Monarch butterflies enjoy nectar from a variety of flowers. However, their caterpillars can feed only on milkweed (Asclepias). So if you plant some now in your yard, you will see the pretty striped caterpillars feeding on it by midsummer.
Milkweed is the common name for Asclepias because plants in this genus have a milky sap. The most familiar species is A. tuberosa, hardy in zones 3 to 9, that grows three feet tall and has orange flowers. It is native to the U.S., as is swamp milkweed (A.incarnata), hardy zones 2 to 9. It has pink flowers and grows up to four feet.
There is also a tropical species known as scarlet milkweed that is hardy only in zone 9. In cold climates it can be grown as an annual, as it is especially showy as well as being irresistible to monarchs and their caterpillars.
There are four stages in the life cycle of a butterfly: the egg, the caterpillar or larva, the chrysalis or pupa and the adult butterfly. However, only the caterpillar and adult butterfly actually eat. The monarch caterpillars eat various species of milkweed, and because it has a bitter taste it makes the caterpillars unappetizing to birds and other predators.
So plant lots of milkweed, as it makes the monarch caterpillars happy and safer in your garden!