Milkweed Identification: Milky Sap and Warty Pods

Common milkweed (Asclepias syriaca) is identified by the sap. Iowa State University Extension's description is direct: all plant parts emit milky sap when damaged. Snap a leaf and a white latex appears at the break.

The rest of the plant is consistent and easy to check. Iowa State records an unbranched stem up to five feet tall, covered with dense soft pubescence, carrying large oppositely arranged oblong leaves up to eight inches long. The USDA Forest Service gives leaves of 6 to 8 inches long and 2 to 3.6 inches wide on a plant about five feet tall that usually occurs in clusters of stout stems.

Flowers come in nearly spherical clusters. Penn State Extension records up to 100 flowers per cluster and one to three clusters per stem, in green, pink, white, purple or lavender, from June through August. The Forest Service calls them very strongly and sweetly scented — on a warm evening you often smell a patch before you see it.

The pod is the other unmistakable feature. Iowa State records seed pods 3 to 4 inches long, 1 to 2 inches wide, and covered with soft prickles; the Forest Service gives about 4 inches, inflated and covered in little finger-like projections. Inside, the Forest Service counts 50 to 100 seeds, each with a white fluffy coma that carries it on the wind.

But seeds are not how most patches grow. Iowa State notes that the majority of common milkweed plants emerge from overwintering roots, spreading by vegetative rootstocks — which is why milkweed appears as a colony of similar stems rather than scattered individuals, and why pulling the tops does not clear it.

The USDA Forest Service places it across most of the eastern United States through the prairie states and southern Canada, in fence rows, on roadsides, in fields, prairies and pastures. Penn State mentions a historical use worth knowing: life jackets were made from the coma of the seeds during the Second World War.

The USDA Forest Service states that in the northeast and midwest, common milkweed is among the most important food plants for monarch caterpillars (Danaus plexippus), and Penn State Extension lists it as a larval host plant for monarch and queen butterflies.

The chemistry is the reason. The Forest Service notes milkweeds contain various levels of cardiac glycoside compounds that render the plants toxic to most insects and animals; Penn State adds that the cardiac glycosides absorbed by monarch larvae make them toxic to birds and other predators. The caterpillar turns the plant's defense into its own.

That also sets the expectation for what you will find on a milkweed patch in summer: chewed leaves, striped caterpillars, aphids, and beetles that are likewise orange and black. Damage on milkweed is usually the point rather than a problem.

If you want to plant milkweed, ask your county extension office or a native plant society which Asclepias species is appropriate for your region, because recommendations differ by state.

Iowa State University Extension names the plant most often confused with milkweed: hemp dogbane, which resembles common milkweed in early spring. Three differences settle it:

• Leaf size — mature dogbane has much smaller leaves
• Stem branching — dogbane branches; common milkweed sends up an unbranched stem
• Stem surface — Iowa State notes hemp dogbane stems are glabrous, meaning hairless, while common milkweed stems are covered in dense soft pubescence

Both plants have milky sap, so sap alone does not separate them — run your fingers along the stem instead.

Safety note. Penn State Extension states that the bark, flower, seed pods, leaves, roots and stems of common milkweed are poisonous, and lists cats, dogs and horses among the animals affected. Do not eat any part of this plant, and do not rely on historical or foraging accounts. If a person swallows part of it, call Poison Control at 1-800-222-1222; if an animal does, call your veterinarian. Wear gloves if you handle cut stems, and keep the sap away from your eyes.