Vernonia (ironweed) is a genus of about 1,000 species of annuals, perennials, climbers, subshrubs, shrubs and trees in the Asteraceae family.  Plants are mainly found in South America but can also be found in North America, Africa, Asia and Australasia.  In the cooler north, the plants are usually annuals or herbaceous perennials.  In the tropics, Vernonia is mainly woody.  The name honors a well known British botanist, William Vernon, who died in 1711.  

 

The perennial ironweeds used as ornamentals in mixed border gardens or wild gardens have upright stems bearing alternate leaves.  Flowerheads are clusters of purple or reddish pink (there is a rare white flowered species) that become rust-colored with age.  Perhaps the rust color is how it got the name ironweed.  Plant it in your garden if you want butterflies and hummingbirds.

A popular species found in eastern gardens in the United States is New York ironweed (Vernonia noveboracensis).  Hardy in zones 5 through 9, this four to nine foot tall plant blooms from August to September. The moister the soil, the taller the plant gets.  Tracy DeSabato-Aust, in her book The Well-Tended Perennial, describes several ways of pruning the plant, while still small, in order reduce the height by several feet.  

"One such method is to cut down the plants to the ground when they reach 2 ft. tall.  (They) may have about a 3-week delay in bloom but will flower nicely….."  

Often used in drifts in the back of gardens, NY ironweed spreads three to four feet.  It likes full sun and likes medium to wet soil that is slightly acidic.  It is deer tolerant and doesn't mind clay or wet soil.  It will self-seed, especially in moist areas, if the 3-4" purple flower heads are left to go to seed.  At the end of the growing season, the stems can be cut down to the ground or left for winter interest and cut down in early spring.

There are some dwarf varieties such as Vernonia lettermannii 'Iron Butterfly' with unusual fine feathery leaves.  It is thirty to thirty-six inches tall.  Vernoniea fasciculata is four feet tall.  On the other hand, giant ironweed (Vernonia gigantea) is the tallest species and has been known to grow up to ten feet.

Courtesy of NYBG Plant Information Service