Most seeds of temperate plants should be vernalized (cold treated); this ensures a higher germination rate than if seeds are sowed without this pre-treatment. Many of the southern species, such as tropical milkweed, will grow without cold treatment. The most successful means of milkweed vernalization is through stratification. By stratifying or subjecting seeds to a cold/moist environment for a short period of time, you simulate the conditions of a seed's natural break of dormancy that occurs when the seeds spend the winter in the ground. To stratify, first, obtain a substrate. Peat has been found to produce the best results. In addition, peat/clay also works well. Secondly, moisten the substrate with water and place the seeds in the cold soil. Store the seeds in a dark place (a refrigerator crisper works well) with a temperature of approximately 5°C for a minimum of 3 weeks up to 3 months. To allow for natural stratification, sow collected seeds directly into a mulched bed in the fall, and the seed will germinate the following spring.
If you have grow lights or a greenhouse, it is best to start your milkweed seeds indoors for a couple of months before you are able to transplant them outdoors. Fill the seedling trays approximately ¾ with potting soil (light, well-drained soils work best for most species) and scatter 3-4 seeds per cup, and then cover the seeds with an additional ¼ inch of soil. The soil is then fully saturated with water and placed either in a sunny window or directly under the grow lights; they need a lot of light and warmth to germinate and grow. It's best to keep the temperature at 26/24°C day/night with a 16-hour photo phase. Keep the soil moist, but don't overdo it. If the seedlings are too wet, fungal growth can occur and kill the seedlings. The seeds will take approximately 10 days to germinate. Once there are 4 true leaves on the seedlings (the seedlings will be approximately 3 inches tall), the plants can be transplanted into your garden. Most milkweed species do best in full sunlight, so choose an open area with lots of sun. Plant the seedlings 1-2 feet apart. The seedlings should be watered frequently; mulch can be used to help hold in the moisture around the plants.
For indoor use, plant the seeds just beneath the soil surface using a rather deep pot, as they have a long taproot. Once the plants are in the seedling stage, fertilize them once a week. To encourage fullness and more leaves, you can pinch off the top set of leaves (when there are at least two sets of leaves) to promote branching. It takes at least a month for the plant to be ready for the larvae to eat. Once the plant is big enough, you can simply place the entire plant, pot and all, into the cage. After the larvae have eaten the leaves, simply cut the plant off about two inches above the soil, and new shoots will grow in 3-4 weeks.
When planting seed outdoors, keep in mind that all plants have optimal soil temperatures for germination, which makes propagation a little more difficult. It is best to plant the seeds as early as possible, but make sure that you plant after the last frost. Sow milkweed seeds by scattering them on the soil surface 1/4-1/2 inch apart, and then cover with about 1/4 inch of additional soil. Water frequently after planting until plants become established. Many species need to be vernalized (cold-treated) before planting. See Monarch Watch's milkweed propagation guide for recommendations on growing milkweed.
Read more about planting milkweed