Rosemary seed takes 15 to 25 days to germinate under ideal conditions. Plant the seed about 1/4 inch deep in a well-draining germination mix. Water thoroughly and place the seed tray on a heating mat. Only about 30 percent of the seeds can be expected to germinate even under the best conditions, so plant seeds thickly. Starting rosemary from seed can be tricky, as germination rates are usually poor and it takes several months before the plants produce any usable leaves. A combination of heat, light, moisture and the right type of planting medium are necessary to successfully grow this herb in the garden or in pots on a windowsill.