The Natural Resources Defense Council and the state Tollway Authority have agreed to plant milkweed along 286 miles of roadways.
It's part of a program to help boost the population of monarch butterflies, whose declining numbers have alarmed environmentalists and others.
Much of the decline is blamed on habitat destruction. Monarchs lay their eggs only on milkweed, which is the sole source of food for the caterpillars that later develop into the distinctive orange-and-black butterflies.
Monarchs in the eastern United States migrate thousands of miles to Mexico for the winter.
NRDC attorney Rebecca Riley says Illinois should do everything possible to save the monarch, which also is the state insect.