Photos courtesy of Chicago Botanic Garden

Ironweeds inhabit a broad swath of the United States, from the Mid-Atlantic to the Midwest and from Minnesota south to Texas. While many are tall, even towering in height, all ironweeds offer an abundance of purple-hued flowers in late summer and early fall. Their value as a food source for pollinators is irrefutable — scores of bees, butterflies and numerous other insects feverishly work the flowers throughout the late bloom season. Ironweeds are extraordinary ecological plants, due to their indigenousness and importance as powerhouse pollinator plants, but they are great garden plants, too. At a glance, some ironweeds may seem too tall for many gardens, but recent breeding has resulted in a number of compact hybrid selections. The shorter ironweeds may have given up some size but have lost none of their ornamental appeal or draw for pollinators. Vernonia spp. are in the aster or daisy family (Asteraceae), but lack the showier petal-like ray florets common to the composite inflorescences of coneflowers (Echinacea spp.), asters (Symphyotrichum spp.) and sunflowers (Helianthus spp.). For several weeks beginning in mid- to late summer, purple to magenta capitula bloom in many-flowered inflorescences measuring up to a foot or more across. Leaves are typically dark green and tend toward lance-shaped but can be willow-like to filiform. Conversely, silver ironweed sports bright silvery white linear leaves. Leaf size enhances the robustness of some ironweeds; for example, giant ironweed (V. gigantea) reaches 8 feet tall, but the 10-inch long leaves make it seem even larger. On the other hand, at 3 inches long and barely wider than a sliver, the linear leaves of narrowleaf ironweed (V. lettermannii) look more like Arkansas blue star (Amsonia hubrichtii) than any of its kin.

Most of the commonly cultivated species have woody, fibrous root crowns and stiff stems ranging from several feet to 10 or more feet in height. Ironweeds are generally easy to grow in full sun and moist, well-drained soils but are often adaptable to light shade and drier soils, and some species are drought-tolerant once established. Silver ironweed, for example, is best grown in lean, gravelly soil or decomposed granite, in full or half-day sun. Ironweeds tend to grow taller in moist conditions.

Many ironweeds are hardy to at least USDA Zone 5 or colder, while others are native to warmer places in the Southeast and westward to Texas. Ironweeds are typically long-lived, growing into large clumps over time, but rarely need division. Deadheading reduces unwanted seedlings, which can be prolifically produced, especially in moist areas; however, deadheading removes a food source for late-season songbirds. Powdery mildew and rust can infect foliage in late summer or fall — some species are more susceptible than others. Disease levels can be severe, thus deleteriously affecting plant health. The bitter-tasting leaves are usually not palatable to most grazing mammals including deer. Ironweeds are both a boon and a challenge to gardeners — their size can be daunting for average gardens, but their late-blooming purple flowers attract a host of pollinators. Ironweeds put on an impressive show in native and naturalistic landscapes, meadows and formal gardens.

In early summer, the dark green foliage provides a handsome backdrop for a variety of earlier -blooming perennials; whereas, late-season bloomers such as sunflowers (Helianthus spp.), goldenrods (Solidago spp.), and big bluestems (Andropogon gerardii cultivars) make stellar floral companions. A shorter stature and feathery foliage sets narrowleaf ironweed (V. lettermannii) apart from other species, and provides a pleasing textural contrast with bolder plants. Despite the large number of species — upward of 1,000 herbaceous and woody plants from the Americas, Asia, Africa, and Australia — ironweeds are not widely cultivated and still are uncommon in home gardens.

The evaluation study

The Chicago Botanic Garden (USDA Hardiness Zone 5b, AHS Plant Heat-Zone 5) undertook a comparative trial of Vernonia species and cultivars from 2012 through 2018. The goal of the trial was to determine the garden-worthiness of a variety of cold-hardy ironweeds. The trial group consisted of 17 taxa in all—representing ten species with eight associated subspecies, cultivars, or hybrid selections. Plants were acquired commercially or were grown from wild-collected seeds; seed-grown species exhibited variable traits within a taxon and included V. arkansana, V. baldwinii, V. fasciculata, V. gigantea, V. gigantea ssp. gigantea, V. glauca, V. missurica and V. noveboracensis. The ironweed trial was originally initiated in 2009 but was interrupted in 2010 by a renovation project in the evaluation garden. Due to significant changes in bed design, all plants were transplanted to pots and moved to the production nursery in June 2010. The original 11 taxa were replanted in the trial garden in September 2011; the official restart of the trial began with data collection the following spring. Several new taxa were added to the trial between 2012 and 2015 including V. angustifolia ‘Plum Peachy’, V. gigantea ssp. gigantea ‘Jonesboro Giant’, V. noveboracensis ‘White Lightning’, V. ‘Southern Cross’, V. ‘Summer’s Surrender’ and V. ‘Summer’s Swan Song’.

Five plants of each taxon were grown in side-by-side plots for easy comparison of ornamental traits and landscape performance. The evaluation garden was openly exposed to wind in all directions and potentially received up to 10 hours of full sun daily during the growing season, which averaged 175 days per year for the 2012-2018 trial period. The clay-loam soil had a pH of 7.4 during this period, and although typically well-drained, the site occasionally retained excess moisture for short periods in all seasons. Maintenance practices were kept to a minimum, thereby allowing the plants to thrive or fail under natural conditions. Trial beds were irrigated via overhead sprinklers as needed, mulched with composted leaves once each summer, and regularly weeded. Moreover, plants were not deadheaded, fertilized, winter mulched, or chemically treated for insects or diseases. Plants were cut back to near the base in late winter before new growth began.

In the trial, the ironweeds were regularly observed for their cultural adaptability to the soil and environmental conditions of the full sun evaluation garden; diseases and pests; winter hardiness and survivability; and ornamental qualities associated with foliage, floral display, and plant habits. All taxa were evaluated for a minimum of four years, except for ‘Plum Peachy’, which died during the second winter in two different trials, and ‘White Lightning’, which had been in the garden for only three years when the trial was terminated in autumn 2018.

Vernonia ‘Southern Cross’

Top-rated ironweeds

Four ironweeds received five-star excellent ratings, including V. gigantea ssp. gigantea ‘Jonesboro Giant’, V. lettermannii ‘Iron Butterfly’, V. ‘Summer’s Surrender’ and V. ‘Summer’s Swan Song’. These top-rated ironweeds featured superior ornamental traits such as strong vigorous habits, handsome foliage, heavy flower production, winter hardiness, and disease resistance.

‘Jonesboro Giant’ was the largest ironweed in the trial, reaching 144 inches tall and 60 inches wide. ‘Jonesboro Giant’ differed from the subspecies in being significantly taller and narrower in habit, and flowering seven to 10 days earlier. The rigid stems were upright at all times, although the plants relaxed a bit in October during peak bloom. The fine-textured inflorescences and upper stems were dark burgundy. Flower production was consistently heavy, with smallish, ¾-inch-wide purple flower heads; the inflorescences were commonly more than 12 inches wide. The large, dark green leaves were generally healthy, with only minor powdery mildew observed. The late bloom period of ‘Jonesboro Giant’ — late September to early November — was occasionally truncated by early frosts in October; the historical frost date at the Chicago Botanic Garden is October 15. In addition, the large leaves were sometimes tattered by strong winds, especially on the upper half of the stems.

The soft, needle-shaped leaves of ‘Iron Butterfly’ had a similar feathery appearance to another Arkansas native — spring-blooming blue star (Amsonia hubrichtii) — rather than to any of the other ironweeds. ‘Iron Butterfly’ originated in the University of Georgia’s trial garden, and was selected for its vigorous growth, compact habit, and floriferous nature. At 33 inches tall, ‘Iron Butterfly’ was 10 inches shorter than the species and had a tighter habit. Otherwise, the purple flowers were the same color and size as the species, and both taxa were equally floriferous. ‘Iron Butterfly’ was less prone to opening up in the center in heavy rainfall but was not untouched by this issue; damage was always more significant on the species, which was also less likely to rebound than ‘Iron Butterfly’. Powdery mildew and rust were never observed on ‘Iron Butterfly’ or the species. These taxa were the latest of the ironweeds to emerge in the spring. ‘Summer’s Surrender’ is a hybrid cross of V. lettermannii and V. arkansana made by Jim Ault at the Chicago Botanic Garden in 2010. It inherited the bushy habit of V. lettermannii and the larger plant size and capitula of V. arkansana; the olive-green linear leaves — 5 inches long and ½ inch wide — were intermediate between the two species. ‘Summer’s Surrender’ was 48 inches tall and 74 inches wide with a densely broad habit after five years, and it had a passing resemblance to ‘Southern Cross’. From early September to early October, dark purple florets, packed into 1-inch-wide flower heads, were generously produced in airy inflorescences. ‘Summer’s Surrender’ was resistant to powdery mildew and rust.

‘Summer’s Swan Song’ is a hybrid created by Dr. Ault’s crossing of V. lettermannii and V. angustifolia ‘Plum Peachy’. Similar in bushiness and fine texture to ‘Iron Butterfly’, ‘Summer’s Swan Song’ is a slightly larger plant that resists lodging because of its elongated floral branches; the interlocking of the floral branches is a unique trait that helps hold stems upright on rainy days. Deep purple florets in 1-inch -wide heads were plentiful from early September to mid-October. The feathery foliage was moderate to dark olive-green with red petioles, up to 5 inches long and less than a quarter inch wide, and disease-free. After five years in the trial, ‘Summer’s Swan Song’ measured 36 inches tall and 40 inches wide. ‘Summer’s Swan Song’ is hardy in Zone 4, despite the marginal cold-hardiness of ‘Plum Peachy’.

The comparative trial at CBG is in USDA Hardiness Zone 5b.

Worthy of more consideration

Ironweeds may be uncommon garden plants but are obvious choices for ecological and naturalistic landscapes, especially for pollinator gardens. The sheer number and variety of insects drawn to their profuse display of late-season purple flowers is astonishing. The only plant group with greater insect visitation in the Chicago Botanic Garden’s trials were mountain mints (Pycnanthemum spp.), and in particular, silvery-leaved P. muticum.

Ironweeds are often overlooked as garden plants due in part to their large size, and perhaps, being native plants, they are not readily available in average garden centers. Furthermore, the dearth of innovation in breeding and selecting new cultivars has until recently exacerbated the matter. Fortunately, the introduction of new compact hybrid cultivars, such as ‘Southern Cross’, ‘Summer’s Surrender’, and ‘Summer’s Swan Song’, has created excitement in the gardening world. Beyond the strong ornamental attributes, easy culture and adaptability to a variety of cultural conditions are merits of ironweeds.

Read the report in full at chicagobotanic.org/sites/default/files/pdf/ plantevaluation/no45_vernonia.pdf.

About the author: Richard G. Hawke is the plant evaluation manager and associate scientist at the Chicago Botanic Garden.