Plant milkweed in the spring along with your nectar plants. The milkweed will be critical to the butterflies in the Fall when they migrate south en route to Mexico.
While the nectar plants in your garden are a necessary food source for adult butterflies, they use milkweed to lay their eggs in the fall. When the eggs hatch into caterpillars, the caterpillars eat the milkweed until they morph into butterflies.
So, plant milkweed in the spring with yoru nectar plants so butterflies have all they need to eat and enhance their cycle of life.
A cateriller on milkweed.