Growing carrot seeds (Daucus carota var. sativus) takes a bit of pre-planting preparation and careful monitoring during germination, but it all pays off when you can start pulling those first, crunchy, sweet carrots from the warm, mid-summer soil. Carrots are technically a biennial hardy in U.S. Department of Agriculture plant hardiness zones 4 through 10, but they are grown so commonly as an annual that most seed companies and sources list them as such.
Time and Temperature
Carrot seeds take seven to 21 days to germinate. At 75 degrees Fahrenheit, the optimal soil temperature for carrot seeds, germination can take as little as seven days, but you don't have to wait that long to plant. Carrot seeds will germinate reliably in soil anywhere from 50 F to 85 F and can even germinate in soil as cold as 40 F. Use seeds less than three years old.
Begin with Good Soil
Clear the bed of visible stones, weeds and sticks, then spread compost evenly over the surface. Use an even 2- to 3-inch deep layer of vegetable-based, seasoned compost. Dig the soil 12 inches deep, breaking up clumps and mixing the compost with the soil as you go. Add additional nutrients by measuring out 1 cup of 10-10-10 fertilizer -- for each 10 foot row you intend to plant -- and scattering it evenly over the soil. Mix the fertilizer into the top 3 to 4 inches of the soil. A garden fork works well to break up clumps but is labor-intensive. To speed up the process, consider renting or purchasing a tiller.
Sow carrot seeds directly in the garden in a full-sun area -- at least six hours per day -- two to three weeks before the last frost date. Plant the seeds 1/2 inch deep and space them 1/2 to 4 inches apart. Seeds spaced 1/2 to 1 inch apart require thinning after germination. If you're growing carrots in rows, space the rows 12 to 24 inches apart. You can continue planting carrot seeds every two weeks through spring and summer.
Plant the last of the fall crop in late summer, 12 weeks before the first frost in the area.
Carrot seeds are poor candidates for starting indoors as they don't like to be disturbed and they do like the cool soil in spring and fall. You can, however, outdoors if you lack garden space. Choose a 12- inch-deep container -- or deeper -- filled with standard potting soil. Pick a container at least 18 to 24 inches wide.
Consistent moisture is important during germination. Water the soil at least 3 inches deep right after planting. Then, during the seven to 21 days it takes carrot seeds to germinate, water daily, misting the surface to keep it from drying out. Carrot seeds tend to germinate erratically so don't worry if a few overachievers pop up within the first week, followed by the rest of the crop.
Thin carrot seedlings when they grow 2 inches tall. Pull out extra seedlings until the remaining plants stand between 1 and 4 inches apart. In containers, thin the seedlings to a spacing of 2 to 3 inches.
Continue watering when the top of the soil starts to dry out keeping it moist at least 3 inches deep. Weed by hand during germination and through the growing season. Pull weeds carefully so you don't disturb roots.
Five to six weeks after planting carrot seeds or when the green tops stand 4 inches tall, scatter 10-10-10 fertilizer on the soil along the row. Use 2 level tablespoons per 10-foot row. Water after fertilizing until the soil feels damp 3 inches deep.
The true carrot harvest starts anywhere from 50 to 80 days from germination. If you can't wait, though, you can pull the first baby carrots as soon as 30 days after germination.