Nice and Bill Johnson, Department of Botany & Plant Pathology,
Ironweeds are a familiar site in Indiana pastures.
They are often 3 to 5-foot tall with purple inflorescence, although
they can reach heights of 10 feet tall. In a pasture that has been
heavily grazed, they are often the only plant of substantial size
left. The ironweeds are a group of native perennials that can be
troublesome in pastures throughout the US. They belong to the genus
Vernonia spp. One of the most common ironweeds found in the Midwest
is tall ironweed, (also known as giant ironweed (Vernonia
gigantea (Walter) Trel. ssp. Gigantean)1. For a breakdown of
species found in Indiana see Table
The ironweeds can produce a large amount of seed,
but seed production appears to be variable. Tall ironweed was reported
to produce between 1460 to as much as 14,550 seeds per plant3.
Control by mowing may be affected by mowing date
and the environmental conditions. In a study conducted in Missouri
investigating mowing regimes of western ironweed, single mowing
events often lead to higher stands4. Western ironweed that was
mowed initially on May 20th then repeated when regrowth reached
6 to 8 inches reduced from 87% to 19% of the original populations4.
When control efforts are initiated, herbicides
are often used. The use of glyphosate has been reported to have
good to excellent control of ironweed5,6. However, control with
glyphosate may provide variable results. In a study by Peters and
Lowance (1979) control was reported to range from 2% to 88% depending
of year6. The authors attributed low control to rainfall and the
poor growing conditions of those years. For sufficient efficacy
on perennial weeds, glyphosate requires the weed to be translocating
efficiently. Glyphosate efficacy is increased when weeds are growing
in good conditions. Glyphosate can only be applied as a directed
application, such as a wiper application or as a spot application
for it can injure or control desired grasses and legumes.
Herbicides containing the active ingredients
dicamba [Banvel, Clarity], 2,4-D [many], and triclopyr [Garlon
4 Ultra, Garlon 3A] were investigated for the control of tall ironweed3.
Dicamba alone did not control tall ironweed greater than 76% and
had 81% or greater regrowth. The use of triclopyr alone or triclopyr
plus 2,4-D provided control 91% and up. In the triclopyr alone
or with 2,4-D, regrowth did not exceed 39% of the original stand.
Two years after the study, control of tall ironweed was maintained
Crossbow, a premix of triclopyr and 2,4-D can
be used to control tall ironweed at 2 qt/A broadcast or in a hand-held
applicator at 1% v/v. Milestone at 5 to 7 floz/A or ForeFront at
2 to 2.6 pt/A will have good activity on ironweed5.
USDA Plant Database. Accessed October 15, 2008.
N. Britton and A. Brown. 1970. An Illustrated
Flora of The Northern United States and Canada. Vol. 3 pp 350-353.
R.K. Mann, S.W. Rosser, and W.W. Witt. 1983.
Biology and Control of Tall Ironweed (Verona
altissima). Weed Science
E.J. Peters and S.A. Lowance. 1978. Effects of
Multiple Mowing on Western Ironweed (Vernonia
baldwinii) and Gray
Goldenrod (Solidago nemoralis). Weed Science 26:190-192.
M.M. Loux, A.F. Dobbels, J.M. Stachler, W.G.
Johnson, G.R.W. Nice, and T.T. Bauman. 2008. Weed Control Guidelines
for Ohio and Indiana. The Ohio State University and Purdue University
[WS-16 p. 167]
E.J. Peters and S.A. Lowance. 1979. Herbicides
for Renovation of Pastures and Control of Tall Ironweed (Vernonia
altissima). Weed Science 27:342-345.