Your growing parsley seeds should be ready for harvest as soon as they are a few inches (5 to 10 cm.) tall and are growing vigorously. Just snip the outer stems from the plant and it will continue to grow throughout the season. At the end of its growth cycle, the plant will produce a seed pod, at which time harvesting your own parsley seeds is possible. Keep in mind that parsley crosses with other parsley varieties, however. You need at least one mile (16 km.) between varieties to get the reliable seeds. Just allow the seeds to mature and dry on the plants before harvesting them. They can be kept in a cool, dry area for up to two to three years and retain their viability.