Gardeners should cut cilantro before it flowers when the leaves get to 6 inches. Once flowers develop, the cilantro leaves can acquire a bitter taste and gardeners should allow the flowers to open, then dry so they can harvest coriander seeds. Coriander just refers to the seed from cilantro.
When flowers develop, let them open and wait until the seed pods turn brown, then cut the plant off at the base. Put the plant in a paper bag to dry, then remove the seed pods for homegrown coriander.
Cilantro grows from late spring to early summer, and the plants tend to flower in mid summer. To get the most from your plant, cut the foliage at 6 inches, then let it grow and flower, and cut the second time for coriander.