Ornamental · Asteraceae
Ironweed
Vernonia noveboracensis
Give it full sun and damp ground, and ironweed crowns tall wiry stems with deep purple flowers the pollinators chase.
Full sun
Every 3–7 days
USDA 5–9
4–8 ft · 1.2–2.4 m
Overview
Ironweed is a tall native perennial of streambanks, wet meadows, and low ground across the eastern United States, and up close it is all height and texture. A member of the daisy family, it carries flat clusters of small deep purple flowers at the very tops of its stems from mid-summer into fall, held well above the border. The name is earned in the hand: the stems turn tough and wiry once they reach full height, stiff enough to resist a snap.
The plant runs up to 4 to 8 feet on those upright stems, which makes it a back-of-the-bed or rain-garden plant clear of shorter neighbors. It roots in moist to wet soil that would rot many other perennials, and the crown survives cold to -20°F (-29°C) before pushing new stems each spring. The foliage is bitter, so deer and livestock pass it by while hungrier plants get browsed.
Those flat purple heads are what draw the eye and the insects. They feed butterflies and bees as the summer flowers fade, and when the flowers dry and the stems go hollow and brown, native bees creep inside the standing stalks to shelter through winter.
Care
The short version: ironweed wants full sun and steady moisture in rich, slightly acidic soil that never fully dries out. Skip the fertilizer, cut the tall wiry stems back by half in late spring to keep them upright, and leave the spent stalks standing over winter for the bees.
Light
Ironweed wants full sun to build strong, self-supporting stems and stand tall. It will take partial shade, but the stems come up weaker and floppier there, leaning over and needing a cut back.
Water
Keep the soil evenly moist and don't let it dry out. This streambank native is at home in medium to wet ground, even the occasional flood, so water it through a dry spell rather than waiting for the leaves to wilt.
Soil
Plant it in rich, moist soil on the acidic side, at a pH of 5.0 to 6.5. It sinks its roots into heavy clay and damp, rain-garden ground that would drown other perennials, which makes it a natural fit for low ground.
Temperature
Ironweed is tough across a wide range, from -20°F to 95°F (-29°C to 35°C). That cold hardiness carries the crown through hard winters, and fresh stems push up again every spring.
Humidity
Ordinary outdoor humidity suits it fine, so there's no number to chase. What it really wants is moisture at the roots, not on the leaves or in the air.
Feeding
Feed it lightly, or not at all. Ironweed is a native that gets by on little to no fertilizer, and a yearly topdress of compost is plenty. Too much nitrogen just makes the stems soft and floppy.
Pruning
Cut the wiry stems back by about half in late spring or early summer to keep the plant shorter and steadier. Deadhead the spent purple heads before they set seed if you'd rather skip volunteers. Leave the hollow dead stems standing 12 to 24 inches over winter, since native bees nest inside them, then clear them once they break down.
Size & spacing
Ironweed climbs on its stems to 4 to 8 feet (120 to 240 cm) tall, so it belongs at the back of a bed. Give each plant 3 to 4 feet (90 to 120 cm) of room to spread into a full clump.
When to plant
Ironweed keeps to the seasons. Cold-stratify the seed over winter and start it indoors in late winter, set plants or divisions out after the last spring frost, and the deep purple heads open on the stem tips from July through September.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Sow seed and cold-stratify for about 4 to 8 weeks; start indoors in late winter, roughly 8 to 10 weeks before the last frost.
- Plant: Transplant or direct-sow after the last spring frost once soil has warmed; divide established clumps in spring.
- Harvest: Deep purple flowers from mid-summer to mid-fall, roughly July through September.
Hardiness
USDA hardiness zones: 5–9
Temperature: down to -29°C
Troubleshooting
Ironweed is an easy plant with only two quirks worth watching, and both show plainly. The tall stems can bow, and a scatter of seedlings can rise where you never planted them. Match what you see below, then step in early.
Tall stems bowing and leaning after rain
The tall, wiry stems are flopping, bending in a slow arc at the base or middle rather than snapping. It happens most in shade or in soil richer than ironweed wants, once the plant has pushed up to 4 to 8 feet and the load of leaves and flower heads outweighs the stem. Set it in full sun, hold back the feeding, and cut the stems by about half in late spring, and the regrowth comes back shorter, thicker, and upright.
Fix:
- Cut stems back by about half in late spring or early summer to keep plants shorter and bushier
- Site in full sun rather than partial shade
- Avoid fertilizing rich soils
Small seedlings crowding the ground nearby
Those low green rosettes ringing the parent are the mark of a heavy self-seeder, sprung from the ripe flower heads of the year before. A single mature plant sheds enough seed to speckle the whole bed with volunteers by spring. Clip the faded heads before the seed sets, and lift the young seedlings while their roots are still shallow and loose in the soil.
Fix:
- Remove flower heads before seed develops
- Pull or transplant unwanted seedlings while young
Toxicity
Good news for pet owners and gardeners: ironweed is safe all round. The ASPCA doesn't list Vernonia as toxic, and nothing in the coarse leaves, wiry stems, or purple flowers will harm cats, dogs, livestock, or people. The only thing that keeps animals off it is the bitter taste of the foliage.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Ironweed is safe around cats, carrying no toxic compounds in its coarse leaves or stems. A curious bite of that rough, bitter foliage might bring a little stomach upset, as any tough leaf can, and nothing worse.
Dogs None
Dogs are safe alongside ironweed, since not a part of it is poisonous. A chewed leaf or a stripped strip of stem is no cause for concern.
Horses Safe
Horses take no harm from ironweed. The sharp, bitter taste of the leaves turns them off before they eat much, and the little they sample does nothing to them.
Rabbits Safe
Rabbits can browse ironweed safely, since nothing in the leaves or stems is toxic. The bitter flavor of the foliage usually sends them nibbling elsewhere.
Livestock Safe
Cattle, goats, and sheep come to no harm from ironweed. The bitter leaves keep them from grazing it, which is exactly why its stems stand untouched in pastures cropped bare around them.
Birds Safe
Birds are safe around ironweed, and the dry, seed-packed heads draw finches and other small species to pick at them late in the year.
People Safe
Ironweed poses no danger to people. It isn't grown to be eaten, but handling its rough stems and leaves and gardening around it carry no risk.
Varieties
Ironweed spans a group of native species plus a few chosen cultivars, each with its own stem height and flower. These are the ones you're most likely to meet.
Vernonia fasciculata (prairie / common ironweed)
A Midwestern prairie and wetland species that lifts flat-topped clusters of purple flowers on its stems and earns a place on the Xerces pollinator list.
Vernonia gigantea (giant ironweed)
It towers to nearly 10 feet with purple flower heads that pale to white only on rare plants, and livestock leave it alone, so it stands tall where pastures are grazed bare.
Vernonia lettermannii 'Iron Butterfly'
A compact threadleaf cultivar of just 2 to 3 feet, clothed in fine, needle-thin fernlike foliage that suits a smaller garden far better than the towering species.
How to propagate
Ironweed is simple to multiply, whether you split an established clump at the crown or catch the seed its dry heads cast so freely.
DivisionEasy
Spring, as new growth emerges · One growing season
The simplest way to make more ironweed is to dig a mature clump in spring, as the fresh shoots push up through the crown, and split it apart, which also trims a big plant down to size. Set the divisions straight back into moist soil, and they root and settle in over a single growing season.
SeedEasy
Sow in fall or cold-stratify and start in late winter · Blooms typically in the second year from seed
Ironweed casts its own seed so freely that raising it from seed is the other easy route. Sow the seed in fall and let winter chill it, or give it a cold, moist spell and start it indoors in late winter. Seedlings grown this way usually put up their first purple heads only in their second year.
Myths
Ironweed is poisonous to pets and livestock and should be kept out of pastures.
The ASPCA doesn't list Vernonia as toxic, and it is treated as a non-toxic plant. What holds cattle and other livestock back is the bitter taste of the foliage, not any poison, and that plain distaste is why its purple-topped stems stand untouched in overgrazed pastures while everything around them is cropped to the ground.
Ironweed must be staked because it always flops.
The bowing stems come from shade or over-rich soil, not from any weakness built into the plant. Grow it in full sun on average ground, or cut it back by half in late spring, and its wiry stems hold straight and tall with no stakes at all.