It is Ironweed (Vernonia spp.) season and I am READY. These beautiful, tall, vibrant purple native flowers are a welcome sign of late summer and a BOON to many insects. They are a valuable late season nectar source for many insects and butterflies, and their pollen is a required resource for specialist long-horned bees.
Ironweed is a perennial wildflower in the Asteraceae family. Unlike the asters, ironweed flowers possess no showy ray florets (think daisy petals) and are comprised of only clusters of disc florets (think center of a sunflower). But I am missing nothing... because these bundles of tiny tubular purple flowers pack a punch! Tall and brightly purple, they certainly catch the eye. These disc florets are bundled into a compact flower head or "capitulum". The overall inflorescence can range from 6 to 16 inches in diameter. Depending on the species, the inflorescence can be densely held or loosely blowing in the breeze. They provide pollen and nectar late in the season or insects and is a host plant for the American painted lady butterfly (Vanessa virginiensis). All while having great visual appeal for the garden or prairie. They bloom July through September in Ohio. There are several species of Ironweed, but Giant Ironweed (Vernonia gigantea) is the most common we may encounter in Ohio.
Leaves can reach 9 inches long and 2 inches wide. Leaves are long and narrow, lanceolate or willow-like in shape, with serrated margins. They are arranged alternately on tall rigid stems that do not branch until the formation of the flower heads. Stems can reach 8 feet in height. It is important to learn to recognize the leaves and stems. This is a tall and late season bloomer. As such, it takes awhile for them to reach maturity and flower. All that time growing with no sign of a flower may trick unsuspecting gardeners into thinking it's undesirable when in fact it's a late season firework. I myself had a wild population I was weed-whacking along my fence until I realized what was popping up and let them do their thing!
The plant grows in clumps due to its short thick rhizomes and fibrous roots. It spreads readily by seed and can quickly fill in a space with purple wonder late in the season. They rarely need divided, but can be for sharing with friends. Ironweed prefers full sun to light shade and moist conditions. It is great for your wetter sites, stream banks, or rain gardens. It is considered easy to grow, as long as you don't accidentally weed it out. It can becomes a problem in grazing pasture because it has bitter foliage and is often controlled in favor of more tasty forage. It is this bitter flavor that also makes it somewhat wildlife resistant compared to the other landscape buffet options.
The U.S. Forest Service notes that there are 17 species of ironweed in North America. Here in Ohio, we are most likely to see giant ironweed, (Vernonia gigantea), as described above, but we also find New York ironweed (V. noveboracensis) with its hair-like flower bracts; Missouri ironweed (V. missurica) with its dense white hairs one the underside of its leaves; and smooth or prairie ironweed (V. fasciculata) with its shorter stature and densely compacted flower heads.
These species may hybridize in the wild and so misidentifications are likely. The common names are also often misapplied, interchanged, and numerous. For example, giant ironweed is also called tall ironweed. Tall ironweed is also called prairie ironweed. and so on.
NEW YORK IRONWEED (V. noveboracensis)
New York ironweed can be distinguished from its cousins by the presence of hair-like tips on its flower bracts. Even when the flower opens, those stringy hairs remain under the flower head. Its habitat is much like that of giant ironweed, often found in wet soils in full sun along streams, or in prairies and pastures.
MISSOURI IRONWEED (V. missurica)
Missouri ironweed is similar in size and look to tall ironweed, growing to about 7 feet tall max. But the underside of its leaves are excessively pubescent with white hairs, more so than other ironweeds. Stems are also pubescent.
SMOOTH IRONWEED (V. fasciculata)
Also called prairie ironweed, V. fasciculata is a shorter species with densely packed clusters of flowers described as a corymb. Corymb is a term describing where the outermost flowers have longer floral stems or pedicels than the inner most flowers, resulting in all the flowers laying evenly at roughly the same height or plane. Like the inflorescence of broccoli. Mature height is listed at 4 to 5 feet max with shorter leaves than giant ironweed that are smooth with no hairs.
Any of these species would be a welcome addition to your yard for pollinators and late season color. Plant breeding has also brought about shorter hybrids that may be more pleasing to the garden scape, but don't forget the irrefutable value of the straight native species to the pollinators that need them. Some new hybrids include 'Summer Surrender' and 'Summer Swan Song' have a bushy habit. V. noveboracensis 'White Lightning' has white flowers. And 'iron butterfly', a cultivar of Narrowleaf ironweed (V. lettermannii), is proving to be a buggy favorite in some pollinator garden designs. But the wild native species of giant ironweed has suited me for now! They can easily pop up on your yard without even having to buy them, just learn to recognize the young stems and let them make it to August to start seeing those pops of purple. If you're adding a new garden bed, consider including ironweed!
Melissodes denticulatus is a native long-horned bee (the male antenna are longer than other species of bee). It is a solitary ground-nesting bee in the family Apidae. They are easy to ID once you start spotting them. The pollen of ironweed is uniquely WHITE. And Melissodes carries buckets of it on a thick, fluffy region of their hind legs called the scopa designed for pollen transport. They are black with gray hairs and brilliant blue eyes.... isn't she beautiful? I get right in there with my camera to photograph these beauties, and they're not bothered, not aggressive, and have never stung me. As solitary bees they are working, single moms, out there collecting pollen for their offspring. They don't have queens and worker bees like social hive bees to do the work of building and provisioning nest cells. As with most native solitary bees, she is doing all the work herself and she doesn't have time for you!
Melissodes as a genus are generally are fall fliers. Melissodes denticulatus here flies July through early September, in synchrony with the ironweed bloom. She is considered a oligolectic bee, collecting pollen from only a few picky sources. Some specialists are one hit wonders, requiring only one host plant (monolectic) and if they go, so goes the bee. All will generally visit other nectar sources, but specific pollen is required to successfully rear their larva to maturity. While other asters may possibly be on the menu for M.denticulatus, it is truly ironweed that is supreme for these girls. On sunny days, ironweed can be swarming with them. There are other Melissodes species that specialize on other asters or groups of asters such as sunflowers and thistles. So while you are enjoying the brilliant purple flowers, take a peek to see if our denticulate long-horn bee is visiting your ironweed and... plant some more!
To learn more about specialist bees and their needed host plants, check out Jarrod Fowler's Pollen Specialist Bees of the Eastern US. This intense guide shares the phenological emergence of many species, and the host plants from which they were recorded collecting pollen.