Controlling Unwanted Plants

Knocking Back the Competition

When invasive plants spread into forest preserve prairies, woodlands and wetlands — even ponds and lakes — they steal space, sunlight and nutrients from native plants and rob wild animals of vital habitat. Fortunately, the Forest Preserve District has several tools it can use to knock back these unwanted invaders.


Prescription Burns

Prescription burns are carefully set fires that trained Forest Preserve District employees use in late fall and early spring to remove invasive plants and improve conditions for native species. 


Why fire?

  • It turns dried plants into ash, which allows nutrients locked inside the plants to quickly reenter the soil and fertilize new growth in the spring.

  • It prevents dead grasses and leaves from piling up. Too much accumulated plant “litter” can lower the temperature of the soil, which makes it harder for seeds and sprouts to develop.

  • It kills or stunts nonnative shrubs such as buckthorn and honeysuckle. These plants have only been in the area for a century or two and can’t bounce back after fires as easily as native species, which have developed deeper roots or thicker bark.

What's the “prescription”?

The “prescription” is two sets of directives. The first outlines the site’s overall management plan. Ecologists use observations and data to determine what’s growing at a site and which tools — fire, mowing, brush cutting, herbicides, etc. — can improve the density and variety of native species. Ecologists may decide to use fire if:

  • They’ve cut invasive buckthorn and honeysuckle but are seeing new sprouts, which fire can knock back.
  • They’ve applied herbicides to invasive plants and burning the dead vegetation will expose the soil and allow the dormant invasive seeds to germinate so that a second more exhaustive treatment can be applied prior to putting in native seed.
  • There’s a mix of native and invasive grasses and burning will clear the entire site and allow them to better target and eliminate invasives as they sprout in the spring.

The second is a safety plan that dictates if a burn is a go on a particular morning.

  • What’s the humidity? Is there rain in the forecast? Moisture on the plants will make it difficult to burn.
  • What’s the wind speed and direction? If there’s a busy road east of the site, the Forest Preserve District can’t burn if winds are out of the west.
  • Are there specific native plants or animals present and will the prescription keep them safe?
  • Are the prescribed number of trained fire crew members required to safely conduct the burn available?
  • Are all firebreaks prepared?
  • Is equipment to safely conduct the burn available?


What happens before the prescription burn season?

After selecting sites and outlining the prescriptions, the Forest Preserve District applies for burning permits from the Illinois Environmental Protection Agency and, if needed, local fire departments. It also mails postcards to people who live within 1,000 feet of each burn site. (Depending on the size, a forest preserve can have more than one burn site.) Residents with health conditions that smoke can aggravate can request calls the morning of the burn.

What happens on the day of a burn?

In the morning the Forest Preserve District reviews site prescriptions and determines which, if any, it can burn that day. If a burn is a go, it notifies local fire departments and the DuPage County sheriff’s office and calls residents who received postcards and requested calls.

Once the burn begins:

  • Employees post signs at entrances and along trails to ensure visitors remain at safe distances.
  • Employees follow strict protocols on lighting, controlling and monitoring each fire. 
  • The Forest Preserve District remains in touch with fire departments.
  • Crews remain on-site after the fire burns out to look for flare ups. At prairies and wetlands they may stay for an hour; in woodlands they may need to remain for several.

Watch this video of prescribed burn at Timber Ridge Forest Preserve.


To see a Forest Preserve District crew in action, watch our video of a prescription burn.


Selective Clearing & Mowing

Selective clearing is typically done in forests and savannas. Crews remove invasive trees and shrubs, such as European buckthorn and honeysuckle, and selected, aggressive native trees and then burn the brush piles on site. They then carefully apply herbicides to the cut stumps so the plants cannot grow back. Crews may also use heavy-duty mowers to cut shrubs and trees.

Work is done primarily in winter to protect native grasses, forbs, and animals and to lower the risk of fire hazards from burning brush piles. When heavy equipment is used, frozen ground in winter prevents machinery from severely disrupting the soil, too.

Studies show native plants as well as populations of birds and butterflies can recover once selective clearing and mowing removes the weedy vegetation.


Aquatic Plants

In healthy lakes and ponds, native aquatic plants do a lot. They filter nitrogen and phosphorus from runoff, help control unwanted algae blooms and reduce shoreline erosion. They provide food, shelter and breeding habitats for a variety of wildlife (ask any angler what that means for finding great fishing spots!) and even keep invasive aquatic plants from taking hold.

But aquatic invasives can spread into ponds and lakes nonetheless, stealing space, sunlight and nutrients from native plants and creating headaches for anglers and boaters alike. In certain cases, depending on the types of plants, how prevalent they are, and which native plants are nearby, the Forest Preserve District may use one or more strategies to control them.

Chemical controls are typically used the most because they’re the most effective way to immediately control established plants. Whole-lake treatments can affect even desired native plants, but spot treatments can target smaller areas of nonnative invasive — even nuisance native — plants. With proper long-term planning, chemical controls give short-term results that bridge to larger changes within the watershed.

If the Forest Preserve District determines uses chemical controls that have water-use restrictions, it always posts signs on site. 

Biological controls such as grass carp can be effective, but these options can create an “all or nothing” result. Grass carp, for instance, tend to prefer native plants over nonnatives and can create more favorable conditions for the spread of invasives.

There are physical and mechanical mechanisms, too. Ecologists sometimes hand-pull plants or use tools or heavy machinery to pull and pulverize them, but these methods are often labor intensive and less efficient. If there’s an existing water-control structure, ecologists may draw down the water level. Barriers are somewhat successful, but because invasives tend to be more resilient to these mechanisms than natives, this technique may help invasives spread while tamping down native plants.