Vernonia spp. and cvs.

USDA Hardiness Zones: 4 to 9

Conditions: Full sun to partial shade; moist, well-drained soil but adaptable to drier and/or gravelly soil

Bloom Time: Mid- to late summer through mid- to late fall

Troubles: Powdery mildew and rust

In an objective trial, there should be no favorites, but Missouri ironweed (V. missurica) was mine. Bright violet flowers sprout from plump purple buds, putting on one of the best shows of late summer I’ve seen. Its lance-shaped leaves, up to 7 inches long, are fuzzy and silvery due to the dense white hairs on the undersides. As a toucher, I never pass Missouri ironweed without caressing the velvety leaves. Compared to other ironweeds, I’ve always thought there is something more refined about Missouri ironweed than its stoutness suggests.

New York ironweed (V. noveboracensis) was my introduction to the genus, so I have a soft spot for it. Of all the ironweeds in the trial, it was the gangliest in its youth, although over time it develops a strong bushy habit. In autumn, cavorting monarch butterflies are especially brilliant against the deep purple flowers. Its dark green, lance-shaped leaves are big—up to 8 inches long, a little coarse, and borne on stiff, purplish stems. New York ironweed had the distinction of remaining rust free during our trial—an attribute shared only with narrowleaf ironweed. Although New York ironweed grows naturally in wet sites, it is adaptable to drier conditions, too.

From a distance, western ironweed (V. baldwinii) looks a little like Joe-Pye weed (Eutrochium spp. and cvs, Zones 4 to 9), but its attractive, lance-shaped leaves—rough on the surface and softly hairy on the underside—are not whorled like its cousin’s. The same white hairs clothe the arrow-straight stems, which branch in midsummer to hold purple flowers in loose, flattened clusters. The scales of the floral bracts recurve at the tips, giving the rosette a barbed appearance, but it’s not at all prickly. Given the smaller flower heads—less than an inch wide—the floral display is not as vivid as Arkansas ironweed, but it’s as bountiful as any ironweed. The upright habit is handsome and narrow—well, at 50 inches wide, what passes for narrow for an ironweed. Contrary to warnings, western ironweed did not spread or create a colony during seven years in the trial.

Arkansas ironweed (V. arkansana) is readily recognizable by the long, thread like appendages on its floral bracts, which is undoubtedly the origin of its alternative common name: curly top ironweed. Purple flowers in large heads to 1¼ inches wide look like pincushions atop the tall, stiff stems in late summer. Like other species in the trial, these plants came from seed that was wild-collected in the Ozarks; being seed-grown, foliar and habit traits varied slightly among the plants. The narrow, willow-like leaves—up to 7 inches long—were occasionally dusted with late-season powdery mildew and rust. Hummingbirds have always been rare sightings in the trial gardens, but they joined in with the butterflies and bees on this ironweed.

New Kids to Keep an Eye On

Bringing ironweeds down in size was going to happen, whether through selective breeding or letting nature have its way. ‘Southern Cross’ (V. ‘Southern Cross’) is a shorter selection from Intristic Perennial Gardens that came from seed sown as narrowleaf ironweed. Its dark green, willow-like leaves are wider, so the unknown parent certainly had broader leaves. The mystery parent passed on some color too—‘Southern Cross’ has purplish stems, and its uppermost leaves have a bronze cast. Bright purple flowers in heads up to an inch across are plentiful from late summer to early fall. Next to its giant cousins, ‘Southern Cross’ is almost a dwarf—expected to be only 4 feet tall and wide.

In a “glass-half-empty” moment, I imagined that the flowers of ‘White Lightning’ New York ironweed(V. novaboracensis ‘White Lightning’) might be a not-so-pretty muddy white like Joe-Pye weed. Imagine my delight when the flowers opened pure white in late summer! ‘White Lightning’ is a seed strain developed by Jelitto Perennial Seeds, and it blooms the first year from seed. Like other ironweeds, though, the habit is a little sparse the first year or two. We haven’t grown it long enough to achieve full size, but it is estimated to reach 6 to 8 feet tall and 3 to 4 feet wide. So far, the dark green leaves have been rust free, but powdery mildew has been troublesome at times. Happily,butterflies and bees make no distinction between white and purple—we’ll see if gardeners do.

While waiting to be christened, ‘Summer’s Swan Song’ (V. ‘Summer’s Swan Song’) garnered a lot of attention as ‘20100308’ in our trials. Early on, we admired the deep red stems and purple-tinged, olive green leaves—the shape comes from narrowleaf ironweed and the coloring from ‘Plum Peachy’ tall ironweed (V. angustifolia ‘Plum Peachy’). Jim Ault of Chicago Botanic Garden (CBG) was the expert matchmaker. From early to midfall, deep purple flowers are plentiful on long, flowering branches—a unique trait of this cultivar. Unlike other ironweeds, the longer branches form an interlocking network that holds the stems upright on even the windiest days. ‘Summer’s Swan Song’ is the smallest of the hybrid cultivars right now, topping out at a compact 36 inches tall and 40 inches wide. Despite the marginal cold-hardiness of ‘Plum Peachy’, ‘Summer’s Swan Song’ is hardy in Zones 4 to 9.

‘Summer’s Surrender’ (V. ‘Summer’s Surrender’), is also the result of careful breeding—arising from a cross between narrowleaf and Arkansas ironweed made by Jim Ault at CBG. ‘Summer’s Surrender’ has a passing resemblance to ‘Southern Cross’, but, at 48 inches tall and 74 inches wide, it has a much broader, mounded habit. The 5-inch-long, linear leaves are dark green and especially lovely; as a bonus, they are resistant to powdery mildew and rust. From early to late fall, ‘Summer’s Surrender’ marks the turning of the seasons with an impressive show of dark purple flowers in broad, airy clusters.

Basics

Ironweed 101

Ironweeds are long-lived perennials that rarely need division. The common name may be a nod to the iron-like rigidity of the stems or more likely refers to the rusty hairs on the cinnamon-colored seeds. Here’s what else you need to know:

Habit: Plants tend to grow taller in moist soils, although some are drought tolerant once established.

Pests and Disease: Powdery mildew and rust can infect foliage in fall. Their bitter-tasting leaves are not palatable to most mammals, including deer, but rabbits occasionally browse narrow-leaf ironweed and ‘Plum Peachy’ tall ironweed.

Pruning: To reduce plant size, cut back new stems by half in late spring; in our trials, plants were up to 18 inches shorter when sheared.

Flowers: Ironweeds are in the aster family but lack their flamboyant, petal-like ray florets. Instead, upward of 50 or more tubular disk florets crowd into compact flowerheads enclosed by leafy bracts that are unique to a species and helpful in identification. Some flowerheads measure a foot across or more.

Pollinators: It’s nearly impossible for me to picture the flowers without a mélange of bees and butterflies hovering overhead, as they are so much a part of the floral show.

Foliage: The dark green foliage tends to be lance shaped but can be willow-like or feathery. Leaf size enhances the bigness of some ironweeds; for example, giant ironweed tops out at 8 feet tall, but the 10-inch leaves make it seem even larger.

Seed Heads: Deadheading reduces unwanted seedlings; however, if left in place the seedheads add extra interest in late fall and in winter.

Ironweed Trial Result

Rating key

★★★★ Excellent

★★★ Good

★★ Fair

★ Poor

*Plant height includes flowers.