I first encountered the butterfly weed (Asclepias tuberosa) in a garden center in Germany when I was maybe eight or nine and I remember being very excited about it. The only Asclepias species I had hitherto encountered was the tropical Asclepias curassavica, which is commonly grown as a container plant in Germany. Enamored of exotic plants, little me was ecstatic to find another member of the genus that was just as showy as its tender cousin and supposedly fully winter hardy. I bought a small specimen and have been growing Asclepias tuberosa ever since, first in my family's garden in Germany and then in our two subsequent gardens in Michigan. In each place, it has proven not only hardy but virtually carefree.
Asclepias tuberosa last summer in my family's current Michigan garden, Zone 6a
Origin: The butterfly weed is native the eastern half of the United States as well as the southwestern states including California and the eastern provinces of Canada. In many parts of its range the plant can often been seen flowering on highway embankments, where the bright orange or sometimes yellow flowers stand out even from afar and at great speed.
USDA Hardiness Zone: Asclepias tuberosa is listed as hardy to Zone 4a.
A yellow-flowered cultivar in the current Michigan garden
Size: Most plants I have ever encountered, as well as all that I have grown, have been about 1' (ca.30 cm) tall and equally wide. However, the species is supposedly quite variable and some specimens reportedly attain heights of up to 3' (ca. 90cm).
Flowering Time: Depending somewhat on the local climate, Asclepias tuberosa will typically begin to flower in mid-summer. The main set of flowers will normally open over the course of three or four weeks but the occasional inflorescence will often continue to form throughout the rest of the summer and early fall.
Light Requirements: Adapted to sunny and dry meadows and grasslands, butterfly weed needs full sun, the more the better.
A typical specimen as seen from above
Soil Requirements: This plant will adapt to most soils, provided they are not excessively wet.
Siting in the Garden: Asclepias tuberosa should be planted in a sunny, well-drained spot with sufficient space so as not to be crowded or shaded by taller or faster-growing plants. Since the crown of the plant's tuberous root system sits close to the surface and the young shoots in spring are quite brittle it is important to site the plant away from any spot where cultivation or other disturbances might lead to damage.
Care: Plants can be planted throughout spring and summer; some organic fertilizer, such as bone or blood meal, or a generous dose of compost can be worked into the soil to give the plant a head start, though it is not necessarily needed. Plants should be kept well-watered until established and also benefit from a mulch of dead leaves, wood chips, or some other such material. Once settled in, the plants require practically no care, except preventive measures against encroachment from more vigorous neighboring plants and watering in case of extreme drought.
Yellow-flowered cultivar at the front of a mixed perennial border
Propagation: Large plants can be carefully divided and seeds can be collected in late summer and fall when the pods become dry and start to break open. Since in nature the seeds are distributed by the wind and are are equipped with silky fibers to keep them airborne, one has to either be quick once the pods open or encase them in little bags to catch the seeds before they fully open. Generally it is easiest to direct-sow the seeds in the garden; alternatively, they may require stratification.
Use in the Garden: Asclepias tuberosa cultivars come in a number of shades ranging from deep, glowing orange to bright yellow and make a great addition to the front or middle level of perennial beds and borders. They are also great for more naturalistic and informal designs, such as a planting of grasses and wildflowers that imitates their natural meadow habitat. The flowers also are very attractive to butterflies and like other species of Asclepias the plant can serve as a food source for the caterpillars of the monarch butterfly (Danaus plexippus), so this is a great plant to make a garden more butterfly-friendly.