Mr. Smarty Plants consulted our Nursery Manager, Sean Watson, to answer this question. 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 everyday 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!"
Here are a few photos from our Image Gallery of the milkweeds mentioned above: