How soon to start flower seeds?
Fast-growing annuals that bloom in summer (such as cosmos, sunflowers, and zinnias) shouldn’t be started more than 4 to 6 weeks before the last spring frost, otherwise they’ll get too big for their growing container and have soft, weak foliage and overgrown roots by the time you can plant them out into the garden. Garden experts agree that calendula is the undisputed champion of fast-growing flowers. Germination occurs just five to 10 days after planting, and blooms appear within six to eight weeks.