Milkweed is a lifeline for monarch butterflies because it provides food and, even more important, a place to reproduce.
Milkweed plants are the only host plants for monarchs, meaning they're the only plants on which monarchs will lay their eggs. When caterpillars hatch from those eggs, they feed on the milkweed leaves.
Besides providing nourishment, the leaves help protect monarchs from predators. Chemical compounds in the milkweed plant make the caterpillar poisonous to many of its enemies, which are usually repelled by the insect's foul taste. That poison stays in the insect's body when it turns into a butterfly.
Milkweed also provides nectar that helps fuel the adult monarchs' long flights to and from Mexico. It's not the only plant monarchs will feed on, but it's the only one that does double duty for the butterflies as both a host plant and food plant.
Growing milkweed is easy, but you want to start by choosing the right kind. More than 100 milkweed species grow in North America, but only about one-fourth of them are good for supporting monarch butterflies, according to information from the U.S. Forest Service.
Some types of milkweeds, called swallow-worts, can fool monarchs into laying their eggs on them. But the caterpillars don't get what they need from the leaves to develop into butterflies.
The three types of milkweed the educational outreach program Monarch Watch recommends are swamp milkweed, common milkweed and butterfly milkweed, also called butterfly weed.
As its name implies, swamp milkweed (Asclepias incarnata) is best for wetter sites. Swamp milkweed grows about 3 1/2 feet tall and produces white, pink or mauve flowers.
Butterfly weed (Asclepias tuberosa) is better suited to drier sites. It reaches only about 2 feet tall and has bold orange flowers.
The common milkweed (Asclepias syriaca) is the most adaptable, suitable for soils that range from a little dry to a little wet. It's also the most fragrant of the three and the tallest, stretching to 4 1/2 feet or more. Its flowers range from light pink to reddish-purple. Common milkweed spreads fairly aggressively by underground rhizomes. So if you want to keep its spread in check, plant it in a bottomless pot sunk in the ground.