Here's a guide to planting Asclepias fascicularis (Narrow-leaf Milkweed) in the USA, covering direct sowing and starting seeds indoors:
Site Selection
Sunlight: Narrow-leaf milkweed thrives in full sun, needing at least 6-8 hours of direct sunlight daily. It can tolerate some light shade, especially in very hot climates, but flowering and growth will be best in full sun.
Soil: This milkweed prefers well-drained soil. It can tolerate a variety of soil types, including sandy, loamy, and clay soils, as long as drainage is good. It is particularly well-suited to drier, leaner soils.
Soil Preparation
Amendments: Asclepias fascicularis is adapted to relatively poor soils and doesn't require rich soil. In fact, too much fertility can be detrimental. If your soil is very poor, you can amend it with a small amount of compost, but avoid adding excessive fertilizers.
pH: Narrow-leaf milkweed prefers a neutral to slightly alkaline soil pH (around 6.5-7.5).
Planting
Direct Sowing (Fall or Spring)
Timing:
Fall: In many areas, fall sowing is ideal. The seeds will naturally stratify over the winter and germinate in the spring.
Spring: Sow seeds in the spring after the last frost has passed.
Depth: Plant seeds about 1/4 inch deep. They need light to germinate, so don't bury them too deeply.
Spacing: Space seeds or seedlings about 1-2 feet apart.
Starting Seeds Indoors (Spring)
Timing: Start seeds indoors 6-8 weeks before the last expected frost.
Containers: Use seed starting trays or small pots.
Depth: Plant seeds about 1/4 inch deep.
Light: Provide ample light for seedlings. A sunny window or grow lights work well.
Stratification and Scarification
Stratification: Asclepias fascicularis seeds benefit greatly from stratification. This process helps break seed dormancy and improve germination rates.
Natural Stratification: Fall sowing provides natural stratification.
Artificial Stratification: If sowing in spring, stratify seeds in the refrigerator for 4-6 weeks before planting. Place seeds in a moist paper towel or in a bag with slightly damp sand or vermiculite.
Scarification: Scarification (lightly scratching or nicking the seed coat) can also improve germination, especially if you haven't stratified. Gently rub the seeds with sandpaper or a nail file. Be careful not to damage the inner part of the seed.
Watering
Keep the soil consistently moist during germination and early seedling growth.
Once established, narrow-leaf milkweed is quite drought-tolerant. Water deeply but infrequently, allowing the soil to dry out somewhat between waterings. Overwatering can lead to root rot.
Maintenance
Weeding: Keep the area around milkweed plants free of weeds, especially when they are young.
Pest and Disease Control: Watch for aphids and other common garden pests. Milkweeds are the host plant for monarch butterflies, so avoid using pesticides whenever possible.
Deadheading: You can deadhead (remove spent flowers) to prevent the plant from self-seeding if you don't want it to spread. However, leaving the seed pods provides food for insects and birds.
Invasiveness
Asclepias fascicularis is native to the western United States and is not considered invasive in its native range. It can spread by rhizomes (underground stems) and self-seeding, especially in disturbed areas. However, it is not generally considered aggressive or problematic. In some areas outside its native range, it might be considered potentially invasive, so check with your local extension service or native plant society.
