Carrot Planting Time. Carrots are a cool-weather crop best grown in spring, early summer, and autumn. Sow carrots in the garden 2 to 3 weeks before the last average frost date in spring. Succession crops can be planted every 2 to 3 weeks until about 12 weeks before the date of the average first frost in autumn. Where winters are mild grow carrots in autumn and winter. Carrots require from 50 to 80 days to reach maturity; baby carrots can be harvested in about 30 days.