Gardeners who grow fennel in a home garden have fennel leaves, fennel stems and fennel seeds to harvest throughout the summer. Clip the leaves from a fennel plant as soon as they begin growing tall and feathery. Remove fennel stems from just above the crown of the plant when they flatten slightly. Collect fennel seeds when they turn brown in color because they will separate easily from the seed heads at this stage.
Use the pruning shears to trim feathery fennel leaves from the stems when they begin growing actively and the stems are at least 6 inches high. Trim away the top 3 inches of the leaves from each stem, leaving the rest of the leaves to continue growing.
Cut fennel stems from the fennel plant approximately 80 days after planting. Use the pruning shears to remove the largest stems just above the crown. Some gardeners opt to make the fennel stems larger by removing the seed heads from the stems two or three days before you want to remove the stems. When you remove the seed heads, the fennel plant will redirect its energies into growing larger stems.
Remove seed heads from the fennel plant when the seeds turn brown. Clip off the seed heads with the pruning shears and place them into the paper bag. Leave the seed heads in the sealed bag for one or two weeks until they dry completely and then carefully brush the seeds from the seed heads to save the seeds. Store the seeds in a small plastic container or glass jar.
Pull the entire fennel plant from the soil when the bulb is approximately 3 inches wide for the most tender and flavorful fennel vegetable. Cut the tap root from the bottom of the fennel bulb and discard it.