Most caterpillars are specific about the plants they feed on, and female butterflies will lay their eggs only on the caterpillar food plant. Since you cannot have butterflies without caterpillars, be sure to provide a variety of caterpillar food plants to meet the needs of the species in your area. Some plants, like milkweed and alfalfa, provide both nectar for adults and food for many species of caterpillar. In addition to providing nectar for adults and food plants for caterpillars, it is important to include sources of shelter in your garden. Plant your garden in a sunny place to increase blooming, and provide rocks for butterflies to bask on in the sun.
Butterflies bask to heat up their wing muscles for flight.
Also, be sure not to use pesticides in or near your garden. The following list of plants and shrubs includes nectar sources and caterpillar food plants that are available in the rocky mountain region that should work well in a butterfly garden. Many of them will work outside the region as well, but check with your local university, natural history museum or zoo for your specific area. When in the Denver area, be sure to visit the Butterfly Pavilion and Insect Center, 104th and U.S. 36 in Westminster, Colorado. Open Tuesday through Sunday, 9am to 5pm.