The Milkweed species is so crucial to the life cycle of the Monarch butterfly that Monarchs and their kin, like the Queens and Tigers, are called "Milkweed Butterflies." In Britain the Monarch is called simply The Milkweed.
The Monarch butterfly ranges all over the United States and Canada, but is limited in the Pacific Northwest unless there is a long and warm summer. Monarchs are famous not only for the beauty of their tiger-colored wings, but also for their multi-generational migration to Mexico, where they overwinter. Sometimes the trees there are so full of Monarch butterflies that even heavy limbs break off. Monarchs are also known for being one of the rare insects that cross the Atlantic ocean.
The Monarch is found just about everywhere in its range, including meadows, gardens, parks and roadsides. Fortunately, milkweed plants also find these habitats to their liking.
Monarchs use milkweed as both a source of nectar and as a host plant for their larvae. After mating in the spring, the female lays her eggs on the milkweed plants leaves. The eggs hatch and the black, yellow and white striped caterpillars eat their own egg case for a burst of energy, then start eating the milkweed. The milkweed not only gives the caterpillar food, but also toxins called cardenolides, which makes both the caterpillar and the adult butterfly distasteful to potential predators. After about two weeks the caterpillar turns into a chrysalis. The Monarch chrysalis is uncommonly beautiful. It is pale-jade green with dots of what look like gold paint. After about two weeks the butterfly emerges. Monarchs live anywhere from one to seven months depending on the time of year they hatch.
There are over 140 species of milkweed, including the Common milkweed (Asclepias syraica), the Tropical milkweed (Asclepias curassavica) and the Swamp milkweed (Asclepias incarnata). Flowers can appear in shades of white, pink, mauve, orange, yellow or scarlet. The Common milkweed is a perennial that grows in full sun, but can also tolerate some shade. It can be invasive and is considered a weed in many areas. The flowers, which grow in umbels, appear in late spring or early summer. The seeds are famous for having long parachutes made of pappus, or floss. They burst out of conspicuous seed pods and are carried away on the wind. Pappus has been shown to have such good insulating qualities that it's now used to stuff pillows.
Common milkweed can grow from three to six feet tall. It propagates through rhizomes. Swamp milkweed is not invasive, prefers soil that's a bit damp, and can be found on the edges of bodies of fresh water. The flowers bloom later than the common milkweed. Butterfly weed (Asclepias tuberosa) has orange flowers and grows to three feet tall. It needs full sun as well and is a popular garden plant.
All milkweeds exude a milky sap that gives them their name. This sap irritates the skin and milkweeds are considered to be poisonous to humans. Yet, in ancient days, milkweed was prized for its medicinal properties. Indeed, the species is named after Asclepius, who was the god of healing in ancient Greece. The sap, though irritating in itself, is still used to treat poison ivy.
Gardeners who want to grow milkweed plants to attract the Monarch butterfly should know that they do well in Zones 4 to 10. They're excellent in meadow gardens and borders. The Butterfly weed can tolerate drought and the Common and Tropical milkweed plants need an average amount of watering.