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Illinois Milkweeds
What are milkweeds?
• Milkweeds are herbaceous (soft-stemmed), perennial plants.
• They usually have leaves paired or in whorls of four on the stem (exceptions exist).
• Most of them have white, milky sap (exceptions exist).
• Hourglass-shaped flowers are produced in an umbel (central point from which a group of flowers develop) at the stem tip or in the leaf axils in the upper part of the plant.
• Flower color varies by species: white; pink; red; orange; green; red-purple; purple-pink.
• The fruit that develops from the fertilized flower is a pod that contains seeds attached to floss (an exception exists).
How many types of milkweeds grow in Illinois habitats?
• Twenty-four species of milkweeds are found in Illinois, but two of them are not native to the state. This number of species was obtained from Vascular Flora of Illinois: A Field Guide, Fourth Edition by Robert H. Mohlenbrock.
What is the relationship between milkweeds and monarch butterflies?
• Monarchs have a life cycle that includes four stages: egg; larva; pupa; and adult. Monarch larvae feed exclusively on milkweed plants of the genera Asclepias and Ampelamus. If there is no milkweed, there will be no monarchs.
• Female monarchs usually lay their eggs only on milkweed plants so that the larvae will have an immediate food source when they hatch from the egg. They find milkweeds by using visual and chemical cues.
• Monarch adults often visit milkweed flowers for the nectar they produce, but they are not restricted to these plants as the larvae are.
Where do milkweeds grow in Illinois?
• Some tend to live in wetlands. Others prefer woodland or prairie habitats. Some are generalists and can grow in most habitats.
Are there any endangered or threatened milkweed species in the state?
• Currently, five species of milkweeds are listed as endangered in Illinois with one of those species also listed as threatened federally.
Are milkweeds important to other pollinators, too?
• Many pollinators visit milkweeds for their nectar. Nectar is a sweet solution produced by flowers to attract pollinators.
How can some milkweed species be harmful to monarchs?
• Two invasive milkweed species that are native to Europe have been growing in North America since the 1800s. Black swallow-wort (Vincetoxicum nigrum) can be found in Illinois. Pale swallow-wort (Vincetoxicum rossicum) grows in Illinois’ neighboring states of Indiana, Wisconsin and Missouri and may also be present in Illinois.
• They are known as "dead-end" hosts. Monarch larvae cannot eat them. The monarch larvae hatched from eggs on these plants will die. Experiments have shown that female monarchs will lay eggs on these nonnative plants even when native milkweeds are present in the same area.
• These two swallow-wort species also crowd out native milkweed plants, reducing native plant biodiversity and biodiversity of the animals that depend upon the native milkweeds.
• Both of these swallow-wort species are herbaceous vines with clear sap and opposite leaves. Black swallow-wort flowers are dark purple to black. Pale swallow-wort flowers range from pink to burgundy. Their seedpods contain seeds that are similar in appearance to those of common milkweed (Asclepias syriaca).
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