If you decide to use herbicide, choose your product carefully. Make sure that the herbicide is effective against the species you plan to control. Some herbicides have high persistence. This means that they stay active for a long time after application. This may or may not be desirable. Herbicides with high persistence can cause harm to other species if they travel away from the site of application. Some herbicides are very selective. This means that they should only damage or kill certain types of plants. This could be desirable.
Before using an herbicide, read and understand the entire label. Wear the proper protective equipment (gloves, safety glasses, etc.) and keep spill clean-up materials nearby just in case. Stay aware of weather conditions and do not apply herbicides in weather that could make the chemicals less effective or could make them travel away from the application area.
There are companies that apply herbicides for you. Some of these companies specialize in treating invasive plants. In cases where invasive plants occur in water or a wetland in Wisconsin, only an applicator certified by DATCP may apply herbicides.
There are many different ways to apply herbicides. Here's a short list.
Foliar - Applying herbicide to the leaves of actively growing plants.
Bundle + Cut - Usually used for grasses, applying herbicide to a bundle of stems after gathering them and cutting off the tops of the plants.
Cut Stump - Applying herbicide to the stump of a woody plant after cutting. More information here.
Hack + Squirt - Applying herbicide into downward-angled incisions into the bark and sapwood of a woody plant. More information here.
Basal Bark - Applying herbicide to the lower bark of a woody plant. More information here.
There are many different tools used to apply herbicides. Here's another short list.
Large scale sprayers - This equipment is often used by larger companies or right-of-way managers. Equipment may attach to a tractor, ATV, or other vehicle. This equipment can hold many gallons of herbicide, and can spray large areas in a short time.
Backpack sprayer - Like it says, a backpack sprayer is worn on your back, and you hold the sprayer wand to direct herbicide spray. Backpack sprayers hold a few gallons of herbicide and are good for treating small areas.
Hand sprayer - Handheld sprayers hold less herbicide than backpack sprayers and are also used to treat small areas.
Sponge/wick applicators - Some people apply herbicide with wicks instead of sprayers. This may reduce amount of herbicide used and damage to desirable plants. One example is the Makutu. Some wick applicators are created for large-scale use.
Injectors - Some species can be controlled by injecting herbicide into the stem. Special products are sold for this purpose. Injectors are often best for small infestations, and may reduce damage to desirable plants and amount of herbicide used.
Additives - There are many additives, or adjuvants, that you may wish to add to an herbicide mix. Dye can help indicate which plants have already been treated. Surfactants can help the herbicide stick to and be absorbed by leaves. Bark penetrants can help herbicide be absorbed by wood. Make sure to choose adjuvants carefully based on label directions and site specifications. Herbicides must often be diluted in water or oil. See label for specific directions.
Image from NPS.gov
Cut-leaf teasel after herbicide application.
Timing is important when using herbicides. Often, seedlings or actively growing plants are most susceptible to herbicides. When controlling annual or biennial invasives, you should treat before they produce seeds.
For perennial plants, use an herbicide that will be carried throughout the plant. Herbicide applications to true seedlings or budding plants are effective. Cutting the plant in the summer and treating in early fall can also work well.
Woody species can be controlled with foliar application of herbicides in late spring or early summer, when leaves are fully expanded. Cut stump applications are effective in summer, fall, and early winter. Basal bark applications are effective in fall and early winter. Avoid applying herbicides to cut stumps or bark in early spring, because sap flow can make them less effective.