If seeds are kept in a cool, dry location they can last for several years. Place the seeds in an airtight container and place in the refrigerator. Silica gel packets placed into the seed container will keep the humidity level down. Broccoli, cauliflower, cabbage, collards, kohlrabi, endive, cucumber, lettuce, pepper, radish, sunflower, and tomato seeds can be stored for at least five years. Beans, carrots, celery, parsley, peas, pumpkin and squash can be stored for at least three years. Corn, onion, parsnip and spinach seeds are not recommended for storage for more than one season.