California, Missouri and Maryland among other states. According to the University of Wisconsin Extension, the majority of ageratum sold in the U.S. is hybridized cultivars of two or more species.
Depending on the species and cultivar, an ageratum plants grow up to 2 feet tall with a, equivalent lateral spread. Some cultivars including Blue Jay are as short as 4 inches, but many of the common cultivars grow to approximately 10 inches tall. Ageratum foliage emerges in pairs on opposite sides of the stems. The pointed, oval leaves have serrated edges and are usually between 2 and 4 inches long. The foliage width varies. Some cultivars have broad leaves almost as wide as they are long. The distinct flowers emerge in small clusters. They are spherical and appear to be feathery or fluffy, bearing some resemblance to a cotton ball. Thin, flexible strands extend beyond the bulk of the flower ball, giving the blossoms a unique aesthetic.
Growth and Management
Some ageratum species are annuals including most of those distributed in North America, but some are perennial. They are vigorous propagators and needing removal when they expand to other areas of a garden or lawn. Ageratum thrives in U.S. Department of Agriculture Plant Hardiness Zones 8 to 10 but is able to survive in zones 5 through 7 as well. They succumb to freezing or near-freezing temperatures and are frost intolerant. These plants prosper with full exposure to sunlight but can perform adequately in slight or moderate shade as well.
Pests and Pathogens
According to The Connecticut Agricultural Experiment Station, ageratum plants are susceptible to several ailments caused by various fungi including root rot and Botrytis blight. Root rot is a general condition caused by several groups of fungi including the Pythium and Fusarium genera that affect almost all types of terrestrial plants. The same article published by the CAES lists mealybugs (Planococcus citri) and whiteflies (Trialeurodes vaporariorum) as potential insect pests of ageratum. They are also vulnerable to fungal mildew and infestations of spider mites.