If your intent is to attract pollinator insects or to raise seed,
you should sow seeds into a firm, weed-free seedbed in
early spring after the soil has reached a temperature of 40ºF, and the
danger of a killing frost has passed. Seed should be uniformly sown at a
depth of ¼-inch as germination is also inhibited by light. Sow seeds in
the late fall if you intend to use it as a winter-killed cover crop for
biomass production or to absorb excess nitrates in the soil.
Seeds
germinate within fifteen to thirty days, and germination rates increase
with rising soil temperatures above 37º to 68ºF. At soil temperatures
greater than 68ºF, the percentage germination decreases with total
germination halted when temperature reach 86ºF.
Informational References: