Coriander requires a minimum soil temperature of 55 to 60 degrees Fahrenheit to germinate. Plant coriander in the spring, when the soil has warmed sufficiently. Prepare the bed by spreading a 4-inch layer of compost over the soil and turning it into the top 8 to 12 inches with a garden fork or tiller. Plant the seeds directly into the garden bed. Coriander plants have deep taproots and do not transplant well.
Space coriander seeds 1 inch apart and space multiple rows 1 foot apart. Each coriander seed has the genetic material for two plants, so a single seed will yield two cilantro plants. Sow the seeds on top of the prepared garden bed and cover them with 1/4- to 1/2-inch of additional soil. Water the coriander seeds after planting to settle the soil and stimulate germination.
Coriander plants germinate and grow quickly. When growing cilantro plants for the fresh leaves, the plants can bolt and go to seed frustratingly fast. Once the plants go to seed, it is too late for a cilantro harvest, but the coriander harvest is just beginning. Coriander seeds germinate in seven to 10 days, and the seeds are ready to harvest two to three months after germination.
When growing cilantro for the leaves, gardeners plant in the spring and fall because the plants tend to go to seed quickly in hot weather. For a fast seed harvest, plant cilantro at the beginning of the hot season. Keep the soil damp around the coriander plants throughout the growing season. Thin the plants to 8 inches apart when they are 6 to 8 inches tall. Take the thinned plants to the kitchen to add to summer dishes.
Harvest coriander when the first set of seeds turns brown and begins to dry on the plant. Test the seeds by squeezing one between your fingers. If it cracks open, the seeds are ready to harvest. Cut down the plants at the ground and hang them upside down until all the seeds dry and turn brown. Shake the seeds onto a cloth. Separate out any plant material. Store coriander seeds in a cool, dry place for use in the kitchen or for next year's planting.