Common milkweed (Asclepias syriaca) is a native herbaceous perennial whose main virtue is its appeal to butterflies—especially the monarch, which deposits its eggs on the milkweed. When the caterpillars hatch, they feed on the leaves of milkweed. Common milkweed plants grow to about two to four feet in height, with a thin, vertical growth habit. The long, oblong leaves are light green and grow to about eight inches long. The stems and leaves bleed a milky sap when cut, which gives the plant its name. In late spring to mid-summer, fragrant clusters of pink-purple flowers appear. The flowers produce warty seed pods two to four inches long that split when ripe to cast many fine seeds to the wind.
- Botanical name: Asclepias syriaca
- Common name: Milkweed, common milkweed
- Plant type: Herbaceous perennial
- Mature size: 2 to 4 feet
- Sun exposure: Full sun
- Soil type: Any well-drained soil; tolerates clay soil and poor, dry conditions
- Soil pH: 4.8 to 7.2
- Bloom time: June to August
- Flower color: Pink, mauve, white
- Hardiness zones: 3 to 9 (USDA)
- Native area: Fields and roadsides of eastern North America
Planting for Monarch Butterflies
Milkweed is the single most important source of food for the threatened monarch butterfly, and planting a patch or two in your landscape is an important contribution to the continued existence of the species. The butterflies use the plants for all stages of their lifecycle, so watching the caterpillars feast on leaves, create their chrysalises, then mature and hatch into butterflies can be an entertaining and informative family activity. (But warn children about the toxic nature of milkweed leaves.) With careful observation, you may see all phases of the life-cycle in a single year, from eggs hatching into tiny caterpillars, to caterpillars magically transforming into butterflies, to butterflies laying new eggs.
If you are planting common milkweed to encourage monarch butterflies, create a small patch of milkweed that includes at least six plants. Include a nearby water source for your butterflies; a birdbath or a large potting saucer filled with water will work fine. Planting other pollinator-friendly plants in a comprehensive butterfly garden is a good idea.
It is important not to use pesticides in a butterfly garden, as the same chemicals that kill destructive insects will also kill butterflies and their larva. Most gardeners find, though, that once a garden goes chemical-free, it establishes a good balance of beneficial, predatory insects, as well as providing songbirds with a source of food (many bird species consume large quantities of insects). Many of the most healthy gardens are those that are entirely free of chemical use.
How to Grow Milkweed Plants
Plant milkweed about 18 inches apart; their rhizomatous roots will quickly fill in the space between plants. There's no need to fertilize milkweed plants, as they tolerate poor soils. Your biggest maintenance challenge with them will probably be in containing them. Asclepias syriaca spreads both via seeds and rhizomes, forming colonies.
At the very least, you may wish to remove the seed pods before they open. Otherwise, they will spread to distant corners of your yard (and beyond), thanks to the silky appendages that allow the seeds to waft on the slightest breeze. They are rather like the seeds of dandelions in this regard.
Milkweed prefers full sunlight.
This plant prefers well-drained soil and performs well in dry conditions, as do many wildflowers.
Common milkweed does not need watering except in the driest conditions.
Temperature and Humidity
Tolerates a wide range of temperatures and humidity.
No feeding is necessary with this plant.
Propagating Milkweed Plants
Milkweed spreads easily via rhizomatous roots; cutting root sections and planting them so the roots are just buried will easily propagate the plants.
Toxicity of Common Milkweed
Common milkweed leaves and roots are toxic to humans, but only if it is consumed in large amounts. The plant has actually been a food source for Native American tribes, who carefully prepared it by cooking. Wild-food enthusiasts continue to cook and eat the plant today. However, eating large quantities of unprepared milkweed may cause bloating, fever, difficulty breathing, dilated pupils and muscle spasms, and the result can, in very rare instances, be fatal.
Because the leaves are bitter, it is unusual for people or pets to deliberately eat milkweed, but grazing animals are sometimes poisoned by them. These plants are generally not a serious hazard in the home garden, but children should be kept away or warned.
Pruning Common Milkweed
Remove the seedpods from milkweed to prevent the seeds from spreading on the wind.
Growing From Seeds
Milkweed can also be grown easily from collected seeds planted about one inch deep in the garden.
Many find butterfly weed (Asclepias tuberosa), with its bright orange flowers, a more attractive type of milkweed than the common milkweed. Another type of milkweed is swamp milkweed (Asclepias incarnata), which is a good choice for moist soils.
This is not a particularly attractive plant, but in addition to its value in attracting butterflies, the seed pods can be used in dried flower arrangements. The flowers are also quite fragrant, and gardeners specializing in native species often grow milkweed. Milkweed has a history of medicinal use, but be aware that the leaves and the milk-like substance within are poisonous—it's this milky residue consumed by monarch butterflies that makes them bitter and repulsive to predators.
Milkweed spreads fairly aggressively, and many gardeners and local agencies warn against its use for this reason. It is too aggressive for most mixed border gardens unless you are specifically creating a butterfly garden.
Common Pests and Diseases
There are no serious disease or pest problems with common milkweed. The most serious issue is its aggressive spreading habit. Removing seed pods will prevent the plant from self-seeding far and wide. Periodically, you may need to dig up the plants during fall and discard all but selected portions of the roots to keep the plant tamed in the garden.