The Do’s and Don’ts for Management Invasive Plants in the Spring

Photos by the author.

Invasive plants are a big problem in natural landscapes in Illinois. These species can impact our native plant species and reduce the quality of habitat for wildlife. Managing invasive plants is a year-round endeavor but it pays to know what the right time is to conduct management to be most efficient and successful. As we are heading into springtime, let’s discuss what management activities are best suited for springtime and what activities should be delayed until a different time of year.

What Not to Do

The biggest practice to avoid in the early springtime is herbicide treatments for invasive shrubs and other woody plants. In general, foliar sprays and cut stump treatments of woody plants during the spring green up period are less effective than other times of the year. This period is from the first signs of bud swelling and starting to open all the way through the initial flush of spring growth and stem elongation. Wait until this initial flush of growth in the spring is finished before using cut stump and foliar treatments. Basal bark treatments, where you apply an oil-based herbicide to the lower bark of all stems from ground level to around 14 inches high, can still be effective during this time, however.

What You Should Do

Garlic mustard (Alliaria petiolata) is an invasive forb that has a biennial life cycle, meaning that it will form a rosette of low growing leaves the first year, then develop a tall stem and flower the second year. After the seeds are developed on the second-year plants, they die. This invasive plant flowers very early in the season, often at the same time or before many of our native spring wildflowers. Early spring, just as the plants are starting to form their flowering stalks, is a great time to control garlic mustard. Small populations can be hand pulled. If flowers are present on the stalks when pulled, the plants should be placed in plastic bags and removed to prevent the seeds from continuing to develop and ripen even after the plants have been pulled.

If you are treating garlic mustard with herbicides, take care not to overspray and impact any native species growing nearby. Herbicide treatments can be made to the first-year rosettes, developing second-year plants, and even flowering individuals. If flowering has ceased and fruit has already developed on the plant, some recent research from the University of Illinois has indicated that herbicide treatments can still be effective at greatly reducing the amount of viable seed produced—as long as the treatments were made before the seeds have started to darken and the fruit starts drying.

Some of the invasive shrub species have an extended leaf phenology. That simply means that they tend to green up earlier than our native shrubs and stay green longer. As discussed earlier, treating invasive shrubs in the spring may not be the most effective, but it is a great time to scout for them and mark new locations.

Species such as Amur honeysuckle (Lonicera maackii), autumn olive (Elaeagnus umbellata) and border privet (Ligustrum obtusifolium) are highly visible in the spring at the right time. Marking the locations of these shrubs on a mapping program or in the field with some flagging can help you relocate the plants when the time is right to conduct management.

Another problematic invader that can be managed in the early spring is Japanese stiltgrass (Microstegium vimineum). This invasive grass is an annual species, meaning it completes its life cycle all in one year. Stiltgrass germinates in early spring, grows throughout the summer, and flowers, sets seed, then dies in the fall. Early spring provides an opportunity to treat heavy infestations of stiltgrass with a pre-emergent herbicide. These herbicides are designed to control seeds as they try to germinate and do not impact rooted plants. Research conducted by the University of Illinois indicated that early spring treatments of pre-emergent herbicides, before stiltgrass has germinated. was highly effective at preventing stiltgrass from growing. Depending upon the herbicide and rate used, the period of effectiveness can last from a couple of months to all season long. Care should be taken when using pre-emergent herbicides as they will work on all seeds germinating and not just stiltgrass. This treatment method is best suited for heavy infestations that do not occur in or adjacent to streams and other open water sources.

Inventory Your Tools and Herbicides

Lastly, spring is a great time to take inventory of your invasive species management tools and stores of herbicide. Gas powered tools, such as chainsaws and string trimmers, are commonly used in invasive species management. If the tools sat all winter with gas in them, they likely need to be drained of gas, cleaned and fresh gas added. Hand tools, including loppers and saws, can be cleaned and sharpened if necessary. Sprayers can be checked for leaks and defective parts replaced. Having everything in good working order and ready to go will greatly increase the success of any invasive species management efforts.

Spring is a time of renewal with the forests and fields around us greening up and flowering. It is also a great time to start thinking about what you can do to help protect our natural ecosystems and native species. Controlling invasive species is an important step in that pursuit.

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Chris Evans is an Extension Forestry and Research Specialist with the University of Illinois. A focus of his research and extension activities is invasive species management and forest health. Evans is currently a board member for the Midwest Invasive Plant Network, president of the southern chapter of the Illinois Native Society and a former board member of the North American Invasive Species Management Association and the National Association of Exotic Pest Plant Councils.