If you incorporate host plants, nectar sources and shelter into your garden, you'll be sure to attract butterflies to your garden!
Host plants are what the caterpillars eat. Adult butterflies will seek out these plants to lay their eggs on because they know that the caterpillar cannot travel far and will not survive if placed on a plant that they cannot eat.
Nectar sources are flowering plants that adult butterflies feed on. Almost all flowering plants produce nectar.
The key to a successful butterfly garden is to plant both nectar and host plants so that the butterflies will have a food source in all stages of their life cycles. When selecting host and nectar plants, be sure to seek out native plants that have not been treated with herbicides and pesticides.
Why plant native?
Certain butterflies and plants have developed an evolutionary relationship together. This relationship is described by some as a biological arms race. As a defense mechanism, plants have evolved to produce secondary metabolites in order to deter herbivory (wildlife feeding on plants). In response, each butterfly species has evolved to be resistant to the toxins of just a small number of plants so their caterpillars have something to feed on. For instance, milkweed plants produce a toxic latex which monarch caterpillars have adapted to tolerate and even use as their own defense mechanism.
Few native butterfly species use exotic or non-native plants as their hosts because they haven't created an evolutionary relationship with them. When we only plant lawns and exotic ornamentals such as Butterfly Bush (Buddleia davidii), we often remove the vital food sources for caterpillars and this leads to decreased populations. Ironically, butterfly bush does not serve as a host plant for any native species of caterpillars. To find out which plants are native to your area visit the Native Plants Database.
Shelter
Butterflies are "ectotherms", meaning that they rely on external sources for body heat. In fact, a butterfly's flight muscles must be at least 55 degrees in order to fly. A mixture of trees, sedges and grasses in your garden will provide adequate protection from wind, rain and snow.