Swamp Milkweed Seed Balls for Fall Planting (Asclepias incarnata) (20) With gorgeous pink flowers, Swamp Milkweed is Monarch butterflies' favorite host plant. Scientifically developed process and recipe is more gentle on seeds than others. Provides nutrients and retains moisture for better germination and growth. Milkweed and most perennial wildflowers need to over-winter in the ground before they germinate. A 1/2" diameter seed ball for planting in the Fall- Requires moist soil.