Garden · Seed starting
Cold Stratification Requirements by Species
Our seed stratification calculator uses general bands (grasses and legumes, prairie wildflowers, shrubs, fruit trees) for when you only know the general seed type. This page covers a few of the most commonly grown native species by name, where a more specific published number exists and should be used instead of the general band.
Common milkweed (Asclepias syriaca)
Cold stratification: ~30 days minimum, up to 84 days for the highest germination rates
Oklahoma State University Extension's native milkweed germination guide notes 30 days of cold moist stratification gets 85% germination or better for several native Asclepias species, though some growers stratify longer for a more even germination flush.
Swamp milkweed (Asclepias incarnata)
Cold stratification: ~30-84 days
Similar profile to common milkweed. The Lady Bird Johnson Wildflower Center's germination guidance for milkweeds covers both species in the same general range.
Purple coneflower (Echinacea purpurea)
Cold stratification: ~42 days (6 weeks)
One of the more forgiving prairie wildflowers to stratify, and one of the more commonly grown from seed by home gardeners.
Prairie coneflower (Ratibida columnifera)
Cold stratification: ~70 days (10 weeks)
Noticeably longer than purple coneflower despite the similar common name, worth checking the specific species rather than assuming all coneflowers behave the same.
Why milkweed and coneflower specifically
Both are among the most commonly seed-started native plants in home gardens, milkweed for monarch butterfly host plant gardens specifically, and both are frequently started from seed rather than transplants since they're less commonly sold as potted plants than typical garden annuals. That combination of popularity and seed-starting-first makes getting the stratification window right more consequential than it is for a plant most people just buy as a seedling.
Not seeing your species here
Use the general band in our seed stratification calculator based on the plant type (grass/legume, prairie wildflower, shrub, or fruit tree), and check your specific seed packet or a native plant nursery in your region for a species-specific number if one's published, that always beats a general band.