Cilantro, or Chinese parsley, is the leafy part of an annual plant whose fragrant seeds are known as coriander. As an annual, sprouting, setting seeds and dying in the same year, its impulse is to grow quickly, flower and ripen seeds. Your wish, on the other hand, is to harvest as long as possible. Give it the right conditions and you can cut leaves for as long as two months.
Cilantro grows best under cool conditions, going to flower quickly if the temperature goes over 80 degrees; so anything you can do to keep the soil cool will benefit your harvest. Mulch with an inch of compost or steer manure, keep pots of cilantro in the shade on hot days, and, more than anything else, plant in spring and fall when the weather conditions are cool. You can sow as early as the last frost date for your area.
Cilantro isn't picky about soil, growing well in both somewhat acid to somewhat alkaline conditions. Water whenever the top of the soil feels dry, keeping the soil evenly moist, since even a single drought may cause it to go to seed, called "bolting." Water should drain through the soil, not standing in puddles at the top.
Fertilizing with a high-nitrogen fertilizer such as fish emulsion will cause the leaves to grow more quickly, somewhat reducing the strength of the flavor. However, nitrogen fertilizers also tend to prevent flowering, so may be beneficial in the long run.
Sun And Shade
For leaf production, part shade is best for cilantro. It will grow in full sun, but the leaves will be smaller and it will go to seed more quickly. If you live in a warm weather area, try growing it in full shade. The soil will stay cooler and the plant will not go to seed as quickly. You might try early spring sowing in sun and later ones in shade.
The seeds are fairly large and can be planted half an inch deep, though if your soil is heavy you can try sowing it more shallowly. It is possible to plant indoors four to six weeks before last frost, but the plants, having a long central root, transplant poorly and may bolt more quickly than if planted outside. Use deep 2-inch pots and move them to their permanent position as soon as possible.
Cilantro sprouts in 7 to 10 days so successive sowings, about a month apart, will extend your harvest as long as possible.
Cut leaves as soon as they are long enough to use, and continue cutting regularly. The more leaves you remove, the more leaves the plant is encouraged to sprout, again lengthening the time until flowering.