Overview of Tropical Milkweed
With numerous common names including tropical milkweed, bloodflower, scarlet milkweed, sunset flower, and silkweed, Asclepias curassavica is an interesting member of the milkweed family (Asclepiadaceae) thought to be native to the Caribbean, South America, Central America or Mexico. It is now found worldwide in the tropics. Although it exact origin is unknown, its species name reflects the place where the first specimens were likely collected, Curaçao, in the southern Caribbean.
It is hardy in zones 8b or 9a to 11b, remains evergreen where it is hardy. It is often grown as a frost-tender annual in temperate areas. In tropical and subtropical regions it can be a weed in pastures, fields, along roadsides and in disturbed areas.
This erect, evergreen perennial subshrub has opposite leaves and milky sap, which can be a skin irritant, and all parts of the plant is poisonous if ingested. The plants grow 3 to 4 feet tall, with simple or branched stems. The narrow leaves are 5 to 6 inches long and pointed at both ends. They are medium green and sometimes have white midribs.
The showy orange and red flowers are borne in rounded terminal and axillary clusters (umbellate cymes). In addition to the normal calyx (ring of 5 sepals) and corolla (ring of 5 petals) that other plant flowers have, milkweeds also have a corona that looks like an extra set of petals facing toward the center of the flower, like a crown. The pollen in milkweeds is not loose, but is instead contained in a purse-like sack called pollinia.
On A. curassavica flowers the corolla is typically red and the corona golden orange. The flower clusters are 2 to 4 inches across with 6-15 flowers in each group. Plants will bloom at 4 months old or younger, and will flower continuously from spring to fall in temperate climates and in tropical climates may be in flower at any time of the year. The long stems are excellent for cutting and the flowers have a long vase life.
The flowers are very attractive to butterflies, bees and other insects, as well as hummingbirds. Monarch butterflies use this species as a larval host plant. The striped caterpillars seem to prefer this species over many of the native species in our area that have tougher leaves. They have a higher survival rate and shorter developmental time on A. curassavica than on common milkweed, A. syriaca.
In warm, temperate climates where temperatures do not get cold enough to freeze, tropical milkweed is a problematic species for monarch, and not a beneficial species. The reason is that there is a protozoan parasite of monarchs (Ophryocystis elektroscirrha, OE) that is prone to building up on this species when it is not subject to winter cold temperatures.
Monarchs that eat leaves infected with OE become infected and have lower migration success, reduced lifespan, lowered flight ability, and reduced success in mating. Therefore, tropical milkweed should not be planted in mild temperate climates. It is a restricted plant in parts of California for this reason. In Wisconsin, it is an acceptable species to plant, but for optimum benefit for monarchs, species native to Wisconsin are best utilized. Avoid planting this species in areas with a hardiness zone of 7a or warmer to avoid it being problematic.
Following the flowers, spindle-shaped pods 3 to 4 inches long form. These eventually split open lengthwise to reveal ovate, flat seeds stacked in 2 rows. Each seed has a pappus of fine, white silky hairs on one end. Wind disperses the seeds, which float easily with the attached plumes. The intact pods can be dried to use in ornamental arrangements.
Landscape Use of Tropical Milkweed
This milkweed can be used in containers, borders, massed plantings, and cottage gardens. Because it has thin stems and an open form it looks best mixed with other tall plants, so its flowers peek out from among the other foliage and flowers.
It combines well with other milkweeds, coneflowers (Echinacea purpurea, E. paradoxa, Ratibida pinnata), asters, liatris, and ornamental grasses. It is ideal for use in butterfly gardens and adapts well to container culture. Due to the potential for OE protozoan parasite buildup, do not overwinter potted plants indoors in cold winter climates.
A. curassavica is quite tolerant of different soil types, growing well in dry, moist and wet soils. This species grows best in full sun. It is somewhat drought tolerant, although it grows best with consistent moisture.
In temperate regions, pests are not common, but aphids may colonize the plants, leading to sooty mold (a fungus growing on the honeydew aphids secrete) on the leaves. These pests can be washed off the plants with a forceful stream of water or can be controlled by repeated applications of insecticidal soap. Avoid using insecticides that kill caterpillars, so as not to endanger monarch caterpillars that often develop on the plants. Natural enemies, including lady beetles, green lacewings, and syrphid flies, often keep aphid populations at reasonable levels.
Propagating Tropical Milkweed
Tropical milkweed can be grown from cuttings, but it is quite easy to start from seed. Sow seeds indoors 8 to 10 weeks before last frost to get flowers earlier; plants from seeds sown outdoors may not bloom until late summer or early fall. Seeds typically germinate in 2 to 3 weeks at room temperature.
Seeds can be soaked in water overnight before planting to improve germination. This species is easily transplanted when young. Because it develops a long tap root, it is more difficult to transplant once established. Pinching the young plants will encourage branching and therefore more flower clusters. Plant seedlings outside after last frost date. This species may reseed if the developing pods are not removed.
Cultivars of Tropical Milkweed
A few varieties are available. These include:
- ‘Apollo Orange’ grows about 28 to 36 inches tall and has bright scarlet sepals and yellow corollas. A sister variety ‘Apollo Yellow’ is less commonly available. It has golden yellow flowers.
- ‘Monarch’s Banquet’ is a vegetatively propagated, variegated variety that has grey-green leaves with cream edges and pinkish tips. Plants grow 2 to 3 feet tall and the flowers have scarlet sepals and golden yellow corollas. It was discovered by Charlotte Merritt at Shady Oak Butterfly Farm in Florida.
- ‘Red Butterfly’ (sometimes listed as ‘Red Butterflies’) has scarlet sepals and yellow corollas and grows 2 to 3 feet tall.
- ‘Silky’ series grows 24 to 28 inches tall. The series includes ‘Silky Deep Red’ with red sepals and golden yellow centers and ‘Silky Gold’, with solid, golden yellow flowers.
– Susan Mahr, University of Wisconsin – Madison
Last Update: Allen R. Pyle, UW-Madison Extension, 2026
