There are over 200 species of Asclepias worldwide, and more than 70 of those native to North America; about a half-dozen can be found in parts of Oregon. It is highly useful as both a nectar plant for adult butterflies and as a food source for the growing caterpillars.
The milky substance in the stem and leaves of the plant that give it its common name contains a chemical compound that, when ingested by the caterpillar, acts as a repellant to predators. And some species of the plant are more compatible with specific species of butterflies, in terms of their protective qualities.
This is especially true for the Monarch butterfly and Asclepias speciosa, as it is the only food source for the Monarch caterpillars!
As habitat in the wild is lost to development, maintaining compatible food and nectar supplies for butterflies becomes even more important.
Besides their usefulness as a butterfly plant, the milkweeds are also beautiful. Leaves broad or thin, standing generally 2-3 feet tall (sometimes taller), with rounded clusters of tiny flowers that look like small stars, often in a mixture of creamy white and pink, sometimes deepening to a rich purple.
Asclepias requires excellent drainage and a mostly- to full-sun location; yellowing leaves indicate either too much water or poor drainage. Too much retained water can also result in root rot.
But in general, all species of butterfly weed are virtually pest and disease-free (if you notice the leaves on your plant are being eaten, you may have successfully attracted butterflies to your garden!).
Asclepias speciosa - Showy milkweed
Large (up to 6”) green to gray-green leaves, covered with fine hairs. Blooms generally from late spring to mid-summer. Grows to 3’. This is the native butterfly weed most commonly available for sale. Adaptable to conditions both sides of the Cascades, but excellent drainage a must. Photo credit: Vicki Watkins.
Asclepias fascicularis — Narrowleaf milkweed
Long, narrow leaves in whorls up the stems of this 2.5’ milkweed. Flowers are generally purplish-pink and white. It grows in dry to moist soil, near streams and at the coast — but note, in sandy (fast-draining) soil.
Asclepias cordifolia — Heartleaf milkweed
Elongated heart-shaped leaves, purple flowers arranged in more open, drooping umbels. This one is found in the dry spots of southern Oregon and into California. Extremely drought-tolerant once established, requires next-to-no summer water. Photo credit: First Light
An exception to the rule: There is an Asclepias that grows in wet conditions: Common name swamp milkweed (A. incarnata). Grows 3-4’ tall, narrow green leaves and clusters of bright pink flowers. Not native to Oregon (the farthest west it gets is Idaho, but if you have a wet garden, there is a butterfly weed for you too!)
Common Name: milkweed, butterfly weed
Origin: mostly open, moist to dry, rocky areas, low to mid elevation, throughout the region
Characteristics: Leaves opposite or whorled, on flowering stems 2-3 feet in height. Flowers are unusually formed clusters of five reflexed, fused petals and sepals that give them a star-like appearance. Color is a combination of white to pink to purple. Stems and leaves contain a milky sap that gives the plant its common name. Caution: most species of this plant is poisonous to humans.
Culture: Milkweed thrives in full sun to light shade, and requires very well-drained soil. Grows and spread by underground rhizomes. It is an excellent choice for a butterfly meadow or rock garden.
Pests/Diseases: pests and diseases generally not a problem. Yellowing leaves is a sign of over-water or insufficient drainage.
Important note: The chemical compounds in milkweed sap can cause dermatitis if it contacts the skin, and the plant is considered poisonous to humans.
Single Monarch Egg
Photo credit Forehand.jay
Amazing Monarch Butterflies:
Every autumn millions of Monarch butterflies undertake a 3000-mile migration from Canada and northern United States south into far southern California and Mexico to spend the winter where temperatures are warmer and food sources abundant. Northwest writer Robert Pyle wrote a book about this amazing annual phenomena, called Chasing Monarchs.
Here is more information about Asclepias and its relationship with Monarch butterflies, as well as about the annual migration: