Ironweed

About Ironweed

Ironweeds are late-summer native perennials with flat clusters of fuzzy, deep-purple flowers that feed bees and butterflies, followed by rusty seed heads that may give them their name and seeds for songbirds (NC State, NYBG, Wildflower Center). New York ironweed is a 5-8 ft giant of moist meadows from Massachusetts to Ohio and south to Georgia and Mississippi; prairie ironweed grows in damp prairies and streambanks across the northern plains, such as South Dakota; and narrowleaf ironweed, native only to Arkansas and Oklahoma, gives the compact, fine-leaved 'Iron Butterfly' (Wildflower Center, SDSU, NC State). Deer and other browsers leave them alone (Mt. Cuba Center).

New York ironweed is hardy in zones 5-9; narrowleaf ironweed and 'Iron Butterfly' in 4-9 (NC State, NYBG). Not listed as invasive anywhere we checked, though it self-seeds in moist spots (NYBG).

When to plant

Timing is counted from your average last spring frost (and first fall frost). Sow & Tell works out the actual dates for your ZIP code.

Set out plants in spring or fall. For seed, sow outdoors in fall or give it about 2-3 months of cold, moist treatment in the refrigerator before spring sowing (Wildflower Center, SDSU). Clumps can also be divided, or softwood cuttings taken in late spring (Wildflower Center). No sourced day count to first bloom.

Soil

New York and prairie ironweed like rich, moist to wet, slightly acid soil and suit rain gardens and pond edges; narrowleaf ironweed and 'Iron Butterfly' want average, well-drained soil and handle drought and poor soil (NC State, SDSU, NYBG).

New York ironweed likes moist soil with organic matter; narrowleaf ironweed and 'Iron Butterfly' do fine in average or poor, well-drained soil (NC State).

Feeding

Avoid over-fertilizing: rich soil makes stems bend and flop (NC State on 'Iron Butterfly' and narrowleaf ironweed). Follow a soil test if you feed at all (Penn State).

Harvest

Blooms from July into September (NC State, Wildflower Center); some southern forms run into October (Mt. Cuba Center). Good cut flowers (NC State). To keep tall kinds shorter, cut them to the ground when about 2 ft tall (flowers come about 3 weeks later) or prune to about 2 ft in spring (NYBG, NC State); cut 'Iron Butterfly' back in late May or early June if it splays (Mt. Cuba Center). Trim spent flowers to limit seedlings.

Varieties (7)

NC State and Mt. Cuba Center's 2020-2023 ironweed trial document these 7 named ironweeds; two are selections of related species (tall and giant ironweed).

Good neighbors

Goldenrod

NC State notes that ironweed's late purple flowers look great against yellow goldenrods; this is a design pairing, not a pest-control claim.

Diseases and problems

Rust, Powdery mildew

In the kitchen

Safety: The sources we checked list no toxic parts and no research-based guidance on eating it, pregnancy or allergies; treat it as an ornamental.

Pets: Not listed by the ASPCA (no ironweed or Vernonia entry on its dog, cat or horse lists).