It is officially summer and many Milkweeds are in bloom here in central Ohio. Milkweeds are favorites of many botanists, gardeners, and insects, but did you know we have 13 species (not including hybrids) here in Ohio? Ohio Milkweed species range from extremely frequent to very rare. For example, Common Milkweed (Asclepias syriaca) is found throughout the state in ditches and along roadsides whereas Redring Milkweed (A. variegata), although it is widespread in the southeastern US, occurs in only a few places in southern Ohio.
Milkweeds are well known for their association with Monarch butterflies. Monarch caterpillars are one of the few specialist organisms that can ingest the toxic leaves without severe negative effects to their health. The milky latex that gives Milkweeds their common name is full of cardiac glycosides known as cardenolides that can disrupt the bodily functions of most animals, including humans. Does that make animals that cannot eat Milkweed latex-intolerant? Hmm…
Insects that specialize on Milkweeds have developed molecular resistance to these toxins and may also strategically cut the leaves to change the flow of the latex and allow for easier feeding, or even sequester the toxins inside their body to make them poisonous to predators (1, 2). All of this reading about Milkweed toxicity has me singing System of a Down (Link to relevant song here.)
Another thing that makes Milkweed unique is its complex floral morphology. Milkweed pollen is packaged into paired masses called pollinaria that attach to the feet of pollinators who land on the flower looking for a nectar treat. In the center of each flower is the stigmatic column which contains slits where the pollinaria can be deposited. Each flower has two ovaries that, when the flower is pollinated, turn into the paired seed pods often seen in the fall. The seeds of Milkweed plants have a tufted hairy appendage that aid in their dispersal by wind. (For a more in depth look at Milkweed floral morphology click here)
Common Milkweed (Asclepias syriaca) requires full sun and can withstand disturbance, so it often grows in roadsides or next to agricultural fields. While there has been a large push in the US to plant more Milkweed in response to declining monarch numbers, there is a different sentiment in Europe. Common Milkweed was introduced to Hungary in the 18th century and is now considered invasive in many European countries (3). It has been found to reduce the diversity of native flora and spiders in European grasslands (4, 5).
Swamp Milkweed (A. incarnata) may be easily confused with Common Milkweed at first glance. However, it typically has smaller, pinker flowers and thinner, pointier leaves. Additionally, swamp Milkweed is an obligate wetland species and is only found where soils retain moisture. Monarchs have been found to prefer this species of Milkweed above others in several studies, so if you have a wet area in your yard that you are looking to spruce up, plant some of these guys (6, 7).
Four-leaved Milkweed (Asclepias quadrifolia) is easily distinguished from other species of Milkweed by its whorls of 4 leaves. Unlike Common and Swamp Milkweed, these plants occur in shady forests. They also have a pleasant scent. Interestingly, four-leaved Milkweed occurs throughout the eastern US except it does not occur in the Mississippi river valley, giving it a disjunct distribution (8).
Poke Milkweed (Asclepias exaltata) gets its common name from its similarity to poke weed (Phytolacca americana), which has similar looking leaves (9). Poke Milkweed usually occurs in forest gaps and edges. It has been shown to hybridize with Common Milkweed, though the two species typically are isolated from each other as Common Milkweed requires full sun (10).
Butterflyweed (Asclepias tuberosa) is commonly planted in pollinator gardens and is a favorite of many insects. Unlike other Milkweeds, butterflyweed does not produce a milky sap.
Hemp dogbane (Apocynum cannabinum), while not technically a Milkweed, is a member of the same family, Apocynaceae. Ohio has two species of dogbane, the more common species being hemp dogbane, which gets its latin name from the fibers it produces which are similar to those of cannabis. Hemp dogbane requires full sun and can be found in disturbed sites and roadside ditches. Dogbane produces paired thin seed pods and can also reproduce clonally, meaning one plant can send up many shoots underground. Dense stands of dogbane can actually be just one genetic individual.
Planting Milkweeds is great for many native pollinators, not just monarchs. If you live in Ohio and are looking for a source for Milkweed plants, I would suggest looking into Natives in Harmony which has a great selection of native Milkweeds.
Additional Milkweed photos