Common Milkweed

Silkweed

Asclepias syriaca

Common milkweed plant flowering
Common milkweed plant flowering. Photo courtesy of Circeus. Licensed under CC BY-SA 2.0.
Milkweed is an erect perennial herb that is commonly associated with monarch butterflies. It is easily identified by a sticky white sap that oozes from any part of the plant that is wounded or broken.
Common milkweed seedlings are very fragile and spindly. The cotyledons are oval shaped, dull green, and have a smooth edge all around. True leaves are oblong, waxy, green, and have a pointed tip. The midvein is prominent and white. Seedlings are less common than young shoots that develop from cuttings or perennial roots. Seedlings and new shoots usually emerge during the spring but have also been known to emerge during the summer.
Common Milkweed Seedling
Common milkweed seedlings. Photo courtesy of Phil Westra. Licensed under CC BY 3.0.
As the seedling matures, its stalk becomes sturdy and maintains straight growth. They grow to 5/8 inch thick and turn from green to red in the fall. Leaves are arranged in pairs or whorls (3), always growing directly from the main stalk and separated vertically from other pairs by 4 to 6 inches. The 3 to 10 inch leaves are rounded and smooth along the edges, usually with a tiny point at the very tip. The upper surface is smooth and deep green, while the underside is pale-green and downy.
Mature Common Milkweed
Mature common milkweed plant. Photo courtesy of Derek Ramsey. {{GNUFDL1.2}}
Common milkweed flowers bloom at the beginning of summer and are arranged in nearly spherical clusters of 20-130 flowers around the tips of the stems and axils of the upper leaves. The individual flowers are small and fragrant with the 5 petals organized in the shape of a star. Though each flower is perfect (male and female parts included), it cannot produce seeds without cross-pollination by insects. After pollination, the fleshy, green 5 inch long seed pod is produced. The dried seed pod splits lengthwise in the late summer/early fall and releases the flat brown seeds that are each attached to a tuft of silky hair.
Common Milkweed Flowers
Common milkweed flowers. Photo courtesy of Ryan Hagerty. Licensed under CC BY 2.0.
The common milkweed plant is able to thrive in areas that are recently disturbed, and quickly establish an an extensive root system. It is a strong competitor for water and nutrients as well as a rapidly-spreading weed. Shoots continue to propegate from these perennial roots which are often found below the plow level. This makes the weed very difficult to eradicate completely. It is also toxic to animals when consumed in large quantities.
Mowing weedy areas before seeds ripen prevents further spread of the seeds. Tilling destroys seedlings but will not be able to eradicate the root system from which shoots are re-established. Common milkweed is very sensitive to sunlight and can be controlled through rotation of crops that rapidly establish a thick shade canopy, which suppress the weed.
Herbicides are best used while the weed is actively growing. In spring, the milkweed should be at least 12 to 18 inches tall. Herbicide can also be quite effective when applied early to mid-summer while the weed is in bud. Commonly used herbicides are glyphosate and picloram. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.