By Robert Hartzler
Iowa State University Extension
Disturbances within Iowa’s natural areas make them susceptible to invasion by exotic woody plants. Three common Iowa invasive woody species are buckthorn, honeysuckle and multiflora rose.
Non-chemical control tactics include pulling, mechanical removal and repeated mowing. In many situations, herbicides provide the most effective and economical control option. Three distinct types of herbicide treatments are commonly used to control woody plants.
Cut surface applications are used to prevent resprouting after mechanical removal of the woody plant. The herbicide should be applied shortly after cutting while the wound is still fresh. For larger trees, the herbicide only needs to be applied to the cambium, the tissue directly underneath the bark. Herbicides can be painted onto the surface or applied with a squirt bottle or small sprayer.
Roundup (glyphosate) at a 50 percent dilution (one part Roundup to one part water) is effective against most species. Use only concentrated formulations with at least 40 percent active ingredient for cut surface applications. Tordon RTU (picloram) is a premixed formulation for cut surface treatments. Caution is required with this product since picloram can be absorbed by roots of adjacent trees and plants, resulting in significant injury.
Basal bark applications involve applying herbicide to the lower 12 inches of stems and trunks. The herbicide is applied using an oil based carrier (diesel fuel, kerosene, bark oil) to increase movement of the herbicide through the bark. Dyes formulated for basal applications allow the applicator to easily see coverage of the target and if excessive off-target spray is occurring. Basal applications are most effective on trees with a diameter less than four to six inches since the bark on larger trees may reduce herbicide absorption. Trees can be treated any time of the year, but the stems must be dry at the time of application. The bark should be thoroughly wetted with the spray, but applying until runoff is not necessary.
Several herbicides are labeled for basal bark application and their effectiveness varies with target species. Triclopyr is effective against most invasive woody species, and is sold under the tradenames of Garlon, Remedy Ultra, Tahoe and Tailspin. Only the ester formulation (e.g. Garlon 4) should be used for basal bark treatments because it is oil soluble (important when mixing with oil) and esters penetrate bark more effectively than amine formulations such as Garlon 3A. A mix of 20 ounces of herbicide in one gallon of oil based carrier is effective against most species. Garlon concentrate is available in 2.5 gallon containers that may be more than required for many users. Pathfinder II is a ready-to-use formulation of triclopyr for basal applications that does not require dilution. The combination of convenience and smaller quantity make this type of product suitable for many acreage owners.
Foliar applications require complete coverage of the plant canopy to provide consistent control. They can be made from the time leaves are fully expanded until fall color develops. Mid to late summer application may be less effective if made during periods of extended hot, dry weather. There is a greater risk of the herbicide contacting nearby sensitive vegetation with foliar sprays than with the other techniques, especially when spraying large plants. Applications should be made on relatively calm days (wind speeds less than 10 miles per hour) to minimize off-target movement of herbicides.
Triclopyr is available either as an amine formulation (Garlon 3A) or ester formulation (Garlon 4). For foliar applications, mix one to three ounces Garlon 4 or two to four ounces Garlon 3A per three gallons of water. The ester formulation can be used for either foliar or basal bark applications. Avoid applications of the ester formulation when temperatures exceed 85 F or in areas where the herbicide may come in contact with water (streams, ponds, etc.). The amine formulation can be used in areas where the spray will come in contact with standing water, making it useful in controlling willows that may invade pond edges.
Most products sold for brush control at garden stores are a combination of two or three growth regulator herbicides intended for foliar applications. Products containing triclopyr generally are more effective on woody species than those based on 2,4-D, dicamba or other herbicides. Products available at garden stores typically contain lower concentrations of active ingredient than products sold for agricultural or commercial uses. Follow the label recommendations for proper mixing. These garden store products are suitable for small jobs, but for larger infestations a commercial formulation such as Garlon may be more economical.
The herbicides used for brush are active at very low concentrations. Caution must be used to minimize their movement onto desirable plants. A separate sprayer should be obtained specifically for applying herbicides since it is difficult to clean herbicide residues from the sprayer. Although most herbicides have a relatively low acute toxicity, steps should be taken to minimize your exposure to the chemical. The herbicide label will provide specific information on protective clothing required when using the product (e.g. rubber gloves, long-sleeved shirts, eye protection, etc.).
Acknowledgment: Loren Lown, Polk County Conservation Board, provided valuable assistance in developing this information.