Mexican Milkweed (also called Bloodflower, Swallow-wort, Silkweed, Tropical Milkweed, Indian root, Butterfly Weed, Red Cottonweed, and Scarlet Milkweed) is botanically known Asclepias curassavica.
This tropical, herbaceous perennial (evergreen in milder areas) is hardy to zone 8, but can be grown as an annual in colder areas. Vertical and showy, this plant with its dramatic red-orange blooms can grow to 48 inches! The continuous supply of red-orange and yellow blooms from early summer to early fall is a visual treat for gardeners and also a food source for many winged Lepidoptera, flies and insect visitors.
Since Mexican Milkweed can reach such often unexpected heights, plant it in the middle or back of the border so that it doesn’t block other shorter plants. An alternative is to pinch the plant in the spring to cause more branching and a bushier form.
This South American native prefers slightly acidic to neutral soils. It is very adaptable and is easy to grow in dry, moist and even wet soils. Full sun to partial shade conditions are ideal with the best blooms in sun. Mexican Milkweeds are somewhat difficult to transplant as it has a deep fleshy, tap root. Better success is achieved when young plants are transplanted or container grown plants planted. They are slow to become established but become a tough plant once settled. This is an excellent xeriscape plant that should not be over-watered.
Mexican Milkweed is easy to propagate and many options are available to multiply plants. These include dividing the plant, rooting cuttings or sowing seeds. Seeds are ripe when the long, narrow pod splits open and releases the white fluffy parachutes. The plant easily self-seeds in warmer climates. Seeds are easy to germinate and can be sown immediately.
Pests include spider mites and milkweed (These are the same ones found on oleander) aphids (yellow-orange soft-bodied with black legs). Aphids are unsightly but don’t cause much harm if the plant isn’t under extreme stress. Leaves will turn a mottled yellow/brown color if aphids are numerous. Use a strong spray of water to wash off the aphids if desired (or let the ladybugs munch away if just a few are present). If aphids are numerous their excrement (honeydew) will produce a grayish black fungus called sooty mold. This will reduce the photosynthetic ability of the plant.
There might also be milkweed bugs on plants. These orange-red bugs have black antennae, legs and heads. They eat the seeds and tissue of the milkweed plants. They often gather as groups and are rarely numerous enough to be a problem. These bugs are one of a small group of insects that can tolerate the toxic compounds in milkweeds.
Butterflies and other nectar feeding insects are very attracted to the blooms. This is a very important butterfly host and larval food plant for the monarch butterfly but also is host for the queen and soldier butterflies (Monarch relatives). While migrating north, Monarch butterflies lay eggs on this plant even though it is not native to Texas. The yellow and black striped caterpillar only feeds on Asclepias spp. (Milkweed relatives). Monarchwatch.org lists Mexican Milkweed as the best plant for the garden and best for maintaining monarch butterflies. Experiments show most female monarch butterflies, if given a choice of milkweed types, will pick Mexican Milkweed to lay her eggs. The young caterpillars love the leaves and fortunately the plant is vigorous enough to re-grow the eaten foliage.
Milkweeds have many ethnobotany uses around the world. An internet search will supply plenty of details. The sap is an irritant if contact is made with the skin. Also many parts of the plant are poisonous if eaten by humans. The plant contains a toxin called galitoxin. This is found in all vegetative parts of the plant. This human and livestock toxin though, is helpful in protecting the Monarch butterfly. Monarch caterpillars that have fed on milkweeds ingest this compound which makes them distasteful to predators such as birds. The milkweed compound becomes more concentrated in adult butterflies thereby offering more inherent protection. Birds eating Monarch butterflies have been observed to vomit shortly after ingesting them.
Mexican Milkweed is not a native plant (and probably isn’t even from Mexico!) It is from South America. There is concern in some warmer areas (central and southern Florida) where it is naturalized and has become somewhat weedy through self-seeding. Most do not considered it invasive but there are those that wonder if it will push out the native Milkweeds. In Texas there are about 30 species of native milkweeds and only sightings of Mexican Milkweed in areas in the far south and southwest (Houston and Brownsville).
[Mexican Milkweed is hardy in zones 8 and warmer.]
‘Butterfly Red’ red and orange blooms
‘Silky Gold (syn. ‘Aurea)’ yellow and mango colored flowers
‘Silky Red’ (syn. ‘Silky Deep Red’) dark red and yellow flowers.
‘Silky Scarlet’ scarlet and red flowers