Exotics are plants not native to Illinois. Exotics pose a large risk to the prairie, especially a young or severely degraded prairie, because they compete with the more desirable prairie plants for water, sunlight, and nutrients. Some examples of aggressive exotics to a to an Illinois prairie include leafy spurge, purple loosestrife, and garlic mustard. An annual ground cover like rye may be used during prairie establishment to reduce unwanted exotics. Many weedy species are annuals and can be removed through a couple of prescribed burns or manual removal.
Some weeds are much more difficult to remove and can be removed with selective use of herbicides.
The native prairie plants develop extensive root structure the first few seasons and eventually will outgrow the alien species. Also because of their underground development, each season's new growth is really not due in large part to plants starting from seed, but from new shoots from the underground association of roots, tubers and rhizomes. This complex association of underground growth is what prevented the early settlers from actually using the prairie to grow their crops. They found it easier to clear forested areas to start their farms. The number of weed seed in the soil is variable and may range from 2 seeds per pound of surface soil in grassland to 140 seeds per pound of surface soil in cropland.