Common names
Common milkweed, silkweed, cotton-weed, Virginia silk, wild cotton, silky milkweed, common silkweed, showy milkweed, swallow wort.
Latin name
Asclepias syriaca.
- “Asclepias” from Asklepios, the Ancient Greek god of medicine and healing.
- “syriaca” is Latin for the country of Syria and was so named because Linnaeus mistakenly thought this plant came from Syria.
Family
Apocynaceae – “the dogbane family”. Previously, milkweeds were assigned to their own family, Asclepiadaceae.
Life cycle
Common milkweed is a perennial plant that often forms large colonies through the growth of underground rhizomes. New plants can originate from seeds, and successful seedlings quickly establish a deep taproot system. During the first year of growth, the plant typically has a single stem, may not reach its full mature size, and often does not flower. However, in subsequent years, especially in favorable conditions, the plant can spread horizontally, and more stems will emerge from the rhizome network.
Interestingly, the stems of established common milkweed plants often emerge later in the spring compared to many other plant species. This delayed emergence may be a strategy to avoid frost damage or competition with earlier-growing plants.
Identification
- Seedlings have smooth, light green stems and round cotyledons (seed leaves) 0.5 inches long. The first true leaves are dull green with prominent veins and pointed tips. The leaves appear in pairs across the stem from one another, referred to as an “opposite” leaf arrangement. Vegetative sprouts (Figure 1) are more common than seedlings and have folded leaves that unfold as the stem elongates.
Figure 1. Common milkweed sprouts emerging at the Purdue Student Farm, West Lafayette, IN (Photo by Carlos Lopez).
- Mature plants reach 3 to 5 feet tall on a single, hollow, unbranched, and hairy green stem that turns reddish with age. Leaves are opposite, 3 to 8 inches long, elliptical, and hairless on top but hairy underneath, with a prominent white midvein (Figure 2). Round flower clusters develop in the upper leaf axils in the summer months, followed by fruit. The fruit of a common milkweed is referred to as a “follicle” because it splits at only a single suture. Each follicle is 3 to 5 inches long, teardrop-shaped, bumpy, and spiny. Follicles often appear in pairs on the plant. Mature follicles split open, releasing hundreds of brown, oval seeds with a raised center and thin, papery edges. Each seed has a tuft of silky white hairs that facilitates seed dispersal by wind.
Figure 2. An established common milkweed plant 3 feet tall (left) and a view of leaves growing in an opposite leaf arrangement (right) at the Purdue Student Farm, West Lafayette, IN (Photo by Carlos Lopez).
Figure 3. Developing flower buds (left) and a milky hollow stem (right) of common milkweed (Photo by Carlos Lopez).
- How can I tell common milkweed from hemp dogbane? Hemp dogbane leaves are almost hairless when they are young, but milkweed leaves have fine hairs. Hemp dogbane stems are typically reddish-purple and thinner than milkweed stems; milkweed usually does not branch, but hemp dogbane often does.
Figure 4a. Lower leaf surface and top of hemp dogbane plants (Photo by Carlos Lopez).
Figure 4b. Lower leaf surface and top common milkweed (below) plants (Photo by Carlos Lopez).
Integrated weed management strategy
Cultural and mechanical practices
- Tillage: Repeated shallow cultivation that cuts shoots before they can replenish the roots is required, as the shoots can emerge from roots well below the plow layer. A tilled fallow period from late spring through summer can help get heavy infestations under control, as carbohydrate storage in the roots reaches a minimum in July to September.
- Crop Rotation: Rotate with crops that establish a leaf canopy before common milkweed emergence to maintain a competitive canopy through the season and greatly suppress this weed.
- Ecological Considerations: Common milkweed hosts aphids that provide a food source for parasitic wasps, which can attack and control the European corn borer (Ostrinia nubilalis). Maintenance of low milkweed densities near crop fields may contribute to a balanced landscape management strategy that realizes the ecological benefits of common milkweed while avoiding its agricultural liabilities.
Chemical Control Options
The use of herbicides should be carefully considered, taking into account factors such as the crop being grown, potential environmental impact, and the development of herbicide resistance.
- The post-emergence (POST) herbicides 2,4-D, dicamba, and glyphosate can be used to control common milkweed with variable success.