Ask Mr. Smarty Plants

Asclepias asperula is a species of milkweed native to the Southwestern United States and northern Mexico. Its common names (for the curving and pointed shape of the seedpods) include antelope horns, green-flowered milkweed, and spider antelope horns.

The wikipedia entry for antelope horns outlines the importance of this native plant: "Like several other species of milkweed, A. asperula is a food for monarch butterfly caterpillars. Along with being a source of nutrition for monarchs, the plants also contain alkaloids that the monarchs retain, making them unpalatable and poisonous to predators. For the same reason, A. asperula can be poisonous to livestock and other animals, including humans."

Generally milkweed seeds have a good viability when stored. The seedcoat is hard and will retain moisture for several years. However, storing the seed in a plastic bag in a drawer is not the best place for optimum viability. A sealed glass jar in the refrigerator is a much better place to store seed in the future. Still, it is worth sowing some seed to see if they will germinate for you this spring.

For an earlier question on germinating milkweed seeds, Mr. Smarty Plants consulted our Nursery Manager, Sean Watson. Here is what he said:

"I start them in the Spring or Summer (they like heat to germinate well). The fresher the seed the better. I just sow them and cover with maybe 1/8 inch of sterile seed starting media (peat based) and keep the propagation tray in part shade. They have long tap roots, so it is better to start them in a deep propagation tray (bulb trays work great, or 5" (or deeper) cell seed starting trays). Keep the media moist until it looks like all have germinated that are going to germinate (they tend to germinate very fast/easily). It is best to not transplant them until the taproot begins to thicken (root swells and begins to harden, starts to become a true taproot). This alleviates a lot of the shock they go through when transplanting. After transplanting (we use a container mix/compost/cut with 25% sand, but can just use straight compost), I water them in immediately, and water them every day for the next week while they are becoming established into the pot. After 1 week, allow the top 1/2 inch of soil to dry before giving them a little drink, do this for one more weeks (try to avoid flooding them at this stage – except Asclepias incarnata which is a water species and loves water, keep this species wet). Allow them to dry out 1 inch before watering during the third week. After the third week, you can start giving them a little more sun to harden them off. After a week of hardening off, place the plant in full sun and water only once the top inch of soil dries. You want to avoid watering them when they are dormant (just let the rain water them).

Another trick is to bury the seedling up to the cotyledons when you transplant. The cotyledons should rest just above the soil level, not on the soil. This will allow everything below the cotyledon to root out, thus giving your transplant an instantly longer taproot. I have had success transplanting seedlings of different Asclepias species once I see the first set of true leaves (many books suggest this). I usually transplant our seedlings directly into gallon pots to allow room for the taproot. I have had great success with Asclepias tuberosa in this way, and moderate success with Asclepias asperula. We are trying bulb trays this year and I have a feeling if we leave them in these trays longer so they can fully develop a taproot before transplanting, we may have Asclepias coming out of our ears!"

Our webpage on Asclepias asperula has the following information about propagation.

Propagation Material: Seeds

Description: Root cuttings can be taken in fall or early spring. Seeds may be sown outside in late fall or the following spring. Germination of spring-planted seeds is enhanced by moist stratification.

Seed Collection: Collect seed in June.

Seed Treatment: Stratify 3 months at 40 degrees. Germinates best in warmer half of the year.

Others have had success with starting antelope horns in the fall. Here's part of a Native Plant Society of Texas article by Znobia Wootan on Antelope horns.

Wootan says that it prefers to grow in well drained soil in full sun. “The easiest method for planting is to sow the seeds outdoors in September to November. This gives the seeds the exposure to the moisture and cold temperatures that it prefers, and once the temperature is warm enough in the spring, the seeds will germinate. Beginning in June, aggressively trim back one plant at a time, to provide fresh foliage for butterfly larvae all summer long. Another method includes a careful cold stratification in the refrigerator before spring planting.

Seed is best sown in a greenhouse as soon as it is ripe in the autumn or in late winter. We have also had good results from sowing the seed in the greenhouse in early spring, though stored seed might need 2-3 weeks cold stratification. Germination usually takes place in 1-3 months at 18°C. As soon as the seedlings are large enough to handle, prick them out into individual pots and grow them on in the greenhouse for their first winter. Plant out when they are in active growth in late spring or early summer and give them some protection from slugs until they are growing away strongly.

Division in spring. With great care since the plant resents root disturbance. Pot the divisions up and place them in a lightly shaded position in the greenhouse until they are growing away strongly, then plant them out in the summer, giving them some protection from slugs until they are established.

Basal cuttings in late spring. Use shoots about 10cm long with as much of their white underground stem as possible. Pot them up individually and place them in a lightly shaded position in a greenhouse until they are rooting and growing actively. If the plants grow sufficiently, they can be put into their permanent positions in the summer, otherwise keep them in the greenhouse until the following spring and when they are in active growth plant them out into their permanent positions. Give them some protection from slugs until they are established.