A member of the cabbage family, broccoli will easily crossbreed with any other cabbage family member that is planted nearby. If you want to save broccoli seeds from your garden, plant only one cabbage family vegetable in the garden or grow different plants at least 1,000 feet apart to prevent cross-pollination. For seed production, plant two or more broccoli plants, but don't harvest the florets from those plants. Instead, check the plants daily for pod production after the small yellow flowers wither. Broccoli seed pods look like long lengths of swollen stem.
Pick the pods from the plant when they dry and turn light brown. Harvest the pods over the course of a week as each matures and dries.
Place the pods inside a cloth bag. Beat the bag with a rubber mallet so the pods crack open and release the seeds.
Pour the contents of the bag into a shallow bowl. Sort through the seeds in the bowl and remove pieces of the pod and other plant matter, leaving only the seeds.
Place the seeds in an envelope and mark it with the type of broccoli and the year harvested. Store the envelope in a cool, dry place until you are ready to plant the seeds.