A beautiful garden really comes to life when it is full of butterflies. Colorful blooms of all kinds will attract them. A few well-chosen food plants for caterpillars will maintain a steady butterfly population through the summer and fall. Do not use insecticides in your garden.
Zinnias are one of my favorite nectar plants: butterflies love them. Start with plants from a garden shop in spring, or sow seeds outdoors as soon as the danger of frost is past. Try several kinds and colors -- tall and short, double and semidouble -- and grow them in several sunny spots in the garden. They bloom all summer long and the butterflies will keep on coming.
Include some native perennial plants in your butterfly garden. Purple coneflowers (Echinacea purpurea) supply nectar for butterflies in late summer and early fall. Butterfly milkweeds (Asclepias tuberosa and A. incarnata) attract all kinds of butterflies; handsomely striped monarch caterpillars, with their long, delicate black horns, feast on the foliage.
Parsley and fennel are excellent food plants for the larvae of the black swallowtail. They have voracious appetites. Plant plenty, so there'll be enough for both you and the caterpillars.