Wildflower season is beginning in Central Texas. One of the more interesting and important flowers is the Antelope Horn milkweed plant (Asclepias asperula) also known as spider milkweed, or green-flowered milkweed. From a distance the blooms appear as large green balls, but closer examination reveals the balls are actually clusters of small green and white flowers. The plants are one to two feet tall and are spreading. The leaves of the plant are long and narrow and often folded lengthwise. When the flowers fade, the seedpods grow curved and pointed resembling antelope horns. When the pods burst, the seeds are attached to a silky down that helps disperse the seeds. This down was used in World War II in regular and aviation life jackets.
Milkweeds are host plants for monarch and queen butterflies. Monarch butterflies use Central Texas as their first stop on their long migration north. Milkweeds are important because Monarch Butterfly caterpillars can only eat milkweeds. These caterpillars form chrysalises and then the new Monarchs continue their journey north for the next set of eggs and caterpillars. The flowers also produce nectar with a high glucose content that benefits many other species. Native bees and honeybees are also attracted and benefit from Antelope Horn.
As our pasturelands give way to development, planting milkweeds in gardens is more important. Grow antelope horns using seeds or propagating by root cuttings. Root cuttings can be done in fall or early spring. Plant seeds in either late fall or early spring. Seeds can be collected in June from established plants or can be ordered online. Pretreatment of seeds increases germination. Soak seeds overnight in water. Cold moist stratification involves chilling the seeds at 40 degrees for up to three months. Planting in late fall also allows this exposure to moist cold conditions. The seeds germinate best in warmer parts of the year.
Antelope horns develop large tap roots that allow it to flower even in dry years. It prefers well-drained soil with full sun. When growing Antelope Horn as a cultivated plant, you can trim it back one plant at a time to provide new fresh leaves for caterpillars all summer.
While it is of great benefit to butterflies and other pollinators, it also has some toxicity from cardiac glycosides. Deer and livestock leave it alone because it tastes bad. The toxins do not affect Monarch butterflies but it causes them to taste bad and be poisonous to the predators. Tropical milkweeds do not have the toxins and monarch caterpillars that feed on them do not have the benefits to protect them from predators. The sap can cause skin irritation in humans. Sensitivity varies based upon age, weight and individual sensitivity. However, children are vulnerable because they are curious and their small size can make the effects of a small dose of toxin more potent. The toxicity in the milkweed varies by season, which part of the plant, and the growth stage. The plant also has beneficial medicinal properties. However, the toxic cardiac glycosides are similar to digitalins used in treating heart disease. Native Americans made tea that was used as a tonic to strengthen the heart. Navahos used it to treat bites from rabid animals.
Antelope Horn is not the only milkweed that helps monarch butterflies. Green Milkweed, Butterfly Weed, Common Milkweed, Showy Milkweed, and Swamp Milkweed are other native species that can be planted in landscapes.