Iowa's Herbaceous Roadside Vegetation Threats
Table of Contents
Iowa’s noxious weed list is outdated. It includes several species that no longer pose a threat to agriculture, and it does not include certain plant species now considered troublesome in the state.
The state’s primary herbaceous (non-woody) roadside threats are listed in Table 2. Herbicides remain the most practical means of controlling these weeds in county road right-of-way.
Table 2: Iowa's Primary Roadside Threats and Herbicide Recommendations
| Species | Roadside Manager Recommendations (2011) | Application Schedule and Notes |
|---|---|---|
| Canada Thistle perennial |
| Most effective control: Bud to bloom, or late fall just before frost; mowing increases efficacy of fall treatments. Growth stage is key, so treatment can be flexible:
|
| Musk Thistle biennial |
| Most effective control: Rosette stage during spring or fall. Herbicides with good residual activity (e.g., Milestone, Opensight) appear to be the most effective over the long-term. Adding 2, 4-D helps with burndown. Tank mixing, 2, 4-D, with Overdrive = very rapid burndown. |
| Bull Thistle biennial |
| Same as musk thistle |
| Teasel biennial |
| Most effective control: Rosette stage during spring or fall. For small populations, cut off seedheads, destroy and dispose properly. Use a stout dose of herbicide and monitor. Teasel is tough; flowerheads and seed can be produced after treatment. |
| Poison Hemlock biennial |
| Most effective control: Rosette through pre-bloom, early spring. |
| Leafy Spurge perennial |
| Most effective control: Spring or fall. Plateau works well and is most effective in the fall, just before frost. Pull or respray plants that appear after treatment. Use biocontrol for large infestations: flea beetles (Aphthona spp.). |
| Purple Loosestrife perennial |
| Most effective control: June to August. |
| Wild Parsnip biennial |
| Most effective control: Rosette stage. |
| Japanese Knotweed perennial |
| Most effective control: Summer Use a good surfactant, e.g., MSO with Arsenal, and Liberate with Habitat. |
| Sericea Lespedeza perennial |
| Can use either herbicide all summer. For best results use Garlon prior to branching (~July 4), and Escort in Aug. & Sept. |
| Russian Knapweed perennial |
| Most effective control: Spring. |
| Garlic Mustard biennial |
| Spray rosettes in early spring (late Feb - early April) or late fall when little else is green to avoid non-target plants. Escort XP and Streamline may be effective, but due to residual activity should not be used where non-target species could be affected. |
Notes: The most commonly used brand names are shown here. Many of these products are now available under other (often less-expensive) labels.
Where Garlon (triclopyr) is noted, either Garlon 3A (amine formulation) or Garlon 4 (ester) can be used. Garlon 4 is usually more effective but in hot weather can volatilize, drift and affect non-target species. 3A is non-volatile and usually considered the best choice for hot weather.
Herbicide labels
Labels explain how to use the product effectively while protecting yourself, non-target plants and the environment. Take time to read the labels; it may be the most valuable time spent in weed control.
Adjuvants
Adjuvants are often added to the herbicide solution to increase its effectiveness. These products are put in the water tank at labeled rates.
- Surfactants improve dispersion and reduce surface tension of spray droplets resulting in increased penetration.
- Crop oils and crop oil concentrates also improve dispersion and, by being oil, keep leaf surfaces moist longer than water allowing more time for penetration.
- Stickers help prevent the solution from being washed off leaves.
- Drift inhibitors control drift.
- Antifoaming agents reduce foaming in the tank so it can be filled more easily.