Binomial nomenclature is helpful. Instead of talking about common milkweed or swamp milkweed or tropical milkweed, it helps to know what the specific species is, when seeding and planting. They want different conditions.
Gomphocarpus physocarpus (hairy balls) is in the same family as Asclepias and is considered a milkweed. It is not a cold hardy species and grows similarly to Asclepias curassavica (tropical milkweed). Look at the seed in the photo to see that G. physocarpus seed are shaped like little rods, while A. currasavica are flattened flakes. Seed of both of these species do not need to be cold stratified, since they grow in tropical climates. Just plant them in moist seed starting mix or potting soil, if you are starting them indoors. They are started indoors in temperate zones, because we need more months of growing time, in order to enjoy blooms before summer’s end.
Cold stratification
The milkweeds of temperate climates, including A. speciosa, A. syriacus, A. incarnata, A. puprurea benefit from cold stratification for better germination. Stratification requires moisture. Simply place you seed on a damp (not dripping wet) paper towel and leave it outdoors or in the refrigerator for at least 4 weeks. Longer is fine, if you can’t get to it for awhile. Make sure to add a label (in case you forget what it is), you’ll be using it later when you plant.
Planting
The top photo shows a lot of equipment. LED grow light, heat mat, thermostat, seeding tray with clear dome, rack. I started out with recycles pots and clear takeaway containers over a radiator by a window. I still start in that spot, but for slower growing perennials that are going to be indoors for months, it helps to have better conditions. Light is really important so that the plants don’t get weak and stringy, the right temperature keeps them growing, rather than sitting and rotting.
The smaller kits are cheap and I got these at a dollar store and have reused them for many years. I also substitute the cell insert with little pots. I’ve seen them online recently.
Don’t worry about having fresh seed. Some of the seed I planted where about five years old and they germinated well.
Good potting mix or seed starting mix is critical. For difficult plants, I even sterilize it in the microwave. Bacteria, fungus spores and sometimes fungus gnat eggs will be in any mix. Garden soil and topsoil will have the most pathogens. Seedling indoors are rather like infants in ICU. Clean is good. Seeding mix is just finer, so easier for small seed with small roots to make their way.
Asclepias seed need light to germinate, so they are just pressed gently onto the surface of moist mix. In order maintain moisture,
Transplanting and Growing on
When planting most seed, not just Asclepias, it is common that more than one seed is planted, so that failure will not leave an empty cell/pot. The more seedlings growing together, the more there is competition for nutrients and survival. If you decide to split them, know that Asclepias are sensitive to transplanting. The less you transplant, the steadier they will grow and you reduce the risk of killing them or slowing them down.
With seed starting, it is easiest to move them when they just sprout, before their tap root grows. The top picture shows the stage when it’s easy to pick them off the surface and transplant them. They do just fine at this stage. One or two days later and the tap root will be an inch or two longer. This makes it more challenging to ease it out and more risky to break both root and stem. Even when they are larger plants, they may show their displeasure if you choose to disturb their roots.
Asclepias do best in full sun. You can’t cheat nature. Plenty of light is critical to their success indoors. Room temperature around 75 degrees F is borderline, because the temperature of moist soil will be several degrees lower.
Hardening Off
One last step before you plant them outside. Hardening off the the process of getting them used to the outdoors. Just like you getting a summer tan, don’t do it all in one day. Leave them out a few hours a day, somewhere sheltered from wind and full sun. They will dry to a crisp on a hot, windy day.
Not sure you can do this? You won’t know until you try. Have fun.