Ornamental · Asteraceae
Ironweed
Vernonia noveboracensis
Give it full sun and damp ground, and ironweed tops off late summer with deep purple flowers the pollinators can't resist.
Full sun
Every 3–7 days
USDA 5–9
4–8 ft · 1.2–2.4 m
Overview
Ironweed's a tall native perennial you'll find along streambanks, in wet meadows, and on low ground all across the eastern United States. It's part of the daisy family, and from mid-summer into fall it holds up flat clusters of deep purple flowers way above most of the border. Ever wonder where the name comes from? It's how tough and wiry the stems get once they reach full height.
This is one for the back of a bed or a rain garden, where its 4 to 8 feet won't shade out the shorter stuff up front. It's happy in moist to wet soil that would rot a lot of other perennials, and it laughs off cold all the way down to -20°F (-29°C). Deer and livestock won't touch the bitter foliage, so it keeps its spot even when hungrier plants get nibbled away.
And when it comes to late-season pollinators, few perennials do more. Those purple heads keep feeding butterflies and bees just as the summer blooms peter out, and if you leave the hollow stems standing, native bees will tuck into them all winter.
Care
The short version: give it full sun and steady moisture in rich, slightly acidic soil that never fully dries out. Skip the fertilizer, cut the stems back by half in late spring so they stay upright, and leave the spent stalks standing over winter for the bees.
Light
Ironweed really wants full sun if it's going to grow strong and stand up straight. It'll put up with partial shade, sure, but the catch is weaker, floppier stems that lean and need cutting back.
Water
Keep the soil nice and moist and don't let it dry out. It's a streambank native, so it's totally at home in medium to wet ground, even the occasional flood, which means you water it through a dry spell instead of waiting for it to wilt.
Soil
Stick it in rich, moist soil that leans a little acidic, around a pH of 5.0 to 6.5. It handles heavy clay and soggy rain-garden spots that would drown other perennials, so it's a natural pick for low ground.
Temperature
Ironweed's tough across a huge range, from -20°F to 95°F (-29°C to 35°C). That cold hardiness is what lets it ride out brutal winters and pop back up every single spring.
Humidity
Regular outdoor humidity is plenty, so there's no number to worry about. What it actually cares about is moisture down at the roots, not in the air around it.
Feeding
Go easy, or honestly just skip the feeding. Ironweed's a native that does fine on little to no fertilizer, and a yearly topdress of compost covers it. Too much nitrogen and the stems just flop over.
Pruning
Cut the stems back by about half in late spring or early summer to keep the plant shorter and less likely to lean. Deadhead the spent flowers before they set seed if you'd rather not deal with volunteers. Leave the hollow dead stems standing 12 to 24 inches over winter, because native bees nest in them, then clear them out once they've broken down.
Size & spacing
Ironweed hits 4 to 8 feet (120 to 240 cm) tall, so it belongs at the back of the bed. Give each plant 3 to 4 feet (90 to 120 cm) of room to fill out into a nice full clump.
When to plant
Ironweed follows the seasons pretty closely. Cold-stratify the seed over winter and start it indoors in late winter, put your plants or divisions out after the last spring frost, and those deep purple flowers will show up 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's an easy plant, and it really only has two little quirks, both simple to spot. Sometimes the tall stems lean, and sometimes a big batch of seed sprouts where you never planted it. Just match what you're seeing below and jump on it early.
Tall stems leaning over, usually after a downpour
What's going on here is plain flopping, and it nearly always comes down to too much shade or ground that's richer than ironweed likes. When a plant shoots up to 4 to 8 feet, it starts to bend at the base or middle, and a stake will only prop it up for a while. So move it somewhere sunnier, quit feeding it, and cut the stems back by about half in late spring, and it'll grow back shorter and a whole lot sturdier.
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
Baby plants showing up all over the bed
Ironweed's a real self-seeder, so what happened is that one grown plant scattered enough seed to fill the bed with volunteers by the next spring. Want to stop it? Just snip the faded flower heads off before the seed ripens. And any seedlings you don't want will pull up easily while they're still young, so no big deal.
Fix:
- Remove flower heads before seed develops
- Pull or transplant unwanted seedlings while young
Toxicity
Good news if you've got pets or you're out gardening with the family: ironweed is safe all around. The ASPCA doesn't list Vernonia as toxic, and there's nothing in the leaves, stems, or flowers that'll hurt cats, dogs, livestock, or people. The only reason animals leave it alone is that bitter taste.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cats are totally fine around ironweed, since there's nothing toxic in it. If one takes a nibble, the worst that might happen is a bit of a stomachache, same as with any tough leaf, and that's it.
Dogs None
Dogs can hang out in the garden with ironweed no problem. There's nothing poisonous in the plant, so a chewed leaf or stem really isn't worth worrying about.
Horses Safe
Horses have nothing to worry about with ironweed. It tastes bitter, so they usually steer clear anyway, and the little they'd try doesn't do them any harm.
Rabbits Safe
Rabbits are fine nibbling around ironweed. Nothing in the leaves or stems is toxic, though that bitter flavor tends to send them off looking for tastier greens.
Livestock Safe
Cattle, goats, and sheep won't be harmed by ironweed. They mostly leave it alone because it tastes bitter, and honestly that's why you still see it standing in pastures that have been grazed down to nothing.
Birds Safe
Birds are perfectly safe around ironweed, and here's a nice bonus: the seed heads pull in finches and other little birds late in the year.
People Safe
Ironweed's no threat to people either. Sure, it's not something you'd grow to eat, but handling it and puttering around it in the garden is completely safe.
Varieties
Ironweed's really a whole bunch of native species plus a few nice cultivars. Here are the ones you're most likely to run into.
Vernonia fasciculata (prairie / common ironweed)
This is a Midwestern prairie and wetland plant with flat-topped clusters of purple flowers, and it's good enough for pollinators that it made the Xerces list.
Vernonia gigantea (giant ironweed)
This big one gets up near 10 feet with purple flowers that go white only on the odd rare plant, and since livestock won't touch it, it does great where pastures are grazed bare.
Vernonia lettermannii 'Iron Butterfly'
Short on space? This tidy threadleaf cultivar stays just 2 to 3 feet tall with soft, fernlike foliage that fits a small garden way better than the giant species.
How to propagate
Ironweed's easy to make more of, whether you split up a clump you've already got or just let its eager seed do the work for you.
DivisionEasy
Spring, as new growth emerges · One growing season
The easiest way to get more ironweed? Dig up a mature clump in spring, right as the new growth pushes through, and split it, which also happens to keep a big plant from getting out of hand. Pop the divisions back into moist soil straight away, and they'll settle in over just one growing season.
SeedEasy
Sow in fall or cold-stratify and start in late winter · Blooms typically in the second year from seed
Since ironweed loves to sow itself, seed is the other easy way to go. You can scatter it in fall and let the winter cold handle things, or give the seed some cold, moist chilling and start it indoors in late winter. Just know that plants grown from seed usually wait until their second year before they bloom.
Myths
Ironweed is poisonous to pets and livestock and should be kept out of pastures.
Nope, the ASPCA doesn't list Vernonia as toxic at all, so it counts as a non-toxic plant. What actually keeps cattle and other livestock away is just the bitter foliage, not poison, and that plain old bad taste is exactly why it hangs on in overgrazed pastures while everything around it gets chewed down to nothing.
Ironweed must be staked because it always flops.
Not really, because flopping comes from shade or soil that's too rich, not from the plant being weak. Give it full sun and average ground, or cut it back by half in late spring, and it holds itself up just fine without a single stake.