In a paper just published in Restoration Ecology, Echinacea Project researchers report that establishing Echinacea angustifolia in existing prairie restorations and abandoned agricultural fields requires more than 20 seeds for each plant that germinates and survives to flowering. Plants start flowering about 10 years after sowing. Also, burning the prairie before broadcasting seeds helps emergence and survival.
Wagenius, S., A. B. Dykstra, C. E. Ridley, and R. G. Shaw. 2012. Seedling recruitment in the long-lived perennial, Echinacea angustifolia: a 10-year experiment. Restoration Ecology 20: 352-359. Available here: http://echinaceaproject.org/