Wild About Illinois Milkweeds!
Twenty-four species of milkweeds are found in Illinois. They grow in a variety of habitats. Most milkweeds have leaves in pairs or in whorls of four and sap that is white and milky. Milkweed flowers develop in an umbel at the stem tip or in the leaf axils in the upper part of the plant. Color varies with the species, but varieties include white, pink, red, orange, green, red-purple and purple-pink flowers. Each flower has five petals and five sepals that bend away from the other flower structures and a five-parted cup that supports five horns and hoods. Pollinators visit milkweed flowers for their nectar. The fruit that develops from the fertilized flower is a pod that contains seeds attached to floss.
A close relationship exists between monarch butterflies (Danaus plexippus) and milkweed plants. Monarch larvae only eat milkweed plants of the genera Asclepias and Ampelamus. If there are no milkweeds plants, there will be no monarchs. As of 2020, five species of milkweeds are listed as endangered in Illinois with one of those species also listed as threatened federally.
Kingdom: Plantae - All plants have nucleated cells and have cell walls containing cellulose. Most have chlorophyll and produce their own food.
Division: Magnoliophyta - These are the flowering plants.
Family: Asclepiadaceae - The milkweed family contains herbs, vines, shrubs and trees. These perennial plants have leaves that are opposite each other or whorled along the stem. Flowers may be single or in clusters. The seeds are hairy and may or may not be winged.
bluevine Ampelamus albidus
sand milkweed Asclepias amplexicaulis
poke milkweed Asclepias exaltata
tall green milkweed Asclepias hirtella
swamp milkweed Asclepias incarnata
woolly milkweed Asclepias lanuginosa [state endangered]
Mead’s milkweed Asclepias meadii [state endangered]
oval milkweed Asclepias ovalifolia [state endangered]
white swamp milkweed Asclepias perennis
purple milkweed Asclepias purpurascens
whorled milkweed Asclepias quadrifolia
showy milkweed Asclepias speciosa
narrow-leaved green milkweed Asclepias stenophylla [state endangered]
prairie milkweed Asclepias sullivantii
common milkweed Asclepias syriaca
butterfly-weed Ascpleias tuberosa
variegated milkweed Asclepias variegata
horsetail milkweed Asclepias verticillata
green milkweed Asclepias viridiflora
green-flowered milkweed Asclepias viridis
climbing milkweed Matelea decipiens [state endangered]
climbing milkweed Matelea gonocarpos
climbing milkweed Matelea obliqua
black swallow-wort Vincetoxicum nigrum [nonnative]
What are Milkweeds?
Milkweeds are herbaceous, perennial plants, meaning that they are soft-stemmed plants that die to ground level at the end of each growing season but grow back from the roots the next spring. Most have leaves that are paired on the stem or in whorls of four on the stem, but there are also milkweed plants with leaves alternating on the stem, and those that have so many leaves that it is hard to see a pattern. Most of them have sap that is white and milky. Milkweed plants contain cardiac glycosides. These chemicals are poisonous and affect birds and mammals. Many grazing mammals will not eat milkweeds. The toxicity of milkweeds varies by species, though, and tends to be greater in milkweeds in the southern United States. A few animal species have adapted to eating milkweeds and thrive on them. Monarch larvae feed exclusively on milkweeds of the Asclepias and Ampelamus genera. The poisons accumulate in the body of the larval monarchs and are retained by the monarch in its transformation to the adult. They make monarchs unpalatable to many predators.
Milkweed flowers develop in an umbel at the stem tip or in the leaf axils in the upper part of the plant. An umbel has a central point from which a group of flowers all develop. In some milkweed species the flowers are arranged in a spherical shape, while in other species the flowers droop. Color varies by species, but milkweeds can be found with white, pink, red, orange, green, red-purple and purple-pink flowers. The flowers are often described as having an hourglass shape. Each flower has five petals and five sepals that bend away from the other flower structures. A five-parted cup supports five small horns and hoods. The hoods contain nectar and are arranged around the central flower column. The flower column has slits in it. Inside each slit is an opening where pollen (containing male reproductive cells) must be delivered to fertilize the egg and start the development of a new milkweed plant. Also in each slit is the pollinarium that contains the pollen in packets.
Pollinators, including adult monarch butterflies, visit milkweeds for their nectar. Nectar is a sweet solution produced by flowers to attract pollinators. Milkweeds have a unique system for pollen transfer. When an insect visits a milkweed flower to drink nectar, its leg, antennae or bristles can slip into the slit in the flower where the pollen is stored. The pollen-containing structure clips onto the insect part. When the insect pulls away from the flower, this pollen packet goes, too. The same insect body part may slip inside a slit in the flower column of a different flower. If the pollen packet is placed precisely where it needs to be, that flower will be pollinated. If the pollen packet is not deposited in the exact position required, fertilization will not occur. The amount of precision required may be the reason that milkweeds produce so few fruits. Pollinator populations are declining. As their numbers continue to decrease, the amount of viable seeds from milkweeds will also be smaller, leading to fewer milkweed plants, fewer milkweed flowers and less pollen and nectar for pollinators.
The fruit that develops from the fertilized flower is a pod that contains seeds attached to floss. Seeds with floss are easily dispersed by wind. An exception, though, is provided by the white swamp milkweed (Asclepias perennis). This species’ seeds are dispersed by water instead of wind, and they do not have floss.
Monarchs and Milkweeds
A close relationship exists between monarch butterflies and milkweed plants. 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.
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. Problems occur when female monarchs lay eggs on these two species. They are known as “dead-end” hosts. Monarch larvae cannot eat them. The 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).
What can be done to stop the invasive swallow-worts? The first step is to successfully identify the plants. They are easiest to identify when they are blooming. Plants in full sun produce fruits sooner than those in shaded areas and should be eradicated first to reduce seed dispersal. When swallow-wort has been removed, it is important to plant the area with native plants quickly so that more invasive species do not move in. Digging, cutting, mowing and seedpod removal can help control these plants. Cutting should be used in combination with other techniques, since cutting alone can lead to resprouting. Prescribed burns can help to keep these plants out of an area after they have been removed by other methods. Appropriate disposal of all roots, root parts and seeds is required to ensure that they do not grow. Bag them and either burn them or dispose of them in a landfill. Do not put them in a compost pile.
Many insects and some bats are pollinators. Hummingbirds, some monkeys, some rodents and other animals are pollinators, too. Humans can be pollinators as well! Not all plants need pollinators, but about 85 percent of them do. These plants would not be able to produce seeds without pollinators.
Pollen contains the male reproductive cells. For fertilization to occur, pollen must reach the female reproductive cells of the plant. Animals that move pollen are called pollinators. They are important to both native plants and agricultural crops. In the United States, more than 150 crop plants require pollinators. Pollinators help insure that we have food to eat. It is estimated that one of every three bites of food that we eat is related to the actions of pollinators. Pollinators provide more than $10 billion in economic value annually in the United States, but they are also vital to the existence of native plants and all of the animals that feed on them.
Pollinators are in decline worldwide. Habitat loss, pesticide use, competition from nonnative species and diseases are killing pollinators. We can help pollinators by providing habitat in pollinator gardens. Native milkweeds are important components of pollinator habitats.
- A good pollinator garden has native plants that attract and support native bees and other pollinators at all stages of their life cycle. Included should be flowers that provide pollen and nectar from early spring through late fall. Add flowering trees. Use plants with flowers of varying shapes. Add bunch grasses to the planting for nest sites for ground-nesting bumble bees. Eliminate or minimize the use of pesticides.
- A butterfly garden is a massed planting of butterfly favorite plants in a sunny location that provides food and shelter for all stages of butterfly life. Prairies are the original butterfly gardens. A prairie garden, usu-ally part of a home/school landscape, is of variable size and utilizes native wildflowers and grasses often organized according to height, color, bloom time, etc.
- Prairie Establishment and Landscaping – This publication from the IDNR offers information about select-ing, establishing and maintaining a prairie. It also includes possible sources for native seeds and plants. Other publications such as the Butterfly Gardens brochure are also available through the Publications Web site.
- Try to obtain plants or seeds that were developed within a 50-mile radius of where you live. If that is not possible, select plants and seeds from a source in Illinois or one of Illinois’ border states.
- There is much information about developing gardens using native plants at the Illinois Schoolyard Habitat Action Grant Web page.
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