Carrot Seed are a cool-season crop that thrives in temperatures between 60°F and 75°F (15°C to 24°C). Start your carrot seed outdoors in early spring, as soon as the soil can be worked and is free from frost. In warmer climates, you can also sow carrot seeds in late summer for a fall harvest. Carrots prefer loose, well-drained soil with plenty of organic matter.