Ornamental · Asteraceae
Ironweed
Vernonia noveboracensis
Give it full sun and damp ground and ironweed will reward you, crowning late summer with deep purple flowers the pollinators love.
Full sun
Every 3–7 days
USDA 5–9
4–8 ft · 1.2–2.4 m
Overview
Ironweed is a tall native perennial from the streambanks, wet meadows, and low ground of the eastern United States, and it asks very little of you. It sits in the daisy family, and from mid-summer into fall it lifts flat clusters of deep purple flowers high above the rest of the border. The name comes from the way the stems turn tough and wiry once they reach their full height.
Think of this as a plant for the back of a bed or a rain garden, where its 4 to 8 feet of growth won't crowd the shorter plants in front. It settles happily into moist to wet soil that would rot many other perennials, and it takes cold in stride, right down to -20°F (-29°C), so a hard winter won't set you back. Deer and livestock leave the bitter foliage alone, which means it holds its place even where hungrier plants get browsed.
Few perennials do as much for the garden late in the season. The purple heads feed butterflies and bees just as the summer flowers begin to fade, and if you leave the hollow stems standing, native bees will shelter in them through the winter.
Care
The short version: give ironweed full sun and steady moisture in rich, slightly acidic soil that never fully dries out, and you're most of the way there. Skip the fertilizer, cut the stems back by half in late spring to keep them upright, and leave the spent stalks standing over winter for the bees.
Light
Ironweed loves full sun, and that's where it grows strongest and stands tallest. It will put up with partial shade, but the trade is weaker, floppier stems that lean and need a trim, so give it all the light you can.
Water
Keep the soil evenly moist and try not to let it dry out. This is a streambank native at home in medium to wet ground, even the odd flood, so reach for the hose during a dry spell rather than waiting for it to wilt.
Soil
Set it in rich, moist soil on the acidic side, with a pH of 5.0 to 6.5. It takes heavy clay and damp, rain-garden corners in stride, the very spots that drown other perennials, which makes it a happy choice for low ground.
Temperature
Ironweed is wonderfully tough across a wide range, from -20°F to 95°F (-29°C to 35°C). That cold hardiness means it sails through hard winters and comes back to you every spring.
Humidity
Ordinary outdoor humidity suits it just fine, so there's no number to fuss over. What it truly wants is moisture at the roots, not in the air, and that's a much easier thing to manage.
Feeding
Go light, or skip feeding entirely and don't feel guilty about it. Ironweed is a native that gets by on little to no fertilizer, and a yearly topdress of compost is all it needs. Too much nitrogen only makes the stems flop.
Pruning
Trim the stems back by about half in late spring or early summer to keep the plant shorter and steadier on its feet. Deadhead the faded flowers before they set seed if you'd rather skip the volunteers. Leave the hollow dead stems standing 12 to 24 inches through winter, since native bees nest inside them, then clear them once they break down.
Size & spacing
Ironweed reaches 4 to 8 feet (120 to 240 cm) tall, so give it the back of the bed where it belongs. Allow each plant 3 to 4 feet (90 to 120 cm) of elbow room to fill out into a full clump.
When to plant
Ironweed keeps time with the seasons, and it's forgiving if you're new to this. Cold-stratify the seed over winter and start it indoors in late winter, set your plants or divisions out after the last spring frost, and the deep purple flowers will greet you 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 easygoing plant with only two little quirks, and both are simple to read and simple to fix. Sometimes the tall stems lean, and sometimes a generous crop of seed sprouts where you never planted it. Find what matches below, step in early, and you'll be fine.
When tall stems start to lean after a rain
Try not to worry if the stems tip over. This is just flopping, and it usually means the spot is a little too shady or the soil a bit too rich for what ironweed likes. A plant reaching 4 to 8 feet may bend at the base or middle, and a stake will only hold it for so long. Give it a sunnier home, ease off the feeding, and trim the stems by about half in late spring, and it will come back shorter and steadier.
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
Little seedlings turning up where you didn't plant them
If new plants appear around the parent, take heart, because this is an easy thing to manage. Ironweed is a happy self-seeder, and one mature plant drops enough seed to freckle the bed with volunteers by spring. Snip the faded flower heads off before the seed ripens, and lift or move any seedlings you don't want while they are still young and come up with a gentle tug.
Fix:
- Remove flower heads before seed develops
- Pull or transplant unwanted seedlings while young
Toxicity
Here's good news if you keep pets or share the garden with family: ironweed is safe all round. The ASPCA doesn't list Vernonia as toxic, and there's nothing in the leaves, stems, or flowers that would harm cats, dogs, livestock, or people. The only reason animals avoid it is that bitter taste.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
You can relax if a cat wanders past ironweed, since it holds no toxic compounds at all. A curious nibble might bring on a bit of tummy upset, no different from any coarse leaf, and nothing worse than that.
Dogs None
Your dog is welcome to share the garden with ironweed. There is nothing poisonous in it, so a chewed leaf or stem really is no cause for alarm.
Horses Safe
Horses have nothing to fear from ironweed. The bitter flavor tends to steer them away anyway, and the little they might taste does them no harm.
Rabbits Safe
Rabbits can hop around ironweed with no trouble. Nothing in the leaves or stems is toxic, though the bitterness usually sends them off to nibble elsewhere.
Livestock Safe
Cattle, goats, and sheep come to no harm from ironweed. They tend to leave it standing because the foliage tastes bitter, and that is just why it lingers in pastures grazed bare.
Birds Safe
Birds are completely safe near ironweed, and you may find its seed heads draw finches and other small visitors as the year winds down.
People Safe
Ironweed is no threat to people. It isn't a plant you grow to eat, but tending it and gardening around it carry no risk at all.
Varieties
Ironweed is really a whole family of native species with a few lovely cultivars alongside them. These are the ones you're most likely to come across.
Vernonia fasciculata (prairie / common ironweed)
This Midwestern prairie and wetland native holds flat-topped clusters of purple flowers and has earned its place on the Xerces pollinator list.
Vernonia gigantea (giant ironweed)
Reaching close to 10 feet, this one carries purple flowers that turn white only on the rare plant, and livestock pass it by, so it thrives where pastures are grazed bare.
Vernonia lettermannii 'Iron Butterfly'
If space is tight, this gentle threadleaf cultivar stays just 2 to 3 feet tall with soft fernlike foliage that suits a smaller garden far more kindly than the towering species.
How to propagate
Making more ironweed is easy and rather satisfying, whether you split an established clump or simply let its eager seed do the work for you.
DivisionEasy
Spring, as new growth emerges · One growing season
The friendliest way to make more ironweed is to lift and split a mature clump in spring as the fresh growth pushes through, and it happens to keep a big plant tidy too. Tuck the divisions straight back into moist soil, and they take hold over a single growing season, so you'll have new plants before long.
SeedEasy
Sow in fall or cold-stratify and start in late winter · Blooms typically in the second year from seed
Ironweed sows itself so willingly that seed is a lovely second route to try. Scatter it in fall and let the winter chill do the work for you, or give the seed a cold, moist rest and start it indoors in late winter. Plants raised this way usually take until their second year to bloom, so be patient and the flowers will come.
Myths
Ironweed is poisonous to pets and livestock and should be kept out of pastures.
You can set this worry aside, because the ASPCA doesn't list Vernonia as toxic and it counts as a non-toxic plant. Cattle and other livestock steer clear only because the foliage tastes bitter, not because it poisons them, and that plain distaste is the very reason it persists in overgrazed pastures while the plants around it get eaten to the ground.
Ironweed must be staked because it always flops.
Take comfort here, since flopping comes from shade or soil that's too rich, not from any weakness in the plant. Give it full sun and plain ground, or cut it back by half in late spring, and it stands tall on its own with no stakes needed.