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
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.