Here's a guide to planting Asclepias hirtella, also known as Tall Green Milkweed, in the USA, covering direct sowing outdoors in fall and spring, as well as starting seeds indoors:
Site Selection
Sunlight: Asclepias hirtella thrives in full sun, requiring at least 6-8 hours of direct sunlight daily. More sun usually means more blooms.
Soil: This milkweed prefers well-drained soil. It can tolerate a range of soil types, including sandy, loamy, and clay soils, but it's crucial that the soil doesn't retain excessive moisture. Poor drainage can lead to root rot.
Planting Outdoors
Fall Planting:
This is often the preferred time to sow milkweed seeds.
Plant seeds in late fall (September-November, depending on your location) before the first hard freeze.
Prepare the soil by clearing any weeds or debris and lightly loosening the top layer.
Scatter seeds on the surface of the soil. Since milkweed needs light to germinate, avoid burying the seeds too deeply. A very light covering of soil or even just pressing them into the surface is sufficient.
Water gently but thoroughly. Keep the soil consistently moist until germination.
Spring Planting:
Sow seeds after the last spring frost.
Germination rates may be lower with spring planting compared to fall planting, so scarification and stratification (explained below) are highly recommended.
Follow the same planting steps as for fall planting.
Starting Seeds Indoors
Timing: Start seeds 6-8 weeks before the last expected frost in your area.
Containers: Use seed starting trays or small pots with drainage holes.
Soil: Use a seed starting mix or a well-draining potting mix.
Planting: Sow seeds very shallowly, barely covering them with soil. They need light to germinate.
Watering: Keep the soil consistently moist but not waterlogged. Use a spray bottle or gentle watering can to avoid disturbing the seeds.
Light: Provide ample light. A sunny window may not be enough, especially in early spring. Grow lights are highly recommended for starting milkweed indoors.
Transplanting: Transplant seedlings outdoors after the last frost and when they have developed a few sets of true leaves. Harden them off gradually before transplanting. Space plants about 1-2 feet apart.
Stratification and Scarification
Stratification: Asclepias hirtella seeds benefit from stratification (cold treatment). This helps break dormancy and improve germination rates, especially for spring planting. Place seeds in a sealed plastic bag with a slightly damp paper towel or vermiculite. Store the bag in the refrigerator for 4-6 weeks before planting.
Scarification: Scarification (lightly scratching or nicking the seed coat) is also recommended, especially for spring planting, to improve germination. You can gently rub the seeds with sandpaper or use a nail file to nick the seed coat. Be careful not to damage the inner part of the seed.
Watering
Keep the soil consistently moist during germination and early growth.
Once established, Asclepias hirtella is relatively drought-tolerant. Water deeply but infrequently, allowing the soil to dry out somewhat between waterings. Avoid overwatering, as this can lead to root rot.
Maintenance
Asclepias hirtella requires minimal maintenance.
Deadheading spent flowers is not necessary unless you want to prevent self-seeding.
It's a host plant for monarch butterflies, so avoid using pesticides in the area.
Invasiveness
Asclepias hirtella is not generally considered invasive, but it can spread through self-seeding. If you want to control its spread, you can remove seed pods before they mature.
Additional Tips
Milkweeds are essential for monarch butterfly caterpillars, so planting Asclepias hirtella is a great way to support monarch populations.
Be patient with milkweed seeds. Germination can be erratic, and it may take several weeks for seedlings to appear.
Tall Green Milkweed can get relatively tall (2-4 feet), so consider this when choosing a planting location.
