Cabbage seeds come from cabbage plants that have produced flower stalks. The cabbage plant, once cross-pollinated, produces cabbage seeds out of the core of the plant. People growing cabbage must remove the plant's roots and store them in a cool area to preserve the cabbage seeds for the next season.
Gardeners use cabbage seeds to grow additional cabbage plants in early spring. The seeds ripen slowly and fall off once they are ripe. The cabbage plant can turn brown if not picked or harvested once it ripens. Cabbage seeds in an open garden may cross-pollinate with other plants of the same family, such as cauliflower, broccoli and brussels sprouts.