P&PDL Picture of the Week for
September 22, 2008

The Ironweeds

Glenn Nice and Bill Johnson, Department of Botany & Plant Pathology, Purdue University

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 1.

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 above 90%.

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.

References:

USDA Plant Database. Accessed October 15, 2008. [http://usda.plants.gov].

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 31:324-328.

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.

 

Click image to enlarge

close up of ironweed flowers

Figure 1. Tall ironweed inflorescence. Photo: Glenn Nice

ironweed

Figure 2. Tall ironweed plant. Photo: Ross

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service