Some cut flowers grow best when sown directly in the garden; others need an indoor head start. Knowing which approach to use for each flower saves time and improves results.
Flowers with taproots or that grow so fast there is no advantage to transplanting: zinnias, sunflowers, nigella, larkspur, and cosmos. Direct sow these after your last frost (or in fall for larkspur and nigella) and they will catch up to transplants within weeks.
Slow-growing flowers that need a long head start: lisianthus, snapdragons, stock, celosia, delphinium, eucalyptus, ageratum, rudbeckia. These need 6–12+ weeks of indoor growing time to be ready for the garden.
Marigolds, cosmos, amaranth, strawflowers, and scabiosa can go either way depending on your season length.
Minimize root disturbance. Use soil blocks or deep cell trays. Water transplants well the day before and after transplanting. Transplant on a cloudy day or in the evening. Always harden off before placing in the garden.