One of the tallest flowering plants in my garden is New York ironweed, scientifically known as Vernonia noveboracensis. My familiarity with this genus has been limited, but that changed recently when the Mt. Cuba Center, a prestigious institution for native plant research and conservation, released findings from its extensive four-year study examining various ironweed species—plants that not only attract pollinators but are also largely ignored by deer.

I had the pleasure of speaking with Sam Hoadley, the horticultural research manager at Mt. Cuba Center in Delaware. I eagerly anticipate his visits and the comprehensive reports generated by each plant genus studied at Mt. Cuba, including this latest discussion on ironweeds. (Additionally, Sam will be giving an online presentation on Vernonia on March 5, 2025; click here for ticket details.)

Listen to the February 17, 2025 episode of my public-radio show and podcast below. To stay updated, subscribe to future episodes on Apple Podcasts or Spotify, and browse through my podcast archive here.

A Closer Look at Vernonia with Sam Hoadley

Download file | Play in new window | Duration: 00:27:30

Subscribe: Apple Podcasts | Google Podcasts

Margaret Roach: Hi, Sam. How’s the winter treating you in the mid-Atlantic region?

Sam Hoadley: We’re actually experiencing a genuine winter this time around. It’s been quite chilly, and it seems we’re in for some more snow. It’s the most winter-like season we’ve had in a few years.

Margaret: For those unfamiliar, could you briefly describe Mt. Cuba Center and how long you’ve been conducting trials there? You’re situated near Wilmington, Delaware, in Zone 7A, correct?

Sam: Yes, that’s right. We’re located just north of Wilmington, Delaware, near the Pennsylvania border. Mt. Cuba Center is a botanical garden dedicated to the conservation of native plants. Established by the DuPont-Copeland family, who moved here in the 1930s, they recognized early on their passion for gardening with native species and preserving open spaces, which was essential to them.

With that visionary approach, they later opted to open their garden to the public, aiming to inspire visitors with the beauty and importance of native plants. This guiding vision continues to shape our mission today.

After Mrs. Copeland’s passing, it was her wish that a trial garden be developed at Mt. Cuba Center. Initially a cut-flower space, this area transitioned to its current role in 2002. Since then, we have been conducting plant trials there and have published about a dozen research reports, now available on our website.

Margaret: In this recent trial, you evaluated 45 distinct Vernonia ironweeds, including various species, cultivars, and hybrids. Upon seeing that number, I thought to myself, “Wait, I only know of two!” What sparked your interest in exploring Vernonia? Unlike your previous studies on Echinacea or Carex, which are more widely recognized by the public, what made you focus on this particular genus?

Sam: We generally focus on two main categories of plants for our trials. The first category consists of popular plants like Echinacea, where there are numerous options for consumers. We aim to clarify questions such as which varieties are most effective at attracting pollinators or which are likely to be longer-lasting, thus helping guide consumer choices.

The other category, which includes ironweeds, goldenrods, and Pycnanthemum, aims to highlight underutilized or less-appreciated plants. While there is some availability of Vernonia in the market, we were surprised to discover the variety of this genus during the planning of our trial. This extensive comparison involved plants sourced from both wild-collected seeds and commercially grown examples of the same species, adding more layers to this research. Ultimately, we aim to promote these remarkable plants that deserve greater recognition in gardens across the Eastern United States.

Margaret: You mentioned “throughout the Eastern United States.” I reviewed range maps for this genus and noted that no species seem to exist in the far West. However, numerous varieties occur across the eastern two-thirds of the country, from north to south. I also learned that Vernonia species have been identified on other continents, including Southeast Asia and Africa, demonstrating they are not exclusive to North America.

Sam: Yes, indeed! Interestingly, the largest plant in the Asteraceae family is an ironweed found in Southeast Asia in a tree form—a fascinating fact! However, in the U.S., our native ironweeds originate mainly from the Eastern Temperate Forest region, which comprehensively encompasses the eastern half of the country. These include herbaceous, clump-forming perennials, some of which can grow quite large, while others stay more manageable. A trait they all share is the exquisite pinkish-purple flower heads that we know and love.

Margaret: Correct, they’re part of the Aster family! I’ve had New York ironweed in my garden for ages, and it blooms quite late for me—almost into the fall. Could you clarify whether most ironweeds generally bloom in late summer to fall, or is there a wide range in their blooming periods?

Sam: There is a considerable range among the various species native to the Eastern U.S. The most commonly recognized varieties in our region, such as New York ironweed, typically bloom from late summer into early fall—around August to early September. However, for instance, Vernonia fasciculata from the Midwest commences blooming in early July. In contrast, some southeastern species, including Vernonia gigantea and Vernonia angustifolia, may even bloom as late as October or even November. With a strategic selection of early-blooming, mid-season, and late-blooming varieties, gardeners can create a stunning succession of ironweed flowers in their landscapes.

Margaret: One aspect of your evaluations that intrigued me—especially in recent years—has been your examination of insect interactions. Some insects are generalists, relying on a variety of plants for nourishment, while others have specialized relationships with specific plants. In the case of Vernonia, I read in your report that you observed a specialist insect interacting with these plants.

Sam: Indeed! It was quite captivating. We initiated our plant evaluations in 2019, established the gardens in 2020, and began observing pollinator interactions in 2021. We discovered a native bee visiting the ironweeds, identified as Melissodes denticulatus, a specialist bee that primarily collects pollen from ironweeds. If you notice white pollen on an insect, it usually comes from ironweed, particularly during the late summer to fall period.

The females of this species gather the snowy-white pollen to nourish their larvae during winter, which then hatches the following year. This unique insect demonstrates a mutually beneficial relationship with Vernonia.

We thought it would be interesting to observe whether this bee had a preference among the 45 different ironweed options available to it. We aimed to understand which varieties might attract more or less of this specialist bee by tracking its activity alongside the general insect counts.

Margaret: Which flavor from the Baskin-Robbins assortment of ice cream do they prefer?

Sam: Precisely! This case provides a specific instance of examining how different species and cultivars contribute to pollinator value. Initially, we assumed the locally native species, particularly New York ironweed—which was well-represented in our trials—would attract the greatest attention from this bee. However, we also included hybrids and other species from various regions of the Eastern Temperate Forest. Essentially, we aimed to discover whether this specialist insect had a specific favorite among them.

We continued to monitor the Melissodes denticulatus activity throughout 2022 and 2023 with the assistance of our dedicated volunteer pollinator watch team, who collected most of the data for us. Our findings revealed that the bee is active during a specialized time window that aligns perfectly with the flowering period of New York ironweed—late August to early September, which is not surprising.

Conversely, New York ironweed struggled during the trials due to its specific moisture requirements. These plants naturally thrive in moist environments, but we didn’t provide that level of water in our trials. Therefore, they didn’t perform as expected.

Margaret: That makes sense since we typically see them thriving in wet meadows or along roadsides where moisture accumulates.

Sam: Correctly said! Those cool, moist root zones are essential for the plant’s health. If you have access to similar conditions in your garden, New York ironweed is an excellent choice—offering beautiful blooms and attracting numerous pollinators. However, in the less optimal conditions of our trial garden, it was more susceptible to stress and certain diseases, such as powdery mildew and rust. These health challenges affected both the vigor of the plants and their overall attractiveness to pollinators. Thus, properly situating these plants in your garden is crucial.

Interestingly, another species that thrived during our trials under average garden conditions was Vernonia baldwinii, which is not locally native. This species is native to more central and even western regions, extending just east of the Rockies into Colorado. Despite this, it bloomed around the same time as New York ironweed, and its adaptability to drier conditions made it a favorite among Melissodes.

Margaret: Until now, I was unaware of this fascinating specialist. I’m excited for the upcoming season to observe whether they frequent my ironweed!

Sam: I’m confident you’ll spot them—they’re delightful to watch!

Margaret: You’ve mentioned New York ironweed several times; it is one of the tallest perennials in my garden. While I suspect mine measures around 7 feet, I know it can grow even taller. Are there other larger varieties, such as Vernonia gigantea?

Sam: Yes, the aptly named Vernonia gigantea is indeed massive! We had multiple accessions of this species in our trial garden, sourced from both selections and wild-collected seeds in Alabama. These Alabama forms can reach heights of 9 to 12 feet.

Margaret: That’s astounding!

Sam: Absolutely! In fact, we even had one selection from Plant Delights in Arkansas, named ‘Jonesboro Giant,’ which set a record in our trial garden, reaching an astonishing 15 feet tall in 2022.

Margaret: Fifteen feet for a perennial, and it dies back to the ground in winter—that’s incredible!

Sam: Yes, it’s particularly remarkable how quickly these plants grow. Ironweeds can be a bit slow to emerge in spring, but when they do, their growth is rapid and impressive. Despite its towering height of 15 feet, this plant stood firm even through inclement weather—it was a true showstopper and captivated every visitor in the garden. I’m thrilled to share that it will return to the trial garden this spring.

Margaret: That’s simply wild! As I noted, New York ironweed can reportedly reach heights of up to 10 feet. Do these plants require full sun, or are there other growing conditions to consider?

Sam: All of our ironweed varieties were assessed under optimal conditions of full sun and average garden soil in the trial setup. While some species can endure light shade, like New York ironweed, they won’t thrive or bloom as vigorously as those in full sun.

Vernonia glauca, a less common native variety, can tolerate a bit more shade and can handle drier conditions, serving as an upland counterpart to New York ironweed. There’s also the obscure Vernonia flaccidifolia, or Tennessee Ironweed, which likely has greater shade tolerance than what we tested in the trial setting. Though it didn’t perform exceptionally well, it could be a valuable option for less sunny gardens.

Margaret: The only other species I had heard of, aside from New York ironweed, was lettermannii, which I understand has been available in some cultivars. What caught my attention in the report was your exploration of “the Chelsea Chop” technique—cutting back plants in the spring—to see how it influenced their growth and flowering time. I believe lettermannii responded positively to this method, correct?

Sam: Yes, Vernonia lettermannii indeed performed well! This species typically thrives along river corridors in the Ozark region and often experiences soils that can be somewhat drier and leaner than the trial garden conditions. Consequently, I suspect we were perhaps a bit overly generous in terms of moisture and nutrients for this species. This can lead to lanky growth patterns that lack structure, especially in late summer.

Utilizing the Chelsea Chop effectively remedied this issue by encouraging bushier, fuller growth and enhancing resistance to flopping. While some of the taller varieties, like Vernonia gigantea, showed some improvement with the chop, too, they may not be the best candidates for this practice, as the new growth can be weaker, resulting in breaks.

For shorter varieties such as Vernonia lettermannii and Vernonia angustifolia, this approach is practical for achieving a tidier appearance and can extend the blooming period. For instance, by cutting back one plant while allowing another to grow normally, you could enjoy successive blooms over a greater duration.

Margaret: So you advised cutting back around a third to half of the plant in late May or early June?

Sam: Exactly! We focused on cutting the plants back by half, which varied their final heights but allowed us to gauge the overall impact.

Margaret: Now, let’s shift our focus to some standout varieties. One performer garnered attention—is it ‘Plum Peachy,’ the one with a quirky name given its almost purple flowers? What’s the origin of the “peachy” reference?

Sam: That’s a great question! While it has a catchy name, it does indeed embody the beautiful pinky-purple blooms typical of Vernonia angustifolia. This particular variety, selected by Plant Delights in Georgia, has proven to be incredibly adaptable for gardening, particularly for those with limited space. Unlike some of the taller varieties that can reach heights of 8 to 15 feet, this one grows to about 4.5 to 5 feet, making it more suitable for smaller gardens.

Additionally, Plum Peachy is a true three-season perennial, showcasing distinctive foliage throughout the seasons. Its leaves exhibit a striking purplish hue, particularly along the stems, and the seed heads retain that impressive coloration post-bloom, making it a stunning visual presence when paired with ornamental grasses or in a meadow setting.

Margaret: That sounds absolutely beautiful! Have you featured any of these species on your property at Mt. Cuba or integrated them into any garden borders? I’d love some inspiration on visual pairings as I’ve heard that the taller ironweeds are best suited for the back of a flower border. Are there any other combinations that shine alongside them?

Sam: Yes, we have integrated numerous species into our gardens. New York ironweed thrives beautifully in our natural areas, and one of my favorite plant companions for ironweeds is goldenrods. I can share that our next trial report will focus on goldenrods due for release next year. Combining Vernonia gigantea and Vernonia noveboracensis with taller goldenrods—or perhaps other late-blooming varieties—in your garden can create a breathtaking visual display.

Additionally, species from the Silphium family and native grasses make excellent partners. The purple hues of ironweed blend beautifully with many other plants, making them versatile options for garden enthusiasts. I hope this report encourages gardeners to consider planting these unique species!

Margaret: And it’s worth noting that since these plants aren’t typically favored by mammals, they’re not animal magnets, which is an advantage. They prioritize attracting pollinators instead!

Sam: Precisely!

Margaret: Sam, I see you’re hosting a virtual event where attendees can get tickets for the March 5th webinar, which will delve into ironweeds.

Sam: Exactly! It will be a focused discussion on ironweeds, summarizing our trial outcomes while providing additional insights that may not have been included in the report. I’m sure it will be engaging!

Margaret: I always look forward to our conversations, Sam, and these reports are consistently a delight to receive. I’ll provide the link for readers to access the full online report.

Explore More Native Plant Discussions with Sam Hoadley

(Photos by Mt. Cuba Center; used with permission.)

Prefer listening to the podcast version of the show?

My weekly public-radio show continues to be recognized, rated as a “top-5 garden podcast” by “The Guardian” in the UK, and it began its 15th year in March 2024. Produced at Robin Hood Radio—the smallest NPR station in the nation—it airs locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) on Mondays at 8:30 AM Eastern and repeats on Saturdays at the same time. You can also play the February 17, 2025 episode using the audio player located at the beginning of this transcript. To keep up with future episodes, subscribe via iTunes/Apple Podcasts or Spotify, and explore my podcast archive here.